Proceedings of the San Diego Society of Natural History

MczZ
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BUNODONT ARTIODACTYLS (MAMMAEIA),,..
FROM THE UINTAN (MIDDLE EOCENE)’*">
OF SAN DIEGO COUNTY, CALIFORNIA

by

Stephen L. Walsh

No. 37
30 September 2000
Proceedings of the
San Diego Society of Natural History

7 ISSN 1059-8707
e

Number 37

PROCEEDINGS

of the
San Diego Society of Natural History

Founded 1874

Bunodont Artiodactyls (Mammalia) from the Uintan (Middle Eocene)
of San Diego County, California

Stephen L. Walsh
Department of Paleontology, San Diego Natural History Museum, P.O. Box 121390, San Diego, California 92112-1390

ABSTRACT.—This report describes new specimens of several bunodont artiodactyl taxa from Uintan (middle Eocene) rocks of San Diego County.
Antiacodon venustus was previously known only from the upper part of the Bridger Formation but is now recorded from the early Uintan Friars
Formation and constitutes another “Bridgerian holdover taxon” present in the latter unit. A single distinctive upper molar proves the occurrence of a
second antiacodontine species in the Friars Formation, but more material is needed to identify this taxon. Tapochoerus egressus was previously known
only from the late Uintan part of the Sespe Formation in Ventura County but is now recorded in San Diego County from the upper part of member C
of the Santiago Formation. This occurrence adds to the already high degree of faunal similarity between the San Diego and Ventura County areas during
late Uintan time. Tapochoerus memillini is a relatively small new species of the genus described from late Uintan strata of the Mission Valley Formation
and member C of the Santiago Formation. It occurs lower in member C than 7: egressus. The large helohyine genus Achaenodon was previously known
only from early Uintan strata of the Rocky Mountain region and is now recorded from the Friars Formation. The available specimens are most similar
to A. robustus. Two associated helohyine upper molars from the lower tongue of the Friars Formation are assigned to the rare taxon Parahyus and may
represent a new species of this genus, which was previously known only from the early Uintan(?) of Wyoming and Texas. While the diversity of
bunodont artiodactyls in the California Uintan is significantly greater than previously known, this fauna still lacks numerous “homacodont” taxa that

30 September 2000

are common in Uintan rocks of the western interior.

INTRODUCTION

Eocene artiodactyls from southern California were first described
from the Sespe Formation in Ventura County. Stock (1934a) named
the supposed hyopsodontid “‘condylarth” Hyopsodus egressus from
the late Uintan Tapo Canyon local fauna. Stock (1934b) also named
the hypertragulid genus Simimeryx from the Duchesnean Pearson
Ranch local fauna of the Sespe. Finally, Stock (1936) named the new
subgenus and species Leptoreodon (Hesperomeryx) edwardsi from the
Tapo Canyon local fauna. In his review of the “upper” (now regarded
as middle) Eocene artiodactyls of North America, Gazin (1955) con-
curred with Stock’s analysis of Simimeryx and Leptoreodon (Hespero-
meryx) edwardsi. However, McKenna (1959) recognized that Stock’s
species Hyopsodus egressus actually pertained to a new genus of
“dichobunid” artiodactyl, which he named Tapochoerus.

Golz (1976) reviewed the Eocene selenodont artiodactyls of
southern California. He named the basal oromerycid genus Meryco-
bunodon from the early Uintan Friars Formation and named several
new species of Protoreodon, Protylopus, and Leptoreodon from
Uintan and Duchesnean strata of San Diego and Ventura counties. In
addition, Golz named the new camelid species Poebrodon californi-
cus from the latest Uintan and/or Duchesnean Laguna Riviera local
fauna of San Diego County and recognized the advanced oromerycid
Eotylopus from the Duchesnean of Ventura County. Kelly (1990),
Kelly et al. (1991), and Kelly and Whistler (1994) have updated the
occurrences of various artiodactyl taxa from the Sespe Formation.
The most recent contribution to our knowledge of the Eocene artio-
dactyls of southern California is that of Theodor (1999), who named

a new species of Protoreodon from the late Uintan of San Diego
County.

Golz (1976:17, 27) noted the rarity of bunodont artiodactyls
(“dichobunids’’) in existing California collections but suggested that
“more dichobunids may be added to the Eocene fossil record of Cali-
fornia as collecting continues in Uinta B equivalents in San Diego,
and perhaps animals related to Tapochoerus egressus will be discov-
ered.” During the past sixteen years, PaleoServices, Inc., and the San
Diego Natural History Museum have conducted extensive paleonto-
logical monitoring of construction-related grading operations in San
Diego County. These salvage efforts have yielded important new
collections of Eocene mammals; among them are several taxa of
bunodont artiodactyls. A few specimens of bunodont artiodactyls
have also been located in the San Diego Eocene collections of the
Natural History Museum of Los Angeles County and the University
of California Museum of Paleontology, Berkeley. This report de-
scribes the new material and reviews the paleobiogeographic distri-
bution of middle Eocene bunodont and bunoselenodont artiodacty]
taxa in southern California and the western interior.

Golz and Lillegraven (1977), Walsh (1996), and Walsh et al.
(1996) discussed various Eocene local faunas and geographic col-
lecting districts of San Diego County. Walsh et al. (1996) provided a
reinterpretation of the Friars Formation and Poway Group (Kennedy
and Moore, 1971; Kennedy, 1975; Kennedy and Peterson, 1975) that
is critical to understanding the stratigraphic provenance of the Uintan
mammals of southwestern San Diego County. Briefly, the Friars For-
mation is subdivided informally into three parts: a lower sandstone
and mudstone tongue, a middle conglomerate tongue, and an upper

i)

sandstone and mudstone tongue, all of which appear to be entirely of
early Uintan age. West of the western pinchout of the conglomerate
tongue, the lower and upper tongues of the Friars cannot be distin-
guished; these sandstone-dominated strata are referred to as the Fri-
ars Formation, undifferentiated.

CLASSIFICATION

Gazin’s (1955) traditional classification of North American Eocene
“dichobunid” artiodactyls has been modified by the rather different
arrangements of Gentry and Hooker (1988), McKenna and Bell (1997),
and Stucky (1998). Gentry and Hooker’s (1988) analysis placed
Antiacodon and leptochoerines within the Selenodontia, and helo-
hyines within the Bunodontia. Gentry and Hooker (1988) regarded
Selenodontia and Bunodontia as unranked taxa between order and sub-
order, named the new selenodont suborder Merycotheria to include
Ruminantia and various “dichobunids” (excluding the “Uintan
homacodontines’’), and named the new family Bunomerycidae to in-
clude the Uintan “homacodontines” Bunomeryx, Hylomeryx, Meso-
meryx, Mytonomeryx, and Pentacemylus. Gentry and Hooker (1988)
assigned the Bunomerycidae to the suborder Tylopoda within
Selenodontia, and apparently regarded leptochoerines and Diacodexis
metsiacus as incertae sedis within Selenodontia.

McKenna and Bell (1997) recognized Suiformes, Tylopoda, and
Ruminantia as three suborders of Artiodactyla. They retained
Diacodexeinae, Leptochoerinae, Homacodontinae, and the European
Dichobuninae as subfamily-level taxa within the Dichobunidae; they
raised the Helohyinae to family rank and included the Asian genera
Gobiohyus and Pakkokuhyus within it. McKenna and Bell also placed
the Dichobunidae, Helohyidae, Cebochoeridae, and Mixtotheriidae
within the superfamily Dichobunoidea and assigned the Dicho-
bunoidea to the suborder Suiformes (= Bunodontia).

Stucky (1998) did not recognize the superfamily Dichobunoidea.
He restricted the family name Dichobunidae to European taxa and
assigned various former North American “dichobunids” to “Diaco-
dexis,” the “homacodonts,” and the subfamilies Antiacodontinae,
Leptochoerinae, and Helohyinae. Stucky left the Antiacodontinae and
Leptochoerinae incertae sedis within Artiodactyla and regarded the
“homacodonts” and the Helohyinae (excluding Gobiohyus and
Pakkokuhyus) as basal members of the suborder Selenodontia.

Obviously, the cladistic relationships of many of these early ar-
tiodactyls are unclear, and Stucky’s (1998) conservative approach
has merit in that it calls attention to the magnitude of our ignorance.
Bunodont taxa described here pertain only to the Antiacodontinae
and Helohyinae as envisioned by Stucky (1998). Specimens of the
leptochoerine /barus sp. cf. I. ignotus recorded by Walsh (1996: table
2) will be described elsewhere.

METHODS

Specimens are sometimes introduced in the form “XXXX/
YYYY,” where “XX XX” represents the locality (Loc.) number and
“YYYY” the specimen number. Unless stated otherwise immedi-
ately below, measuring orientations and measuring endpoints of
cheek teeth follow those of Golz (1976:3). For upper molars, the
anteroposterior (AP) axis passed through the apices of the paracone
and metacone. The anterior and posterior width was measured per-
pendicular to the AP axis, from the lingual base of the protocone to
the labial base of the paracone and metacone, respectively. For lower
molars, the trigonid width and talonid width were oriented perpen-
dicular to the AP axis and respectively measured the maximum trans-
verse width of the anterior and posterior halves of the tooth. Depend-
ing on the size of the specimen, measurements of teeth were made
either with an Ehrenreich Photo-Optical Industries “Shopscope”’ to
the nearest 0.01 mm, or with a pair of vernier calipers to the nearest
0.1 mm. Abbreviations are as follows:

Stephen L. Walsh

Dental:

AP, anteroposterior length of upper and lower teeth.

AW, anterior width of upper molars.

C and c, upper and lower canine, respectively.

DP and dp, upper and lower deciduous premolar, respectively.

i, lower incisor.

Land R, left and right, respectively.

P and M, upper premolar and molar, respectively.

p and m, lower premolar and molar, respectively.

PW, posterior width of upper molars.

W, maximum transverse width (for upper and lower premolars).

WTRI, maximum width of trigonid.

WTAL, maximum width of talonid.

Institutional:

AMNH, American Museum of Natural History, New York.

BCHS, Bouxwiller collection of the Muséum d’ Histoire Naturelle,
Basel, Switzerland.

CM, Carnegie Museum of Natural History, Pittsburgh.

DMNH, Denver Museum of Natural History.

LACM (CIT), original collections of the California Institute of Tech-
nology, now housed at the Natural History Museum of Los Ange-
les County, Los Angeles.

MCZ, Museum of Comparative Zoology, Harvard University, Cam-
bridge, Massachusetts.

MPM, Milwaukee Public Museum.

SDSM, South Dakota School of Mines and Technology, Rapid City.

SDSNH, San Diego Natural History Museum.

SMNH, Royal Saskatchewan Museum, Regina.

TMM, Texas Memorial Museum, University of Texas at Austin.

UCM, University of Colorado Museum of Geology, Boulder.

UCMP, University of California Museum of Paleontology, Berkeley.

UM, University of Michigan Museum of Paleontology, Ann Arbor.

USNM, United States National Museum, Washington, D.C.

UW, University of Wyoming, Laramie.

YPM, Yale Peabody Museum, New Haven.

YPM-PU, original Princeton University collections, now owned by
YPM.

SYSTEMATIC PALEONTOLOGY
Order Artiodactyla Owen, 1848
Suborder incertae sedis
Family incertae sedis
Subfamily Antiacodontinae Gazin, 1958

Remarks.—McKenna and Bell (1997:401) placed Antiacodon
and Auxontodon in the subfamily Homacodontinae, but I follow
Storer (1984a) and Stucky (1998) in uniting these two genera with
Tapochoerus and Neodiacodexis in the subfamily Antiacodontinae.
McKenna and Bell (1997) also assigned Antiacodon to the super-
family Dichobunoidea and the latter to the suborder Suiformes. This
conflicts with Gentry and Hooker’s (1988, fig. 9.8) placement of
Antiacodon within the suborder Selenodontia. Stucky’s (1998) con-
servative approach is partly followed here by leaving the familial
and subordinal assignment of the Antiacodontinae in doubt.

Stucky (1998) characterized the Antiacodontinae by their posses-
sion of (1) a strong cristid obliqua that extends to the metaconid apex,
(2) a paraconid subequal in size to or larger than the metaconid on m1—
3, (3) a small cingular hypocone, and (4) a weak accessory cusp
(anterocone) on the anterior cingulum. Characters 1, 2, and 4 are po-
tential autapomorphies for the taxon, although in Tapochoerus the
anterocone is absent or very weak and the paraconid is partially or
completely merged with the metaconid on m2-3. Upper molars of all
antiacodontine genera have a distinct gap between the protocone and

Bunodont Artiodactyls (Mammalia) from the Uintan (Middle Eocene) of San Diego County, California 3

metaconule (one new species of Tapochoerus excepted), but this char-
acter is probably primitive for the subfamily, because it also occurs in
Hexacodus, Homacodon, and Microsus (although not in Diacodexis).

Antiacodon Marsh, 1872

Type Species.—Antiacodon venustus Marsh, 1872.

Included Species.—Antiacodon pygmaeus (Cope, 1872), A.
furcatus (Cope, 1873a), and A. vanvaleni Guthrie, 1971.

Diagnosis —Differs from Auxontodon in having upper molars
that are lower-crowned, with weaker mesostyles, conules, cingula,
and hypocones, and in having a ventral border of the dentary that is
slightly rather than strongly convex. Differs from Neodiacodexis in
lacking crenulated enamel, lacking a cuspule on the posterior cingu-
lum of P4, and in having upper molars with weaker lingual cingula
and less crescentic cusps and conules. Differs from Tapochoerus in
having upper molars with (weak) mesostyles, stronger anterocones,
and stronger labial cingula, and in having a (weak) metaconid on p3,
large separate paraconids on m1-3, and less anteroposteriorly elon-
gated lower molars. No autapomorphies known.

Known Distribution—Late Wasatchian to late Bridgerian (late
early Eocene to early middle Eocene) of the Rocky Mountain region
(Willwood, Wasatch, Wind River, Green River, Bridger, Huerfano,

4

:

and Aycross formations; Stucky, 1998); early Uintan (early middle
Eocene) of southern California (Friars Formation).

Antiacodon venustus Marsh, 1872
Figures 1-3

Diagnosis.—Largest known species of the genus; linear dimen-
sions of cheek teeth ranging from 15% to 43% larger than in
Antiacodon pygmaeus, A. furcatus, and A. vanvaleni. Differs from A.

furcatus in having a relatively weaker postcingulid and a relatively

weaker hypoconulid that is more closely appressed to the entoconid
on ml—2. Differs from A. vanvaleni in having stronger paraconids.
No morphological autapomorphies known.

Referred Specimens.—Friars Formation, undifferentiated:
SDSNH 3784/49223, LM2?; 3784/49550, LM2?; 3784/51379,
Ldp4?; 3784/49170, L dentary fragment with p4—m3; 3784/50565, L
dentary fragment with p4—m3 + alveoli for p2—3; 3784/50576, Lm3;
LACM (CIT) 249-S/56127, Lm3. UCMP V-68101/104316, LM3.

Upper tongue of Friars Formation: SDSNH 3611/45300, Rm2.

Conglomerate tongue of Friars Formation: SDSNH 3621/55891,
L dentary fragment with p4 + m2—3. SDSNH 3824/6087 1-60872,
associated RM1 and RM2, respectively.

Description—No skull, maxillary material, or upper teeth ante-

Figure 1. Stereophotographs in occlusal view of upper molars of Antiacodon venustus from the Bridger and Friars Formations: (A), YPM 37189, L maxillary
fragment with M2—3; (B), UCM 57498 and 57499, associated LM1 and LM2, respectively; (C), MPM 6717, associated LM1-2; (D), SDSNH 49550, LM2?; (E),
SDSNH 49223, LM2?; (F), UCMP 104316, LM3; (G), SDSNH 60872 and 60871, associated RM2 and RM1, respectively. Scale divisions in mm.

4 Stephen L. Walsh

aii

hogosccancaeiuaonstestses

Figure 2. Stereophotographs in occlusal view of lower dentitions of Antiacodon venustus from the Friars Formation: (A), SDSNH 49170, L dentary fragment
with broken roots of p3 + p4—m3; (B), SDSNH 50565, L dentary fragment with alveoli for p2-3 + p4—m3; (C), SDSNH 55891, L dentary fragment with p4 +

m2-3. Scale divisions in mm.

rior to the molars are known. Five isolated upper molars are available
from the Friars Formation (Figures 1D-—G). SDSNH 60871 and
60872 (Figure 1G) almost certainly pertain to the same individual
and can be confidently identified as M1 and M2, respectively. As
expected from maxillary specimens of Antiacodon pygmaeus with
M1-2 in place, the M2 is slightly larger than M1. The paracone and
metacone of M12 are conical and subequal in height and diameter.
The weak postparacrista and premetacrista barely converge but do
not form a distinct centrocrista. A strong ectocingulum is present
along the entire preserved labial border of each tooth and bears a
very weak mesostyle. Distinct, subequal paraconules and meta-
conules are present, with weak preparaconular and postmetaconular
wings. The protocone is conical, and shorter but broader than the
labial cusps. There is a distinct gap between the protocone and the
metaconule. A strong anterior cingulum extends from the antero-
lingual base of the protocone all the way to the weak parastyle. A
weak cuspule (anterocone) is present on the anterior cingulum below
the protocone on all four M1—2s. The hypocone is distinct but much
smaller than the protocone and located at the posterolingual base of
the latter. A posterior cingulum extends from the hypocone apex to
the metastyle area. SDSNH 49550 and 49223 (Figures 1D—E) have a
relatively blunt lingual margin, are slightly larger than SDSNH 60871
(undoubted M1), and are tentatively identified as M2s. M1—2 are

three-rooted. The lingual root is the largest and is essentially cylin-
drical, while the two smaller labial roots are subequal in diameter
and slightly compressed anteroposteriorly.

A single M3 is known (UCMP 104316, Figure 1F). The tooth is
essentially complete, lightly worn, and pentagonal in occlusal out-
line. The presence of a distinct anterior appression facet and the ab-
sence of a posterior facet confirm the tooth to be an M3. The lingual
root is the largest and is essentially cylindrical. The two smaller la-
bial roots are broken away but were subequal in diameter. The para-
cone is essentially conical and is larger and taller than the conical
metacone. A weak rib extends down the anterior face of the paracone
to connect with a very weak parastyle. There is no metastyle. The
strong labial cingulum is damaged but was clearly complete around
the labial border of the tooth. A shallow ectoflexus seems to have
been present. The centrocrista is positioned relatively labially and
connects the bases of the paracone and metacone. A distinct
mesostyle is absent, but this cuspule may have been located on the
damaged labial cingulum. The paraconule and metaconule are rela-
tively large. A strong preparaconular wing is present, but a
postparaconular wing is absent. The metaconule bears both pre- and
postmetaconular wings. The protocone is low and broad, weakly
connected to the paraconule, and separated from the metaconule by a
distinct gap. The anterior and posterior cingula are quite strong, and

Bunodont Artiodactyls (Mammalia) from the Uintan (Middle Eocene) of San Diego County, California 5

Figure 3. Stereophotographs in occlusal view of lower dentitions of Antiacodon venustus from the Bridger and Friars formations: (A), Cast of
YPM 11765, Lm3, holotype of A. venustus; (B), YPM 13272, L dentary fragment with p4—m2; (C), YPM 16861, L dentary fragment with m1—3; (D), SDSNH

50565, L dentary fragment with alveoli for p2—3 + p4—m3. Scale bar = 10 mm.

almost continuous around the lingual base of the protocone. There is
no hypocone and no anterocone.

Three dentary fragments are known, of which SDSNH 50565 is
the most nearly complete (Figure 2B). The condyle and coronoid
process are not preserved. The depth of the ramus below m1-3 is a
nearly constant 9.5 mm and decreases to about 7.0 mm below p3.
The ventral border of the dentary is slightly convex as in Antiacodon
(see Burke, 1969; West, 1984, fig. 7) and not strongly convex as in
Auxontodon pattersoni (see Gazin, 1958). The masseteric fossa dis-
plays a weak ventral division and a deeper dorsal division. The fossa
extends anteriorly to a point below about 2.5 mm posterior to the
posterior end of m3. The ascending ramus starts to rise from the dor-
sal border of the dentary immediately posterior to the posterior end
of m3. A single tiny mental foramen is present below the anterior root
of p3 on SDSNH 49170 but occurs below the posterior root of p3 on
SDSNH 50565 and below the anterior root of p4 in SDSNH 55891.
As seen in SDSNH 50565, there is a short diastema of about 1.8 mm
between p2 and p3.

No lower teeth anterior to p4 are known. The broken roots and/or
alveoli for p3 are present in SDSNH 49170 and 50565 and show that
this tooth was subequal in size to p4. A possible dp4 is tentatively
assigned to Antiacodon venustus (SDSNH 51379, not figured). This
tooth is unlikely to be a p3, because the p3 in Antiacodon pygmaeus
is relatively short anteroposteriorly, with a simple trenchant crown
and very weak metaconid (e.g., MPM 6721; West, 1984, fig. 6). The
crown of SDSNH 51379 is heavily worn, but there was clearly a
distinct metaconid, paralophid, and paraconid. However, there is no
distinctly expanded anterior lobe as in the dp4 of other small artio-
dactyl taxa in the Friars Formation, such as Leptoreodon and cf.
Protylopus. The talonid appears to have had a distinct hypoconid, a
weaker entoconid, and a distinct cristid obliqua. The tooth is
anteroposteriorly elongate, with subequal talonid and trigonid widths.

The anterior and posterior roots have been broken away, and their
thin-walled remnants are filled with matrix. The AP length of SDSNH
51379 is 6.25 mm (minimum value owing to slight damage), and the
maximum width is 3.29 mm.

Three p4s are known, all of which occur in dentary fragments.
The protoconid is tall and conical. A distinct metaconid is present on
the posterolingual flank of the protoconid on SDSNH 50565 and
55891 (Figures 2B—C) but is very weak on 49170 (Figure 2A). A
distinct paraconid is present on 49170 and 50565. Two “paraconids”
are present on SDSNH 55891, occurring atop the anterior and poste-
rior ends of the paralophid. A posterolabial cingulid is strong in all
three available p4s, while the anterolabial cingulid is weak in SDSNH
49170 and 50565 and absent in SDSNH 55891. The talonid lacks
accessory cuspulids. A weak cristid obliqua is present on all three
p4s, extending from the hypoconulid area up the posterior face of the
protoconid.

Two mls are known, both of which occur in dentary fragments
(Figures 2A—B). The trigonid is distinctly narrower than the talonid.
The strong, conical paraconid is taller than the metaconid and proto-
conid. A weak anteriorly convex paralophid connects the anterior
faces of the paraconid and protoconid. An even weaker metalophid
connects the metaconid with the lingual face of the protoconid. A
distinct anterior cingulid is present. The strong cristid obliqua ex-
tends from the hypoconid apex up the labial face of the metaconid. A
weak ectostylid is present in the hypoflexid. The entoconid is conical
and much smaller than the hypoconid. A weak hypoconulid is present
at the lingual end of the distinct postcingulid and is separated from
the entoconid by a narrow notch.

Four m2s are known, three of which occur in dentaries (Figure 2).
They are very similar in morphology to the mls, including the invari-
able presence of a strong paraconid that is well separated from the
metaconid. The m2s do differ from the mls in being slightly larger, in

6 Stephen L. Walsh

TABLE 1. Measurements (in mm) of cheek teeth of Antiacodon venustus from the Friars Formation. Asterisks indicate minimum
value because of slight damage, enamel dissolution, or interdental wear.

P4 M1 M2 M3
W L AP AW PW AP AW PW AP AW PW
SDSNH 49550 _— 6.46 _—
SDSNH 49223 _— 6.73 —
SDSNH 60871 = — Spilah- 5.81 5.94
SDSNH 60872 5.41 6.72 6.71 — — —
UCMP 104316 4.92 5.95 5.62*
Mean: = 6.64 —
p4 ml m2 m3
AP W AP WTRI WTAL AP WTRI WTAL AP WTRI WTAL
SDSNH 49170 5.22 3.32 5.88 3.77 4.36* 6.04 4.07* 4.59 6.80* 3.95 3.56
SDSNH 50565 5.86 3.50 5.50 3.80 4.12 5.66 4.15 4.62 6.86 3°97, 3.83
SDSNH 55891 6.34 3.61 5.93 4.27 4.55 6.85 4.32 4.11
SDSNH 45300 5.77 4.16 4.45
SDSNH 50576 6.47 3.84 3.82
LACM 56127 = — — 6.30 3.56 3.36
Mean: 5.81 3.48 5.69 3E79 4.24 5.85 4.16 4.55 6.66 3.93 3.74

having trigonid widths that are more equal to the talonid widths, and
in having weaker or absent ectostylids.

Five m3s are known, three of which occur in dentaries (Figure 2).
The trigonid morphology is very similar to that seen in m1—2, includ-
ing the invariable presence of a strong paraconid that is well sepa-
rated from the metaconid. However, the m3 entoconid is subequal in
size to the hypoconid, and a strong hypoconulid is present on the
hypoconulid lobe. A short posterolabial cingulid is present between
the hypoconid and hypoconulid. An anterolabial cingulid is com-
plete around the base of the protoconid in SDSNH 50576 but is ab-
sent on SDSNH 49170, 50565, 55891, and LACM 56127. All m3s
lack an ectostylid. A distinct ridge connecting the hypoconulid with
the region between the entoconid and hypoconid is present in SDSNH
50565 but is absent in all other San Diego m3s. Finally, LACM 56127
differs from the other m3s in the sample in having a very weak ento-
conid. Table | provides dental measurements of Antiacodon venustus
from the Friars Formation; Figure 4 shows bivariate plots for p4—m3.

Comparisons.—Stucky (1998) included several genera in the
Antiacodontinae, which are compared to the Friars Formation speci-
mens as follows. The upper molars of Antiacodon venustus from the
Friars Formation differ from those of the only known specimen of
the late Bridgerian species Neodiacodexis emryi in that M1—2 of the
latter range from 2% to 18% larger in linear dimensions, have
strongly crenulated enamel, slightly stronger parastyles, stronger
doubled anterocones, larger paraconules and metaconules, stronger
pre- and postparaconular and metaconular wings, sharper centro-
cristae (giving the paracone and metacone a transversely compressed
appearance), and a deeper ectoflexus (West and Atkins, 1970).

The lower cheek teeth of Antiacodon venustus from the Friars
Formation differ from those of the late Uintan species Auxontodon
pattersoni Gazin, 1958, in that the type specimen of the latter (MCZ
9316, Figure 5) has slightly larger and more anteroposteriorly elon-
gated p4—ml, has a tiny but distinct “entoconid” on p4, lacks an
ectostylid on m1, has a much deeper dentary whose ventral margin is
strongly concave rather than slightly concave, and lacks a short di-
astema between p2 and p3. However, the p4 in MCZ 9316 is other-
wise similar in size, occlusal outline, and paraconid and metaconid
development to the p4s in SDSNH 50565 and 55891. The upper
molars of Antiacodon venustus from the Friars Formation differ from
those of the late Uintan species Auxontodon processus in that the
best-preserved upper molar of the latter species (SMNH P1654.449;
M1 or 2) has a much stronger mesostyle, a more strongly ribbed

paracone and metacone, larger conules, a larger hypocone with a
ridge connection to the protocone, and a nearly complete lingual cin-
gulum (Storer, 1984a, fig. 8D). The holotype m3 of Auxontodon pro-
cessus (P1654.457) differs from the Friars Formation m3s in having
a much larger paraconid than metaconid and in having a much wider
trigonid than talonid. Molars of Auxontodon processus also range
from 17% to 52% larger in linear dimensions than those of
Antiacodon venustus from the Friars Formation.

The Friars Formation specimens of Antiacodon venustus differ
from those of the late Uintan species Tapochoerus egressus in that
the latter has cheek teeth ranging from 12% to 60% larger in linear
dimensions, upper molars that lack mesostyles and have a deeper
ectoflexus, a weaker paraconid and metaconid on p4, a weaker
paraconid on m2, a paraconid merged completely with the metaconid
on m3, a shorter hypoconulid lobe on m3, and more anteroposteriorly
elongated lower molars (see Stock, 1934a; McKenna, 1959).

The early Bridgerian species Sarcolemur furcatus was originally
designated as Antiacodon furcatus by Cope (1873a), and later trans-
ferred to the new genus Sarcolemur by Cope (1875). Subsequently,
this species has often been regarded as a junior synonym of Antiaco-
don pygmaeus (Cope, 1872). See for example Cope (1884:233),
Gazin (1955:23), and McKenna and Bell (1997). In contrast, Guthrie
(1971) and Stucky (1998) retained Sarcolemur as a distinct genus,
with S. furcatus the only known species (but note that Guthrie, unlike
Stucky, regarded Hexacodus Gazin as a junior synonym of Sarco-
lemur). The lower cheek teeth of Antiacodon venustus from the Fri-
ars Formation differ from those in the type specimen of S. furcatus
(AMNH 5008; dentary fragment with p4—m3) in that the latter range
from 11% to 36% smaller in linear dimensions, are more transversely
compressed, have a more transversely compressed protoconid on p4,
and have ml—2 with relatively smaller paraconids, stronger
postcingulids, and relatively taller hypoconulids that are better sepa-
rated from the entoconid.

One of the diagnostic characters of Sarcolemur cited by Stucky
(1998) is the presence of the hypoconulid on the postcingulid in m1—
2 (cf. Guthrie, 1971:86). I am dubious about the significance of this
character, however, because this location of the hypoconulid is com-
mon to all specimens of Antiacodon that I have seen, e.g., MPM
5896, YPM 13275, YPM 16865 (A. pygmaeus), and YPM 13272,
13273, 16861, and 37188 (A. venustus). Nevertheless, the postcin-
gulid is broader and the hypoconulid is better separated from the
entoconid in AMNH 5008 than in these specimens. Stucky (1998)

WTAL (mm)

WTAL (mm)

WTAL (mm)

WTAL (mm)

Bunodont Artiodactyls (Mammalia) from the Uintan (Middle Eocene) of San Diego County, California 7

e oO +
e° e

3.2 MCZ 9316 = type of
Auxontodon pattersoni
3.0
2.8
26 p4
2.4
X AMNH 5008 = type of Sarcolemur furcatus
2.2
46 48 50 52 54 56 58 60 62 64
AP (mm)
44 re) +
MCZ
4.2 9316
e bd oO
4.0
Robinson's
3.8 only m1 e
e
3.6
* e

42 44 46 48

SO) §5:2) 15,495.69 15:8) 6:0) 16:2

AP (mm)

4.8

: °
4.6

0 0?
44 .
4.2
4.0
X AMNH 5008

m2

Ga Ges} CRS} Gar Co) Sil SES Sess Bae BEE EI
AP (mm)
41 fe)
3.9
O° °
37 type of e Pa
A. venustus —@ @
3.5 e
3.3 9
3.1 I Y 13
X AMNH 5008
2.9

50 52 54 56 58 60 62 64 66 68 7.0
AP (mm)

also cited the presence of a strong crest between the hypoconid and
entoconid as a characteristic feature of S. furcatus (see also Guthrie,
1971:86), but, in my opinion, based on an excellent cast of AMNH
5008, this crest is actually quite weak; its strength is exaggerated in
Osborn’s (1902, fig. 16) illustration of the type. An additional char-
acter that may separate S. furcatus from A. pygmaeus is the very
narrow p4 talonid (2.27 mm) in AMNH 5008, compared to the rela-
tively wide talonid in most p4s of A. pygmaeus, e.g., CM 10930
(Burke, 1969), MPM 6721 (West, 1984, fig. 6), and YPM 16865.
However, a relatively narrow p4 is present in USNM 417343 (as-
signed to A. pygmaeus by Emry, 1990:207) and UM 100572 and
100759 (assigned to A. sp., cf. A. pygmaeus by Gunnell, 1998:125).
I again question the generic separation of Sarcolemur from
Antiacodon but retain A. furcatus as a distinct species on the basis of
its narrow p4 and its strong postcingulid and isolated hypoconulid on
m1—2. Note that the p4 in AMNH 5008 is unworn, while the molars
are very lightly worn. Thus, dp4 was apparently shed quite early in
the lives of individuals of A. furcatus; this may cast doubt on my
above identification of the well-worn SDSNH 51379 as a dp4 of
Antiacodon venustus.

The Friars Formation specimens described above are assigned to
the genus Antiacodon on the basis of their nearly horizontal ventral
border of the dentary, their strong well-separated paraconids and
metaconids on m1-3, strong cristids obliqua that contact the meta-
conid, weak mesostyles, lack of crenulated enamel, and relatively
small size. The Friars Formation teeth differ from those of Antiaco-
don pygmaeus in ranging from 4% to 31% larger in linear dimen-
sions (based on measurements of West, 1984, Table 2) and in having
a relatively narrower p4 talonid. The Friars Formation specimens
have only a single mental foramen (although the location of this
foramen varies from below the anterior root of p3 to below the ante-
rior root of p4). In contrast, an additional anterior mental foramen
was reported below the p2 of A. pygmaeus by West (1984:14). Two
mental foramina are also present below the p3—p4 contact and below
the posterior root of p3 on another dentary of A. pygmaeus (YPM
16865).

The Friars Formation teeth are virtually identical in size and
morphology to those of the late Bridgerian species Antiacodon
venustus Marsh, 1872, ranging from about 0% to 15% larger in linear
dimensions, based on specimens in the original YPM hypodigm of
this species (Table 2). To my knowledge, no specimens of A. venustus
have ever been figured (including the type; e.g., Gazin, 1952:76).
Gazin (1955:23) believed that A. venustus and A. pygmaeus were
synonyms, but Gazin (1976:12) apparently changed his mind when

Key to Figure 4
Antiacodon venustus, Friars Fm.

A. venustus, Bridger Fm.

SS
ll

rectangle containing West's (1984)
measurements of A. pygmaeus

L

rectangle containing Robinson's (1966)
measurements of A. p. huerfanensis

Figure 4. Bivariate plots of anteroposterior length vs. talonid width of lower
cheek teeth of various samples of Antiacodon from the Bridger and Friars
formations. Note that specimens of A. venustus from the Friars Formation
tend to be slightly larger than those from the Bridger Formation, but that all
teeth of A. venustus are well separated from West’s (1984) sample of A.
pygmaeus and are generally well separated from Robinson’s (1966) sample
of A. p. huerfanensis.

8 Stephen L. Walsh

Figure 5. Stereophotograph in occlusal view of cast of MCZ 9316, holotype
L dentary of Auxontodon pattersoni, from the Myton Member of the Uinta
Formation. Dentary contains broken roots of il—p2, complete p3—m1, and
partial m3. Scale bar = 10 mm.

he noted that two referred specimens of A. venustus were present at
USNM (see Table 2). I agree with West (1984) and Stucky (1998)
that A. venustus is a valid species, distinctly larger than A. pygmaeus.

The type specimen of A. venustus is YPM 11765, a dentary frag-
ment with slightly damaged m3 from the upper part of the Bridger
Formation (Marsh, 1872:210—211). This tooth (a cast of which is illus-
trated in Figure 3A) is slightly smaller than the Friars Formation m3s
but is otherwise very similar in morphology. It has a weak connecting
ridge between the hypoconulid and the entoconid—hypoconid area,
which is a variable character in the Friars sample. Several other YPM
and USNM lower dentitions of A. venustus are very similar in size and
morphology to the Friars Formation teeth, e.g., YPM 13271 (dentary
fragment with damaged p4—m3), YPM 13272 (dentary fragment with

p4-m2; Figure 3B), YPM 13273 (dentary fragment with m1), 13277
(dentary fragment with m2 talonid + m3), 16861 (dentary fragment
with m1—3; Figure 3C), and USNM 13446 (dentary fragment with m1)
and USNM 364903 (dentary fragment with m3 and associated? p3).
YPM 13271 and 13272 also have a single mental foramen below the
posterior alveolus of p3, as in SDSNH 49170 and 50565. These YPM
and USNM specimens of A. venustus from the upper part of the Bridger
Formation cannot be separated at the species level from the Friars For-
mation specimens. Figure 4 shows bivariate plots of selected measure-
ments of the lower cheek teeth of A. venustus from the Friars and
Bridger formations, along with two samples of A. pygmaeus measured
by Robinson (1966) and West (1984).

Upper dentitions of Antiacodon venustus are poorly known. West
(1984:44) listed YPM 16861 (includes an isolated P4), YPM 37189
(maxillary fragment with M2-3; erroneously listed by him as YPM
13189), and MPM 6717 (associated M1—2), all from the upper part
of the Bridger Formation (Bridger C and D undifferentiated). An
associated M1—2 (UCM 57498 and 57499, respectively) was also
collected in 1994 from Bridger D by UCM personnel and identified
as A. venustus by R. K. Stucky.

The M2 in YPM 37189 (Figure 1A) differs from the undoubted
M2 from the Friars Formation (SDSNH 60872; Figure 1G) in having
a slightly stronger mesostyle and a slightly larger anterocone but is
otherwise very similar in size and morphology. UCM 57498 and
57499 (Figure 1B) differ from the Friars Formation M 1—2s in having
slightly larger conules, slightly larger anterocones, slightly deeper
ectoflexi, slightly stronger mesostyles, and a more pointed lingual
margin. UCM 57499 is about 4% larger in linear dimensions than
SDSNH 60872. The M1—2 in MPM 6717 (Figure 1C) are very simi-
lar in size and general morphology to the M1—2s from the Friars
Formation but differ slightly in having more wrinkled enamel and
stronger mesostyles. The M3 in YPM 37189 (Figure 1A) differs from
the Friars Formation M3 (UCMP 104316; Figure 1F) in having a
smaller metaconule and in being more anteroposteriorly compressed.

The minor differences noted above between the upper molars
from the Friars and Bridger formations cannot be ascribed to taxo-
nomic differences at this time, as some variability is evident within
the Bridger sample. For example, the M1—2 in MPM 6717 differ
from the homologous teeth in UCM 57498 and 57499 in being
smaller, in having a more blunt lingual margin, in having weaker
conules but stronger hypocones, and in having more wrinkled
enamel. The M2 in MPM 6717 also differs from the M2 in YPM
37189 in being wider posteriorly than anteriorly, in having more
wrinkled enamel, and in being relatively shorter in anteroposterior
length. Samples of A. venustus from the Bridger Formation and Fri-
ars Formation remain inadequate to determine confidently the true
range of morphological and size variation present in each popula-
tion, and there is no convincing evidence to suppose that these two
populations should be assigned to different species.

Discussion.—Golz and Lillegraven (1977, table 1) listed a record
of “Diacodexinae?” from the Friars Formation. This occurrence was
based on LACM 56127, an isolated m3 here referred to Antiacodon
venustus. The additional Friars Formation specimens of this species,
described above, were previously identified as “Antiacodon sp. 1”
by Walsh (1996:86). Their refined identification here as Antiacodon
venustus extends the known geographic range of this species from
Wyoming to southern California and the known temporal range into
the early Uintan.

West (1973:150) assigned several specimens in the UW collec-
tion to Antiacodon pygmaeus. However, two of these specimens (UW
1519, L dentary fragment with damaged p4—m1; UW 1556, RM3)
are quite large (Table 2), and are here tentatively referred to
A. venustus on this basis. If correct, these re-identifications would
extend the temporal range of this species into the early Bridgerian,
given West’s (1973) “Bridger B” correlation of UW Locality V-57001
(“Big Sandy”). In numerical terms, the apparent temporal range

Bunodont Artiodactyls (Mammalia) from the Uintan (Middle Eocene) of San Diego County, California 9

TABLE 2. Measurements (in mm) of cheek teeth of Antiacodon venustus from the Bridger Formation. Asterisks indicate minimum
values because of slight damage, enamel dissolution, or interdental wear.

P4 MI M2 M3

W L AP AW PW AP AW PW AP AW PW
YPM 16861 4.17 5.53
MPM 6717 _— — 5.02* _— 6.00 15 6.61 6.80
UCM 57498 6.22
UCM 57499 5.62 6.97 7.02
YPM 37189 5.35 6.59 6.64 4.30% 6.08 5.05
UW 1556 4.12* — 5.24
Mean 6.11 5.37 6.72 6.82 4.22 — 5.15

p4 ml m2 m3

AP WwW AP WTRI WTAL AP WTRI WTAL AP WTRI WTAL
YPM 13271 6.26* 3.47 — 3.47 — 65% = —
YPM 13272 Sai 3.46 4.98 3.67 3.70 5.70 3.96 4.34
YPM 16861 — — 5.15 3.54 4.12 5.44 BI95 4.70 6.21 4.00 3.72
YPM 37188 — = 5.23 3.20 3.55
YPM 13273 5.22 SD 4.28
YPM 13277 = = 4.41 6.30 3.68 3.60
YPM 11765 (type) 6.27 3.66* 3.58
USNM 13446 — = 5.37 3.63 4.17
USNM 364903 6.11 3.59 3.47
UW 1519 5.23 3.62 5.08* 3.26 3.74*
Mean 5.75 B52 5.16 3.46 3.86 5.45 3.94 4.43 6.31 3.73 3.59

“Measurement from Marsh (1872:211). The hypoconulid of YPM 11765 is damaged, and the tooth currently measures only 6.04 mm in anteroposterior length.

(parachron of Walsh, 1998) of A. venustus is determined by the puta-
tive oldest known record (OKR) as represented by the specimens
from UW V-57001 (about 48 million years ago [Ma] based on a new
40Ar/°Ar date from Bridger B reported by Murphey et al., 1999) and
the putative youngest known record (YKR) as represented by the
stratigraphically highest specimens described above from the Friars
Formation (very roughly 45 Ma according to the magnetostrati-
graphic correlations of Walsh et al., 1996, as modified here). Thus,
the parachron of A. venustus is about 3 million years.

The only specimen from Bridger B assigned to A. venustus by
West (1984:44) was YPM 13274, a “batched” specimen lot which
includes an artiodactyl dentary fragment with worn m1—3, unworn
and probably associated LM1 or 2 and LM3 of a small artiodactyl, an
M2 of Hyopsodus sp., and an M1 of the rodent Sciuravus. The lower
molars in the dentary fragment are distinctly smaller than the other
teeth of A. venustus in the YPM collection and are about the size of
those of A. pygmaeus. However, the cristid obliqua is quite weak and
is directed toward the area between the protoconid and metaconid,
rather than to the metaconid. The lower jaw fragment in YPM 13274
is therefore probably referable to Microsus or a small species of
Homacodon. The M1 and M3 included in YPM 13274 differ from
Antiacodon in their lack of a mesostyle and anterocone and in their
stronger hypocones. The size and morphology of these upper molars
are entirely consistent with the figures and descriptions of Microsus
cuspidatus given by West (1984). YPM 37192 (L dentary fragment
with m1—2), assigned by West (1984:44) to A. pygmaeus, also per-
tains to Microsus.

Stucky (1998:366) suggested that the type and only known speci-
men of Neodiacodexis emryi (see West and Atkins, 1970, and below)
might represent the upper dentition of A. venustus. However, as dis-
cussed above, P4—M3 of Bridger Formation A. venustus are known
from YPM 16861, YPM 37189, MPM 6717, and UCM 57498—
57499. The P4 in YPM 16861 is only slightly smaller than the P4 in
the type of N. emryi but differs morphologically from the latter in its
lack of a distinct cuspule on the posterior cingulum and its smooth
rather than crenulated enamel. The M2 in YPM 37189 and the

M1-2 of UCM 57498 and 57499 differ from the M1—2 of NV. emryiin
having smooth rather than crenulated enamel, having more conical
conules with weaker conular wings, lacking an accessory cusp on the
postcingulum, lacking a lingual cingulum (nearly complete in
N. emryi), and in having more pointed lingual margins (as opposed
to the blunt lingual margins of M1—2 in N. emryi caused by the lin-
gual expansion of the cingular hypocone). Given these differences, it
may be concluded that the type specimen of Neodiacodexis emryi
does not represent the upper dentition of Antiacodon venustus.

Tapochoerus McKenna, 1959

Type Species.—Tapochoerus egressus (Stock, 1934a).

Included Species.—Tapochoerus memillini sp. nov.

Diagnosis —Differs from Antiacodon and Auxontodon in having
upper molars that lack mesostyles, generally lack anterocones, and
that have weaker, discontinuous labial cingula. Also differs from
these two genera in its lack of a metaconid on p3 and in having a
paraconid partly to completely merged with the metaconid on m2-3.
Further differs from Auxontodon in having a slightly rather than
strongly convex ventral border of the dentary. Differs from
Neodiacodexis in its lack of a cuspule on the posterior cingulum of
P4 and its lack of mesostyles, anterocones, and crenulated enamel on
the upper molars.

Known Distribution —Late Uintan (middle middle Eocene) of
southern California. Middle member of Sespe Formation, Ventura
County. Member C of Santiago Formation, Mission Valley Forma-
tion, and Miramar Sandstone Member of Pomerado Conglomerate,
San Diego County.

Tapochoerus egressus (Stock, 1934)
Figure 6

Diagnosis.—Linear measurements of cheek teeth ranging from
14% to 47% larger than those of Tapochoerus mcmillini sp. nov.; P4
with stronger cingula and preprotocrista than in 7: memillini; upper

10 Stephen L. Walsh

Figure 6. Stereophotographs in occlusal view of teeth of Tapochoerus egressus from California: (A), SDSNH 62198, LM2, member C of Santiago Formation,
San Diego County; (B), LACM (CIT) 1598, RM2, middle member of Sespe Formation, Ventura County; (C), Cast of LACM (CIT) 1590 (holotype of
T. egressus), R dentary fragment with p4—m3, middle member of Sespe Formation, Ventura County; (D), SDSNH 62199, Rm1 or 2, member C of Santiago
Formation, San Diego County. Scale divisions for A and B in mm; scale bar for C and D = 10 mm.

molars slightly less transverse than in 7. memillini, with distinct gap
between protocone and metaconule; hypoconulid of m3 relatively
shorter than in 7: memillini.

Referred Specimens —SDSNH 4081/62198 (LM2) and 4082/
62199 (Rm1 or 2). Both localities (Emerald Ridge East sites 1 and 2,
respectively) were collected from the upper part of member C of the
Santiago Formation in Carlsbad, northwestern San Diego County,
California.

Description—The M2 is complete and slightly worn (Figure
6A). The paracone and metacone are conical, equal in height and
diameter, and connected by a weak centrocrista. A weak parastyle
and metastyle are present. The ectoflexus is quite deep, and a rela-
tively weak ectocingulum is present along the labial border of the
tooth but is interrupted at the labial base of the paracone. There is no
distinct mesostyle. A strong anterior cingulum extends from the
anterolabial base of the protocone to the parastyle, but a continuous

lingual cingulum is absent. A very weak anterocone is present as a
slight swelling of the anterior cingulum at the base of the “gully”
between the paraconule and protocone. The protocone is conical and
is connected to the strong paraconule by a short preprotocrista. The
paraconule in turn bears short pre- and postparaconular wings, which
extend to the lingual base of the paracone. A strong metaconule is
present, separated by a gap from the protocone. The metaconule bears
short pre- and postmetaconular wings, which extend to the lingual
base of the metacone. The small distinct hypocone is conical and
sends a posterior cingulum to connect with the metastyle. Measure-
ments of SDSNH 62198 (mm): AP = 7.90; AW = 10.07; PW = 9.92.

The m1 or 2 is complete and unworn, with the exception of slight
damage to the hypoflexid area (Figure 6D). The lingual and labial
outlines of the tooth are both distinctly concave in occlusal view. An
anterior cingulum extends from the anterior base of the paraconid to
the anterior base of the protoconid. The conical protoconid is by far

Bunodont Artiodactyls (Mammalia) from the Uintan (Middle Eocene) of San Diego County, California

the largest trigonid cusp. The conical paraconid is located directly
anterior to the conical metaconid. The paraconid is subequal to the
metaconid in size but slightly taller than the latter cusp. The trigonid
is enclosed anteriorly by a distinct paralophid and posteriorly by a
weaker metalophid. The hypoconid 1s by far the largest talonid cusp
and sends a strong cristid obliqua high up the postvallid, which ends
just short of the metaconid apex. The twinned entoconid and
hypoconulid are subequal in height and diameter. A wide postcingulid
extends from the hypoconulid to the posterior base of the hypoconid.
The talonid is open lingually, with a tiny cuspulid in the talonid notch.
Measurements of SDSNH 62199 (mm): AP = 7.76; WTRI = 4.83;
WTAL = 4.88.

Comparisons.—Tapochoerus egressus was previously known
only from the Sespe Formation of southern California, where it is
represented by several specimens from the Tapo Canyon and Brea
Canyon local faunas (McKenna, 1959; Kelly, 1990). SDSNH 62198
is quite similar in size and morphology to LACM (CIT) 1598 (Figure
6B), an M2 of T: egressus described by Stock (1934) and McKenna
(1959). SDSNH 62198 differs from LACM (CIT) 1598 in lacking a
continuous lingual cingulum and in having stronger pre- and
postmetaconular wings, a deeper ectoflexus, and a slightly larger
hypocone. The development of the lingual cingulum is highly vari-
able in upper molars of 7. egressus from LACM (CIT) 180. For ex-
ample, LACM (CIT) 1598 (M2), LACM 40534 (M2), and LACM
(CIT) 5227 (M3) have a strong, complete lingual cingulum, while
LACM 40537 (M1), LACM (CIT) 5226 (M1), 1597 (M1 or 2), 5230
(associated M2—3; see McKenna, 1959: plate 37A), and LACM
40538 (M3) all lack a continuous lingual cingulum. SDSNH 62198
and the M1—2s from LACM (CIT) 180 consistently have a weak
ectocingulum with a distinct interruption at the labial base of the
paracone (McKenna, 1959:128).

SDSNH 62199 is very similar in morphology to the m1—2s in
LACM (CIT) 1587, 1588, 1589, and 1590 (see Stock, 1934), which
are dentary fragments of T. egressus from the Tapo Canyon local
fauna of the Sespe Formation (Figure 6C). SDSNH 62199 is dis-
tinctly longer in AP length than the mls in these four Sespe jaws
(means are 7.24 mm, 4.75 mm, and 4.93 mm for AP, WTRI, and
WTAL, respectively). SDSNH 62199 is more similar in AP length to
the m2s in these four jaws (means are 7.69 mm, 5.16 mm, and 5.25
mm for AP, WTRI, and WTAL, respectively), but the Santiago For-
mation lower molar is more transversely compressed than the Sespe

m2s. Therefore, SDSNH 62199 is identified only as an m1 or 2.
Discussion.—The minor morphological differences between the
Sespe Formation and Santiago Formation specimens cannot be re-
garded as taxonomically significant given the small sample size avail-
able from the latter unit. The two isolated molars described above
represent the first record of Tapochoerus egressus from San Diego
County, thus increasing the number of species known from late
Uintan strata of both San Diego and Ventura counties (Walsh, 1996,
table 3). The ages of SDSNH Locs. 4081 and 4082 (which were prob-
ably collected from the same fossiliferous bed exposed along strike)
are not known with sufficient precision to determine their age rela-
tive to the Tapo and Brea Canyon local faunas of the Sespe Forma-
tion. However, it is interesting to note that these now-destroyed lo-
calities occurred at least 60 m above the local base of member C of
the Santiago Formation (unpublished stratigraphic section in B. O.
Riney Notebook # 24, on file at SDSNH). SDSNH Locs. 4081 and
4082 have also yielded several specimens of the protoceratid artio-
dactyl Leptoreodon leptolophus, a species which is otherwise confi-
dently known in San Diego County only from the latest Uintan or
early Duchesnean Laguna Riviera local fauna (Golz, 1976) and the
faunally similar and geographically proximate SDSNH Loc. 3495
(Walsh, 1996:92). SDSNH Locs. 4081 and 4082 thus appear to be
somewhat younger than certain other late Uintan assemblages from
member C in northern San Diego County, such as the Jeff’s Discov-
ery and Rancho del Oro local faunas discussed by Walsh (1996).

Tapochoerus memillini sp. nov.
Figures 7-9A

Type Specimen—SDSNH 3870/54400, R maxillary fragment
with P3—M3. Mission Valley Formation, late Uintan.

Referred Specimens, Santiago Formation, Member C.—Jeff’s
Discovery local fauna (SDSNH Locs. 3276, 3562, and 3564):
SDSNH 52238 and 52390 (RM1s). SDSNH 52973 (RM2). SDSNH
47644, L dentary fragment with ml—-3. SDSNH 52971, L dentary
fragment with ml. SDSNH 47457 (Rm1?) and 52717 (Lm1?).
SDSNH 52972, Lm2.

Mission Valley Formation —SDSNH 3870/54592, RM2. UCMP
V-6893/96247, RM3.

Miramar Sandstone Member of Pomerado Conglomerate.—
SDSNH 4448/72388, LM1 fragment.

TABLE 3. Measurements (in mm) of cheek teeth of Tapochoerus mcmillini from the late Uintan of San Diego County. Asterisks indicate
minimum values because of slight damage, enamel dissolution, or interdental wear.

P3 p4 MI M2 M3
AP W AP WwW AP AW PW AP AW PW AP AW PW
SDSNH 52238 5.49 6.51 6.64
SDSNH 52390 5.49 6.45 6.63
SDSNH 52973 5.21 7.07 6.97 — — —
SDSNH 54592 5.42 6.97 6.91 — — —
SDSNH 54400 (type) 7.38 4.47 5.33 6.44 5.65* 6.65* 6.89* 5.59* _ a= 4.44* 6.22* 6.09*
UCMP 96247 4.58 6.15 5.54
Mean 5.54 6.54 6.72 5.41 7.02 6.94 4.51 6.19 S12
ml m2 m3
AP WTRI WTAL AP WTRI WTAL AP WTRI WTAL
SDSNH 47457 5.50 3.67 3.71
SDSNH 52717 5.73 3.64 3.72
SDSNH 52971 5.65 3.78* 3.87
SDSNH 47644 5.60 3174: BNi2 5.73 4.00 4.23 6.43 4.07 _
SDSNH 52972 5.64 4.22 4.15 — — _
Mean 5.62 Sort 3.76 5.69 4.11 4.19 _ — —_—

2 Stephen L. Walsh

Figure 7. Stereophotographs of SDSNH 54400, R maxillary fragment with partial P| alveolus, alveoli for P2, and P3—M3, holotype of Tapochoerus memillini
sp. nov.: (A), Occlusal view; (B), Oblique anterior view showing double infraorbital foramina dorsal to anterior root of P3, and cross-section of broken

posterior alveolus for P1. Both scale bars = 10 mm.

Diagnosis.—Linear measurements of cheek teeth ranging from
12% to 32% smaller than those of Tapochoerus egressus (Table 3);
P4 with weaker cingula and weaker preprotocrista than in T: egressus;
upper molars slightly more transverse than in 7: egressus and without
distinct gap between protocone and metaconule; hypoconulid of m3
relatively larger than in T. egressus.

Etymology.—Species named for Mr. Corky McMillin of McMillin
Communities, who provided support for paleontological mitigation
above and beyond the call of duty during the grading of various phases
of the Scripps Ranch North housing development between 1990 and
1995.

Description—tThe holotype maxilla (SDSNH 54400) has two
infraorbital foramina directly dorsal to the anterior root of P3 (Figure
7B). The larger, more dorsal foramen is elliptical in outline, 1.9 mm
in dorsoventral diameter, and 1.3 mm in lateral diameter. Immedi-

ately ventral to this foramen is a smaller circular foramen 0.6 mm in
diameter. No other cranial structures are discernible.

SDSNH 54400 preserves part of the alveolus for a long, narrow
root that is presumably the posterior root of a double-rooted P1 (Fig-
ures 7A-B). That P1 in ZT. memillini was probably double-rooted is
suggested by LACM (CIT) 5233, a very high-crowned, triangular,
double-rooted tooth from LACM (CIT) Loc. 180 that was tentatively
assigned to T. egressus by McKenna (1959:126). I agree that this
tooth probably pertains to T. egressus, and further suggest that it is
almost certainly a P1, as the P2 of most early artiodactyls tends to be
distinctly lower-crowned than the P1. To my knowledge, P1 is not
preserved in place on any available antiacodontine specimen. How-
ever, a double-rooted, high-crowned P1 is present in bunodont forms
such as Helohyus milleri (described as Lophiohyus alticeps, see
Sinclair, 1914: fig. 9; Stucky, 1998:369) and Diacodexis pakistan-

Bunodont Artiodactyls (Mammalia) from the Uintan (Middle Eocene) of San Diego County, California 13

Figure 8. Stereophotographs in occlusal view of composite referred RM1—3 of Tapochoerus memillini: (A), UCMP 96247, M3; (B), SDSNH 52973, M2: (C),

SDSNH 52238, M1. Scale divisions in mm.

ensis (see Thewissen et al., 1983: plate 1, fig. 3a), as well as seleno-
dont forms such as Leptoreodon (see Wortman, 1898: fig. 1) and
Hypertragulus (see Scott, 1940, plate LIV). The P1 is also double-
rooted but relatively low-crowned in Protylopus petersoni (see
Wortman, 1898: fig. 3), Homacodon vagans (crown height unknown;
see Sinclair, 1914: fig. 19), Stibarus yoderensis (see Macdonald,
1955), Gobiohyus orientalis (see Coombs and Coombs, 1977a), and
Messelobunodon (see Franzen, 1983: plate 2, fig. 3). There is a di-
astema of 7 mm between the posterior edge of the posterior alveolus
for Pl and the anterior edge of the anterior alveolus for P2. The dis-
tance between the anterior edge of the anterior alveolus of P2 and the
posterior edge of the posterior alveolus of P2 is 5.9 mm. There is no
diastema between P2 and P3.

P3 and P4 are known only from the holotype (Figure 7A). P3 is
large and anteroposteriorly elongate, with a strong parastylar lobe,

and a weak ridge connecting the anterior flank of the paracone to the
weak parastyle. The paracone dominates the crown and is the tallest
cusp of any on the upper cheek teeth. There is a fairly strong
metastylar crest, atop which sits a very small metacone. The
metastyle is very weak and sends a short cingulum labiad to end at
the posterolabial base of the crown. There is no ectocingulum or hy-
pocone. The protocone lobe is very weak, and the protocone is merely
the lingual end of the posterior cingulum that connects the metastyle
to the protocone lobe. The tooth is triple-rooted, with all roots cylin-
drical and subequal in diameter.

The P4 has a strong conical paracone with a weak anterior crest
that stops just short of the distinct parastyle. A weak anterior cingu-
lum extends from the parastyle toward the lingual base of the proto-
cone but is not complete. The anterior cingulum bears a small indis-
tinct cuspule immediately below the valley between the paracone

Figure 9. Stereophotograph in occlusal view of: (A), SDSNH 47644, referred specimen of Tapochoerus memillini, L dentary fragment with m1—3; (B), Cast
of LACM (CIT) 1590, holotype of 7. egressus, R dentary fragment with p4—m3. Scale bar = 10 mm.

14 Stephen L. Walsh

and protocone. There is a weak metastylar crest on the posterior flank
of the paracone that extends to the weak metastyle. There is no
ectocingulum, although the enamel along the base of the crown is
partially dissolved. A strong posterior cingulum extends from the
metastyle to the lingual base of the protocone. There is no hypocone.
The protocone is strong and conical, with no pre- or postprotocristae.
The tooth is triple-rooted, with the lingual root being the largest.

M1 in the holotype is lightly worn. The paracone and metacone
are conical and equal in diameter, but the metacone is slightly taller
than the paracone. There is no distinct parastyle or metastyle. The
labial base of the metacone extends slightly further labially than the
labial base of the paracone. A weak ectocingulum is present only
between the labial faces of the paracone and metacone, and there is
no mesostyle. The protocone is essentially conical and is connected
to the conules by weak pre- and postprotocristae. The conular wings
are generally weak, with the premetaconular wing being the stron-
gest. Strong anterior and posterior cingula are present but stop just
short of connecting around the lingual base of the protocone. The
anterior cingulum ends lingually in a tiny anterocone, while the
postcingulum ends in a small but distinct hypocone. The lingual base
of the anterocone extends slightly further lingually than the lingual
base of the hypocone. SDSNH 52238 (Figure 8C) is similar to the
M1 in SDSNH 54400 except that the anterior cingulum does not
extend as far lingually; there is no anterocone, and the base of the
hypocone extends slightly further lingually than the base of the pro-
tocone. The single fragmentary M1 from the Miramar Sandstone
Member of the Pomerado Conglomerate (SDSNH 72388, not fig-
ured) is very similar to the M1 in the holotype, although no standard
measurements can be taken.

The M2 in the holotype is missing the lingual third of the crown
but is similar in size and general morphology to M1. SDSNH 52973
(Figure 8B) is an isolated, essentially unworn M2. It confirms that
M2 is similar to M1 in having a weak, discontinuous ectocingulum
and no mesostyle. Unlike M1, the labial base of the paracone extends
slightly further labially than the labial base of the metacone, and the
paracone is slightly taller than the metacone. M2 also differs from
M1 in its lack of a hypocone and anterocone.

The M3 in the holotype is abraded, and its labial face is slightly
damaged. The metacone is shorter than the paracone, and the conules
are weaker than in M1-2. The anterior and posterior cingula are just
as strong as in M1—2 butare less continuous around the lingual flanks
of the protocone. There is no hypocone or anterocone.

An isolated M3 referable to T, memillini is known from a Mission
Valley Formation locality in the La Mesa area. UCMP 96247 (Figure
8A) is complete and well-preserved, with only the paracone showing
minor wear. The tooth is essentially triangular in occlusal outline,
with a large cylindrical lingual root, and two smaller cylindrical la-
bial roots. The presence of a distinct anterior appression facet and the
absence of a posterior appression facet confirm the tooth to be an
M3. The paracone is conical, distinctly taller than the conical meta-
cone, and located well labial to the metacone. A weak rib extends
down the anterior face of the paracone to the parastylar area, although
there is no distinct parastyle. A weak centrocrista connects the para-
cone and metacone. There is no labial cingulum, no mesostyle, and
no metastyle. A small paraconule is present, but it occurs atop the
preparaconular wing, not at the junction of the pre- and postpara-
conular wings. A very weak metaconule occurs at the junction of the
pre- and postmetaconular wings. The protocone is broad, lower than
the two labial cusps, and sends distinct pre- and postprotocristae la-
bially toward the conular wings. There is no hypocone. Distinct
anterior and posterior cingula are present but not continuous around
the lingual base of the protocone and are distinctly weaker than the
cingula in the M3 of the holotype. Finally, UCMP 96247 has a more
pointed lingual margin than the M3 in the holotype.

SDSNH 47644 (Figure 9A) is a dentary fragment with m1-3. Itis
8.7 mm deep below the m2—m3 contact. The m1 has a conical proto-
conid and metaconid. The paraconid is slightly transversely com-
pressed but equal in size to and well separated from the metaconid. A
weak paralophid extends from the paraconid to the anterior face of
the protoconid, and a weak metalophid connects the metaconid with
the lingual base of the protoconid. There is a short weak anterior
cingulid. A strong cristid obliqua extends up toward the metaconid
apex. There are three tiny cuspulids in the hypoflexid. The deep tal-
onid notch opens lingually. The hypoconid is strong. The hypo-
conulid and entoconid are well worn but much smaller than the hypo-
conid. There is a short but wide posterior cingulid. SDSNH 52717
(m1?) is similar in size and proportions to the ml in SDSNH 47644
but has a partially fused paraconid and metaconid, as in m2 of the
latter specimen. In contrast, SDSNH 47457 (m1?) is similar in size
and proportions to the m2 in SDSNH 47644 but has a conical
paraconid well separated from the metaconid.

The m2 in SDSNH 47644 is similar to m1 but is slightly larger,
less transversely compressed overall, and has a more transversely
compressed paraconid that is not well separated from the metaconid.
The hypoconulid is small and equal in size to the hypoconid. SDSNH
52972 is an isolated m2 that differs from the m2 in SDSNH 47644 in
having a trigonid wider than the talonid. SDSNH 52972 also has a
partially fused paraconid and metaconid, and the hypoconulid is taller
than the entoconid.

The m3 in SDSNH 47644 has the paraconid completely fused
with the metaconid. The resulting transversely compressed loph turns
anteriad into a strong paralophid that extends to the anterior base of
the protocone. The hypoconulid is relatively large, but the entoconid
area is damaged.

Comparisons.—The specimens described above were initially
identified as “Antiacodon? sp. 2” by Walsh (1996:86). Although the
teeth are about the same size as those of A. venustus from the Friars
Formation, they differ significantly from both Antiacodon and
Auxontodon in their lack of mesostyles and anterocones and in the
progressive fusion of the metaconid with the paraconid from m1—3.
In these features and in their weak, discontinuous labial cingula, the
specimens closely resemble Tapochoerus egressus. In addition to
being significantly smaller, 7. mcmillini differs morphologically from
T. egressus in that an isolated P4 probably referable to the latter spe-
cies (LACM 40548, not figured; originally identified by D. J. Golz)
has relatively stronger anterior and posterior cingula, a complete lin-
gual cingulum, a weak complete ectocingulum, and a stronger
preprotocrista than the P4 in the holotype of 7: mcmillini. Upper
molars of 7, mcmillini also appear to be slightly more transverse than
those of T: egressus. More importantly, upper molars of 7: memillini
lack a distinct gap between the protocone and metaconule, unlike T.
egressus and all other known antiacodontines. Finally, the single
known m3 of TZ memillini (in SDSNH 47644) has a relatively long
hypoconulid lobe compared to the short hypoconulid lobes in all
known m3s of T: egressus.

Tapochoerus memillini is similar to the Wasatchian genus
Hexacodus in lacking mesostyles and in having a joined or closely
appressed paraconid and metaconid on m2-3 (see Gazin 1952: plate
11; Gazin, 1962: plate 7, fig. 2; plate 14, fig. 1). However, 7. memillini
differs from Hexacodus in having the antiacodontine condition of the
cristid obliqua and a much larger and morphologically “standard”
primitive artiodactyl P3 (Gazin, 1962; plate 7, fig. 2; Stucky, 1998:
fig. 23.3F), in lacking a distinct gap between the protocone and meta-
conule (Gazin, 1962; plate 14, fig. 1), in having an M3 relatively
smaller than M1—2 (Stucky, 1998: fig. 23.3F), in having relatively
narrower lower molars, and in having a larger hypoconulid lobe on
m3 (Gazin, 1952: plate 11, figs. 2-3). The p4 of Hexacodus is very
distinctive (Gazin, 1952: plate 11, fig. 1; Krishtalka and Stucky, 1986:

Bunodont Artiodactyls (Mammalia) from the Uintan (Middle Eocene) of San Diego County, California 15

fig. 7), but this tooth is not preserved in the available sample of
T. memillini.

Tapochoerus memillini is very similar in upper molar morphol-
ogy to Diacodexis, especially in the lack of a distinct gap between the
protocone and metaconule. However, 7: mcmillini differs from
Diacodexis in having a relatively smaller M3, lower molars rela-
tively more elongated anteroposteriorly, and a paraconid completely
merged with the metaconid on m3 (see various figures in Krishtalka
and Stucky, 1985).

The double infraorbital foramina of Tapochoerus mcmillini are
interesting features, but this part of the snout is not preserved in avail-
able specimens of 7: egressus. The infraorbital foramina of SDSNH
54400 are located dorsal to the anterior root of P3 as in Antiacodon
pygmaeus and Diacodexis, but in these taxa the infraorbital foramen
is reportedly single rather than double (West, 1984:8). The skull of
Diacodexis pakistanensis described by Russell et al. (1983) appar-
ently also has a single infraorbital foramen, as does the skull of
Messelobunodon described by Franzen (1981). Interestingly, SDSM
3375 (specimen not seen; assigned to Stibarus obtusilobus by Scott,
1940, plate XXX VII; regarded as the type of Nanochoerus scotti by
Macdonald, 1955; reassigned to Leptochoerus elegans by Edwards,
1976:111) reportedly has a double infraorbital foramen, with both
foramina apparently being subequal in size (Scott, 1940:373). In con-
trast, Scott (1940:377) reported that the infraorbital foramen in
Amherst College 1787 (specimen not seen; assigned by him to the
new leptochoerid species ?Stibarus loomisi; assigned to S. quadri-
cuspis by Edwards, 1976) is “narrow and slitlike and opens above
the space between P2 and P3.” The phylogenetic significance of the
double infraorbital foramen in Tapochoerus memillini is uncertain
given our poor knowledge of the status of this character in other taxa.

Discussion.—tThe holotype maxilla of T. memillini is from the
late Uintan Mission Valley Formation, whereas all known lower
molars of this species are from member C of the Santiago Formation
(late Uintan Jeff's Discovery local fauna). Nevertheless, several iso-
lated upper molars from Jeff’s Discovery are indistinguishable from
those in the holotype; this provides strong evidence that the hypodigm
listed above pertains to a single species.

The specimens of 7. memillini from the Jeff’s Discovery local
fauna (basal part of member C of the Santiago Formation) and the
Mission Valley Formation are evidently older than the specimens of

T. egressus described above from the upper part of member C of the
Santiago Formation. The possibility of an ancestor—descendant rela-
tionship between the two species cannot be entirely ruled out, but 7.
memillini does possess a potential autapomorphy in its unexpected
lack of a distinct gap between the protocone and metaconule. This
Diacodexis-like condition in T: memillini may represent a derived
reversion to a generally more primitive character state.

Because the Jeff’s Discovery local fauna, SDSNH Loc. 3870,
and UCMP Loc. V-6893 are indistinguishable in terms of relative
age, the parachron of 7: memillini is essentially zero. The parachron
of Tapochoerus is determined by the putative OKR of 7. memillini
(about 42.8 Ma, based on a bentonite date from the Mission Valley
Formation obtained by J. D. Obradovich; see Walsh, 1996) and the
putative YKR of 7. egressus (about 40.5 Ma, based on the age of the
Brea Canyon local fauna as inferred from magnetostratigraphic cor-
relation by Prothero et al., 1996). Thus, the parachron of Tapochoerus
is about 2.3 million years.

Antiacodontinae, unidentified genus and species
Figure 10A

Referred Specimen.—SDSNH 3785/49267 (RM1?), from upper-
most part of undifferentiated Friars Formation, early Uintan.

Description—The tooth is complete and unworn, and relatively
wide transversely. The paracone and metacone are tall and conical,
with weak centrocristae and weak ridges extending from their apices
to the parastyle and metastyle. There is a very weak cuspule where
the centrocristae converge (mesostyle?), but the ectocingulum also
shows a distinct elevation directly labial to this cuspule. The
ectoflexus is moderately deep. The strong paraconule has a strong
pre- but a weak postparaconular wing, whereas the strong metaconule
has a moderate pre- and a strong postmetaconular wing. The proto-
cone has conical lingual flanks but a nearly planar posterolabial flank.
There is a weak connection between the protocone and paraconule
but a distinct gap between the protocone and metaconule. There is a
small hypocone. The anterocone is just a weak rise of the anterior
cingulum, and there is no lingual cingulum. The tooth is three-rooted,
with a large cylindrical lingual root and two much smaller labial roots,
slightly compressed anteroposteriorly . Some very weak crenulation
is present on the flanks of the major cusps. Measurements (mm):

Figure 10. Stereophotograph in occlusal view of: (A), SDSNH 49267, unidentified antiacodontine, RM1? (coated with ammonium chloride); (B), Cast of
AMNH 56054, holotype of Neodiacodexis emryi, L maxillary fragment with P4-M2. Scale divisions in mm.

16 Stephen L. Walsh

AP = 5.90; AW = 7.93; PW = 7.99.

Discussion.—This tooth was originally thought to be an M2 of
Antiacodon venustus, but inspection of associated M1—2s of A.
venustus and A. pygmaeus reveals that these teeth are generally
subequal in size, with M2 only slightly wider transversely than M1
(e.g., MPM 6717 and UCM 57498 and 57499; see also West, 1984,
fig. 3; Gunnell, 1998, fig. 20C). In addition, the anterior width of
SDSNH 49267 is slightly less than the posterior width; this suggests
that this tooth may be an MI. SDSNH 49267 is similar to all
antiacodontines (Tapochoerus mcmillini excluded) in having a weak
connection between the protocone and paraconule but a distinct gap
between the protocone and metaconule. SDSNH 49267 differs from
Friars Formation specimens of A. venustus in being 9% to 19% larger
in linear dimensions than the largest available M1—2s of this species.
SDSNH 49267 also has stronger conular wings than all known upper
molars of A. venustus and is more similar in this respect to the type
and only known specimen of Neodiacodexis emryi (AMNH 56054;
Figure 10B). However, SDSNH 49267 has only very weakly crenu-
lated enamel, lacks a lingual cingulum, and is about 16% wider than
the M1 and 8% wider than the M2 of N. emryi.

SDSNH 49267 is similar to CM 19785 (an isolated M3 from the
late Uintan of Badwater, Wyoming; see Black, 1978:232) in its pos-
session of relatively sharp cusps and crescentic conules. However,
CM 19785 has a strong lingual cingulum (absent in SDSNH 49267)
and belonged to an animal somewhat larger than the one represented
by SDSNH 49267. Black (1978:232) suggested that CM 19785 could
pertain to “Auxontodon gazini” (presumably meaning A. pattersoni
Gazin), but this tooth is about the same size as the M3 of Tapochoerus
egressus (see McKenna, 1959:132). Storer (1984a:74) stated that CM
14552 (Black, 1978: fig. 2) and CM 19785 appear to represent
Auxontodon, and Stucky (1998:365) also implicitly assigned them to
A. pattersoni, but corroboration of these reasonable suggestions re-
quires more material.

SDSNH 49267 differs from Tapochoerus egressus in being more
transverse, in having weaker cingula, a smaller hypocone, slightly
stronger conular wings, and in ranging from about 12% to 29%
smaller in linear dimensions. SDSNH 49267 differs from 7. memillini
in ranging from 4% to 15% larger in linear dimensions than the larg-
est M1—2s of the latter species, in having a weak mesostyle, in hav-
ing stronger conules, and in having a distinct gap between the proto-
cone and metaconule. Finally, SDSNH 49267 differs from M1—2 of
Auxontodon processus Storer, 1984a, in being more transverse, in
having much weaker conules and a weaker mesostyle, in lacking a
connection between the protocone and hypocone, and in lacking a
complete lingual cingulum. More material of this taxon must be col-
lected to identify it confidently.

Finally, note that an M2 and M3 from the early Uintan part of the
Devil’s Graveyard Formation were assigned by Runkel (1988, figs.
24A, B) to an unnamed new species of Texodon. However, as
Runkel’s illustrations show, TMM 42952-60 and 42952-61 are quite
different from the M2-3 in the holotype of 7: meridianus West, 1982,
and seem more likely referable to Antiacodon, Auxontodon, or the
indeterminate taxon described above (note the presence of an
anterocone and a distinct gap between the protocone and metaconule,
although Runkel’s specimens do appear to be somewhat smaller than
homologous teeth of Antiacodon venustus). Stucky (1998:365) has
already suggested that TMM 41745-1 (a DP4? from the late Uintan
Serendipity local fauna assigned to Texodon meridianus by West,
1982) may be referable to Auxontodon.

Subfamily Helohyinae Gazin, 1955 (from Marsh, 1877)

Remarks.—The subfamily Helohyinae is used here in the re-
stricted sense of Gazin (1955) and Stucky (1998). McKenna and Bell

(1997) included the Asian taxa Gobiohyus and Pakkokuhyus within
the family Helohyidae (e.g., Coombs and Coombs, 1977a; Holroyd
and Ciochon, 1995), but Stucky (1998:369) suggested that the re-
semblance of these taxa to North American helohyines was due to
convergence. Stucky (1998) proposed that helohyines were part of
the basal selenodont radiation, but here I adopt an even more conser-
vative approach and leave this subfamily incertae sedis within Artio-
dactyla.

Achaenodon Cope, 1873b

Type Species.—Achaenodon insolens Cope, 1873b.

Diagnosis.—Differs from Helohyus in having cheek teeth about
100% larger in linear dimensions than the largest known species of
this genus (H. lentus), in its derived loss of pl, and in its derived loss
of the paraconid on m1. Differs from Parahyus in having lower cheek
teeth at least 15% larger in linear dimensions, in retaining the primi-
tive condition of having relatively unelongated lower molars (and
M3), and in its derived loss of a paraconid on ml. Differs from an
unnamed helohyine genus (Stucky, 1998) in having cheek teeth about
100% larger in linear dimensions, in its derived loss of pl, apparently
derived crowded tooth row, and derived loss of the paraconid on ml.
Autapomorphies include large size and loss of the paraconid on m1.

Known Distribution—Early Uintan (middle middle Eocene) of
North America. Washakie and Tepee Trail formations of Wyoming,
Uinta Formation of Utah, and Friars Formation of California.

Achaenodon sp. cf. A. robustus Osborn, 1883
Figure 11

Referred Specimens. Conglomerate tongue of Friars Forma-
tion.—SDSNH 3621/55890, fragmentary C1. SDSNH 3737/47730,
partial skull with LC1 + P3—M1 + M3, and associated mx and RM3.

Friars Formation undifferentiated —SDSNH 3784/50566, Lm1.
3784/50578, RM3. SDSNH 3788/54582, R dentary fragment with
roots of p34, heavily worn and damaged m1—2, roots of m3, plus
associated Lp4. SDSNH 3851/54581, R dentary fragment with m2-3.

Description —SDSNH 47730 is a highly distorted and crushed
partial skull containing variably damaged, heavily worn LC] + P3—
M1 + M3 (Figure 11A). The infraorbital foramen is located dorsal to
the middle of P3, as in the type skull of A. robustus figured by Osborn
(1883) and Peterson (1919). C1 is mostly intact but the enamel has
been dissolved away. P2 is absent but was apparently double-rooted,
as indicated by damaged alveoli that were partially crushed by an-
teroposterior compression between C1 and P3. There was apparently
a diastema about 10 mm long between P2 and P3, although it is pos-
sible that this feature is a result of distortion.

The P3 is two-rooted, and its long axis is oriented more nearly
parallel to the AP axis of the molars, rather than being distinctly ob-
lique as in the type of A. robustus. The crown consists of a single
large cusp, and the posterior end is the widest part of the tooth. Weak
cingula are present at the labial and lingual bases of the crown, but
the anterior and posterior bases of the crown are too worn to tell if
cingula were also present there.

The P4 is dominated by a tall conical paracone and a much
smaller, conical protocone. There is no metacone, but a small
anterocone is located on the anterior cingulum midway between the
anterior bases of the protocone and paracone. There appears to be a
continuous cingulum around the labial, anterior, and lingual bases of
the crown. The anterior occlusal outline is markedly concave, and
the posterior occlusal outline is convex. The parastylar lobe is strong-
er than the metastylar lobe, and the lingual lobe 1s stronger than that
in the P4 of the type of A. robustus, as illustrated by Osborn (1883)
and Peterson (1919). P4 is apparently three-rooted, with the lingual

Bunodont Artiodactyls (Mammalia) from the Uintan (Middle Eocene) of San Diego County, California 7

Figure 11. Stereophotographs in occlusal view of specimens of Achaenodon
sp. cf. A. robustus from the Friars Formation: (A), SDSNH 47730, partial
skull containing heavily worn LC1 + P3—M1 + M3; (B), SDSNH 54581, R
dentary fragment with worn m2-3. Both scale bars = 5 cm.

and anterolabial roots subequal in size. P4 is only slightly worn,
whereas M1 is heavily worn, indicating a relatively late time of erup-
tion of P4.

M1 is heavily worn, with no details of the crown discernible.
There is a distinct ectoflexus, and the lingual face of the protocone
extends further linguad than the lingual face of the hypocone. The
tooth is three-rooted, but the lingual root has a distinct lingual bifur-
cation. The two labial roots are compressed anteroposteriorly. M2 is

missing but, judging from the three alveoli, was distinctly larger than
M1, as in the type of A. robustus.

M3 is also heavily worn. The hypocone and metacone are much
smaller than the protocone and paracone. The labial face of the para-
cone Is heavily worn. A weak posterior cingulum is present below the
hypocone but is not continuous around the lingual base of the proto-
cone. The M3 on SDSNH 47730 lacks the posterior cuspule present
on the M3 of A. wintensis, as illustrated by Peterson (1919, plate
XXXIX, fig. 4). Although Peterson’s (1919, plate XX XIX, fig. 2)
illustration of the M3 of A. robustus shows no sign of a posterior
cuspule, Osborn’s (1883, plate VI, fig. 2) illustration of the same
specimen shows a small but distinct cuspule immediately postero-
labial to the hypocone. Another isolated, heavily worn M3 from the
Friars Formation (SDSNH 50578; not figured) differs from the M3
in SDSNH 47730 in having a continuous ectocingulum but is too
worn to tell if a posterior cuspule was present. The lingual root of
SDSNH 50578 is transversely compressed, while the labial roots are
anteroposteriorly compressed.

The isolated ml (SDSNH 50566, not figured) is an unworn tooth
cap without roots and is identified as such because it is slightly nar-
rower transversely then the m2 in SDSNH 54851. The trigonid is
slightly wider than the talonid. The protoconid and metaconid are
conical and subequal in size and connected by a weak, anteriorly
convex paralophid. There is no paraconid, but a distinct anterior
cingulid extends from the anterolabial base of the protoconid to the
anterolingual base of the metaconid. A short ridge extends postero-
linguad from the posterior face of the protoconid to meet the anterior
end of the weak cristid obliqua. A similar ridge extends posterolabiad
from the posterior face of the metaconid but does not reach the cristid
obliqua. There is a short labial cingulid in the hypoflexid. The hypo-
conid and entoconid are conical and subequal in size and connected
both by a weak hypolophid and a posteriorly convex postcingulid.
There is a small talonid notch between two weak spurs descending
from the apex of the entoconid and the posterolingual flank of the
metaconid.

The m2-—3 in SDSNH 54581 (Figure 11B) are well worn. The m2
is similar in size and morphology to the m1 described above, except
that the trigonid is slightly narrower than the talonid, and the spur on
the posterior flank of the metaconid is stronger. The latter trend con-
tinues in m3, which has a distinct metastylid at the posterolingual
base of the metaconid.

A heavily damaged right dentary fragment (SDSNH 54582, not
figured) is from an old individual, as shown by the extremely worn
m1—2. The dentary is relatively shallow, being only 57 mm deep
below the p4—m1 contact and 59 mm deep below the m2—m3 con-
tact. It bears a small mental foramen below the anterior root of p4,
and a much larger mental foramen below the anterior root of p3, as in
the type specimens of Achaenodon insolens (AMNH 5143; see Cope,
1884, plate LVII) and A. robustus (see Osborn 1883, plate VI;
Peterson 1919, plate XXXIX). SDSNH 54582 also has a short di-
astema of about 8 mm between p3 and p4, as is also apparently the
case in the type of A. robustus. The associated Lp4 of SDSNH 54582
(not figured) is complete but heavily worn from occlusion with P4,
displaying a deep posterior wear facet similar to that on the p4 of the
type of A. robustus, as illustrated by Osborn (1883) and Peterson
(1919). Table 4 provides dental measurements of Achaenodon from
the Friars Formation.

Discussion.—Golz and Lillegraven (1977, Table 1) listed a Fri-
ars Formation occurrence of “Achaenodontinae?” from LACM (CIT)
Loc. 249. This record was based on LACM 55989, a right navicular
identified by D. J. Golz. The dental specimens described above con-
firm the presence of Achaenodon in the early Uintan of southern
California. Three species of the genus have been recognized.
A. insolens is known from the middle unit of the Adobe Town Mem-

Stephen L. Walsh

TABLE 4. Measurements (in mm) of cheek teeth of Achaenodon sp. cf. A. robustus from the Friars Formation. Asterisks indicate

approximate values owing to minor damage.

P3 P4 MI M2 M3
AP W AP W AP AW PW AP AW PW AP AW PW
SDSNH 47730 = 25.2* = 18.6 AO? YS) Mike Ws) ASLO — = —_ 25.3* 30.0% —
SDSNH 50578 Ay? DYESS) 22.8
p3 p4 ml m2 m3
AP W AP W AP TRI TAL AP TRI TAL AP TRI TAL
SDSNH 54582 — — 28.3* 19.4
SDSNH 50566 27.9 21.4* 19.4
SDSNH 54581 28.0 20.8 Pla] 37.6 23.7 21.5

ber of the Washakie Formation of Wyoming (McCarroll et al., 1996,
table 1) and the Wagonhound Member of the Uinta Formation of
Utah (Stucky, 1998). A. robustus is known only from the middle unit
of the Adobe Town Member of the Washakie Formation (McCarroll
et al., 1996, table 1). Finally, A. uintensis is known only from the
Wagonhound Member of the Uinta Formation (Stucky, 1998). Gazin
(1955) questioned whether A. uintensis was specifically distinct from
A. robustus but tentatively retained them as separate species. Prothero
(1996) recorded only A. uintensis from the Uinta Formation, but
Stucky (1998) recorded both A. uintensis and A. insolens (e.g.,
Osborn, 1895:105; Peterson, 1919:79-80).

Eaton (1985:355) noted that the P3 in the type skull of A. robustus
was double-rooted, while the P3 in UW 13991 (assigned by him to A.
sp. cf. A. robustus) was broader and triple-rooted. The P3 in SDSNH
47730 is double-rooted as in the type of A. robustus. The Friars For-
mation specimens are also similar to A. robustus in that they have
teeth that are more similar in size to those in the type of A. robustus
(see Gazin, 1955:40) than to those in the type of A. wintensis (see
Gazin, 1955:41). However, the dentary of SDSNH 54582 appears to
be shallower than the dentaries in the types of A. insolens and A.
robustus and comparable in depth to the dentaries of A. uintensis
figured by Peterson (1919, fig. 11 and plate XLVI, fig. 4). The Friars
Formation specimens are also similar to A. robustus in their lack of a
posterior accessory cuspule on M3 (absent or weak in the type of A.
robustus, unknown in A. insolens, well-developed in CM 3182, a
referred specimen of A. uintensis) and in their possession of a short

diastema between p3 and p4 (present in the type of A. robustus; absent
in the type of A. insolens and a referred dentary of A. insolens illus-
trated by Peterson [1919, fig. 11]; absent in a referred dentary of
A. uintensis figured by Peterson [1919, plate XLVII, fig. 4]; and
absent in YPM 19817, a dentary from the Tepee Trail Formation
figured and identified as Parahyus vagus by Lewis [1973] but re-
identified by McKenna [1980] as Achaenodon sp.).

Obviously, the populational reality of the above characters is
questionable in view of the small sample sizes available of these
three nominal species. Nevertheless, the Friars Formation specimens
appear to be more similar to A. robustus in most characters and so are
tentatively compared to this species. It is also worth noting that A.
robustus Osborn, 1883, has priority over A. uintensis Osborn, 1895,
should the two names ever be regarded as synonymous.

Parahyus Marsh, 1876

Type and only known species —Parahyus vagus Marsh, 1876.

Diagnosis.—Differs from the largest known species of Helohyus
(H. lentus) in having cheek teeth ranging from 5% to 80% larger in
linear dimensions, in its derived loss of pl, and its derived, relatively
more elongated m3 and M3. Differs from an unnamed helohyine
genus (Snyder, 1993; Stucky, 1998) in having cheek teeth ranging
from about 19% to 100% larger in linear dimensions, in its derived
loss of p1, its lack of diastemata, and its derived, relatively elongated
m3 and M3. Differs from Achaenodon in having relatively more elon-

Figure 12. Stereophotograph in occlusal view of: (A), Associated RM2-3 of Parahyus sp. from the Friars Formation; (B), Cast of YPM-PU 10027, RM2 of
Helohyus lentus (holotype of Parahyus aberrans Marsh) from Bridger D. Scale bar = 2 cm.

Bunodont Artiodactyls (Mammalia) from the Uintan (Middle Eocene) of San Diego County, California 19

gated lower molars that are at least 15% smaller in linear dimensions,
in primitively retaining a paraconid on m1, and in having a relatively
elongated m3 and M3. Possible autapomorphies are the relatively
elongated m3 and M3.

Known Distribution —Early Uintan(?) of Wyoming (Washakie
Formation), early Uintan of trans-Pecos Texas (Devil’s Graveyard
Formation), and early Uintan of southern California (Friars Forma-
tion).

Parahyus sp.
Figure 12A

Referred Specimen.—Lower tongue of Friars Formation:
SDSNH 3893/55150, associated RM2-3.

Description —The M2 (Figure 12A) is roughly square in occlusal
outline and slightly wider than long. The protocone, paracone, meta-
cone, and hypocone are all conical and subequal in size. There is a
distinct conical paraconule fused to the anterolabial base of the pro-
tocone, and a weak metaconule seems to have been present. A short
anterior cingulum extends from the anterior base of the protocone to
the anterior base of the paracone. A strong ectocingulum originates
from the anterolabial flank of the protoconule, extends around the
labial bases of the paracone and metacone, and merges with a weak
postmetacrista at the posterior base of the metacone. There is a dis-
tinct ectoflexus in the labial outline of the tooth but no mesostyle. A
short spur extends posterolabiad from the hypocone apex and ends at
the junction of the postmetacrista and posterior cingulum. The latter
extends around the lingual base of the hypocone and ends at the
posterolingual base of the protocone. The protocone extends much
farther linguad than the hypocone, so the lingual outline of the tooth
is slightly concave. The apices of the paracone and metacone are
slightly worn, while the apices of the protocone and hypocone are
moderately worn. The tooth is three-rooted. The lingual root is trans-
versely compressed and slightly larger than the labial roots, both of
which are anteroposteriorly compressed. Measurements (mm): AP =
16.3; AW = 20.5; PW = 18.2.

The M3 (Figure 12A) is more anteroposteriorly elongate than
M2, with a distinct posterior lobe. The protocone and paracone are
the largest cusps and are conical and subequal in size (the posterior
flank of the paracone 1s broken away). A very weak paraconule seems
to have been present at the anterolabial base of the protocone. A dis-
tinct cingulum is present around the entire circumference of the tooth.
The metacone and hypocone are conical and about half the size of the
protocone and paracone. A small conical metaconule is fused to the
posterolabial base of the hypocone, and another small conical conule
is present immediately posterior to the metacone. Moderate wear is
evident at the lingual bases of the paracone and metacone and along
the anterior cingulum, with only slight wear on the apices of the main
cusps. As in the M3 of Achaenodon, the lingual and posterolabial
roots are subequal in size and transversely compressed, while the
anterolabial root is slightly smaller and anteroposteriorly com-
pressed. Measurements (mm): AP = 19.8; AW = 18.9; PW = 15.6.

Comparisons —SDSNH 55150 formed the basis for Walsh’s
(1996:86) record of cf. Parahyus sp. from the Friars Formation. The
M2 of SDSNH 55150 is similar to the M2 of the very large early
Uintan helohyid Achaenodon (e.g., Peterson, 1919, plate XX XIX) in
being essentially square in occlusal outline, with four subequal, coni-
cal cusps, a strong paraconule, and a strong ectocingulum. However,
SDSNH 55150 is much smaller than Achaenodon, having linear di-
mensions of M2 ranging from only 55% to 64% of those of the M2 of
A. robustus and A. uintensis (see Gazin, 1955:40-41). The M3 in
SDSNH 55150 is also grossly similar in morphology to, but distinctly
more anteroposteriorly elongated than, known M3s of Achaenodon.
The SDSNH 55150 M3 has two accessory posterior cusps, while the
M3 of A. uintensis has a single posterior accessory cusp, and that of

A. robustus has none (Peterson, 1919, plate XXXIX). The M3 in
SDSNH 55150 has linear dimensions ranging from only 53% to 74%
of those of the M3s of A. robustus and A. uintensis.

The M2 of SDSNH 55150 is also similar in crown morphology
and occlusal outline to Marsh’s (1894, fig. 2) type specimen of
Parahyus aberrans from Bridger D (YPM-PU 10027; Figure 12B).
YPM-PU 10027 was identified as an M3 by Marsh, but crown pro-
portions and presence of interdental wear facets on both anterior and
posterior faces indicate that it is an M2. YPM-PU 10027 was cor-
rectly referred to the late Bridgerian taxon Helohyus lentus by Gazin
(1955:42-43), who described it only as an upper molar. The M2 in
SDSNH 55150 ranges from 15% to 18% larger in linear dimensions
than YPM-PU 10027 (AP = 14.2 mm; AW = 17.4 mm; PW = 17.3
mm). The latter tooth has a distinct cuspule on the lingual cingulum
between the protocone and hypocone that is absent in the SDSNH
55150 M2. Another M2 of H. lentus is present in YPM-PU 10084
(Sinclair, 1914, fig. 13; West, 1984, fig. 19). This tooth is again simi-
lar in overall morphology to the M2 in SDSNH 55150 but differs
from the latter in having a complete cingulum below the lingual base
of the protocone, an accessory cuspule on this cingulum, and a uni-
formly convex lingual occlusal outline. Linear dimensions of the M2
in SDSNH 55150 are 5% to 11% larger than those in the M2 of YPM-
PU 10084 (West, 1984: table 8).

West (1984:47) implied that no M3s of H. lentus are known.
However, other species of Helohyus have M3s that are very different
from the M3 in SDSNH 55150 in that they lack a distinct posterior
elongation and are similar in occlusal outline to M2; i.e., their trans-
verse widths exceed their anteroposterior lengths (Sinclair, 1914:
figs. 14-15; West, 1984: fig. 18B). Furthermore, the mean ratio of
AP length to trigonid width for three m3s of H. lentus is 1.68, which
is virtually identical to the ratio of 1.67 obtained for a large sample of
m3s of H. plicodon (West, 1984, tables 6 and 8). On the basis of this
evidence, it may be predicted that the currently unknown M3 of H.
lentus will not be significantly elongated and so will be unlike the
M3 in SDSNH 55150.

Snyder (1993) discussed two specimens of an unnamed new
helohyid genus from early Uintan strata of the Sand Wash Basin,
Colorado. The new genus was separated from Parahyus by having
four rather than three lower premolars, having more complex lower
premolars, and having these premolars separated by diastemata (see
also Stucky, 1998:370). Associated with the lower jaws of DMNH
1764 are several skull fragments, including a left maxillary fragment
with heavily damaged M2-3 (part of DMNH 2925). The M2 in
DMNH 2925 is similar to the M2 in SDSNH 55150 in having finely
crenulated enamel and a weak cingulum around the labial base of the
metacone. However, the M2 in DMNH 2925 differs from the M2 in
SDSNH 55150 in having a slightly stronger centrocrista connecting
the paracone and metacone, a slightly stronger rib on the posterior
flank of the metacone, and a uniformly convex (rather than slightly
embayed) lingual margin. The M3 in DMNH 2925 is almost com-
pletely broken away, but enough of the base of the crown remains to
show that this tooth was not significantly anteroposteriorly elongated,
unlike the M3 in SDSNH 55150. Consistent with this observation is
the ratio of anteroposterior length to trigonid width (1.69) of the in-
tact m3 in DMNH 1764. This value is almost identical to that noted
above for Helohyus lentus and is much less than that for the m3 in
Parahyus vagus (see below). Finally, the M2—3 in SDSNH 55150
range from 19% to 36% larger in linear dimensions than the M2-3 in
DMNH 2925. Given the above differences, SDSNH 55150 is prob-
ably generically distinct from DMNH 1764 and 2925. The M2 in
DMNH 2925 is more similar to the M2 of Helohyus lentus in size and
general morphology (Figure 12B), but the latter tooth has a deeper
ectoflexus.

The M2 in SDSNH 55150 resembles BCHS 516, the holotype
M1 or 2 of Buxobune daubreei from the Lutetian (middle Eocene) of

20 Stephen L. Walsh

Bouxwiller, France, in its essentially square occlusal outline, distinct
lingual and labial cingula, and conical cusps (Sudre, 1978: plate 1,
fig. 2; Sudre et al., 1983: fig. 15h). The M2 in SDSNH 55150 differs
from BCHS 516 in that the latter tooth has a weaker paraconule, a
stronger ectocingulum, and stronger ribs on the anterior and poste-
rior flanks of the paracone and metacone. BCHS 516 also has linear
dimensions that are only about 42% of those of the M2 in SDSNH
55150. The Bouxwiller tooth is perhaps more similar in overall mor-
phology to the M2 of Helohyus lentus and the M2 in DMNH 2925
but is distinctly smaller than these teeth as well.

The teeth in SDSNH 55150 are similar in size and general mor-
phology to those of Choeropotamus from the middle and late Eocene
of Europe (e.g., Viret, 1961: fig. 11) but differ from this genus in
having much weaker cingula, in lacking a mesostyle on M2, in hav-
ing an anteroposteriorly elongate rather than anteroposteriorly com-
pressed M3, and in having fewer accessory cusps on M3.

The M2-3 in SDSNH 55150 are grossly similar in size and gen-
eral morphology to the homologous teeth of the Duchesnean—
Chadronian entelodont Brachyhyops Colbert, 1938. The M2-3 in the
type skull of the small Duchesnean species B. wyomingensis are ex-
tremely worn (Colbert, 1938:94; Scott, 1945:245). However, two
maxillary fragments (CM 11989 and 12079) referred to B. wyomin-
gensis by Wilson (1971: fig. 2) show that the M2 of this species is
more quadrate, has a much stronger cingulum that extends all the
way around the crown, and has a much weaker hypocone than the
M2 in SDSNH 55150. The M3s in CM 11989 and 12079 lack a hy-
pocone, have a stronger circumcoronal cingulum, and.are also
anteroposteriorly compressed, in contrast to the hypocone-bearing,
anteroposteriorly elongated M3 in SDSNH 55150. The M2-3 of the
Chadronian species B. viensis are similar in morphology to those of
B. wyomingensis but significantly larger (see description of SMNH
P1251.1 by Storer, 1984b). The possibility that SDSNH 55150 repre-
sents an early entelodont cannot be completely dismissed, and this
would seem to be plausible given Effinger’s (1998: fig. 24.4) indi-
cated Uintan temporal occurrence of Brachyhyops. However, the age
of this alleged Uintan record is unsubstantiated. The locality in ques-
tion is SB 26B (Mariano Mesa; Baca Formation, New Mexico),
which is probably of Duchesnean age (Lucas, 1992:99). Stratigraphic
control is lacking in the Baca Formation, and some fossil localities
are apparently Uintan, while others are apparently Duchesnean
(Lucas and Williamson, 1993:146).

Despite the fact that upper teeth of the large early Uintan(?)
helohyid Parahyus vagus are unknown, SDSNH 55150 seems to be
more similar to this species than to any of the other taxa discussed
above. In particular, the anteroposteriorly elongated m3 in the type
specimen of Parahyus vagus (YPM-PU 10972; AP/WTRI = 2.05;
see Peterson, 1919, fig. 12; Gazin, 1955:43) suggests that M3 in this
species was also anteroposteriorly elongated, probably more so than
in most specimens of Achaenodon. Note, however, that the m3 in
YPM 19817 is also somewhat elongated anteroposteriorly (AP/
WTRI = 1.84). This large lower jaw from the early Uintan Tepee
Trail Formation of Wyoming was identified as Parahyus vagus by
Lewis (1973) and re-identified as Achaenodon sp. by McKenna
(1980:339).

To estimate the size of the unknown lower teeth of the species
represented by SDSNH 55150, note that in the associated skull and
jaws of the type specimen of Achaenodon robustus, the AP length of
m2 is about 98% of the AP length of M2 (Gazin, 1955:40). By apply-
ing the same proportions to the M2 in SDSNH 55150, it can be pre-
dicted that the m2 in this individual would have an AP length of 16.0
mm. This figure is in turn about 76% of the AP length (21.1 mm) of
the m2 in the type specimen of Parahyus vagus as measured by Gazin
(1955:43). P. vagus was noted by Gazin (1955) to be intermediate in
size between the late Bridgerian Helohyus lentus and the early Uintan
Achaenodon. SDSNH 55150 in turn appears to be intermediate in

size between Helohyus lentus and Parahyus vagus and may well rep-
resent a new species of the latter genus, although additional material
is required to corroborate this suggestion.

As discussed by Gazin (1955:42-43), the type specimen of
Parahyus vagus was probably collected from the Washakie Forma-
tion of Wyoming, but its exact stratigraphic provenance and age are
unknown (see also Stucky, 1998:370). The only other previously re-
ported specimen of Parahyus is a p3 from the early Uintan of Texas
assigned to P. vagus by (West 1982:15). TMM 42287-15 is from the
basal Tertiary conglomerate of the Devil’s Graveyard Formation
(Wilson, 1986:371).

Note that SDSNH Loc. 3893 occurred relatively low in the lower
tongue of the Friars Formation, in association with a micromammal
assemblage typical of the early Uintan Poway fauna. Although no
other ungulate taxa characteristic of the Uintan were collected from
SDSNH Loc. 3893 (e.g., Amynodon, Leptoreodon, Protoreodon,
Achaenodon), the site was primarily a microvertebrate locality, and
the largest-bodied taxon represented is Parahyus sp. Because the rela-
tive ages of the type locality of P. vagus, TMM 42287, and SDSNH
Loc. 3893 are unclear, the OKR and YKR of this genus cannot be
even tentatively identified. Thus, the parachron of Parahyus is inde-
terminate.

DISCUSSION

Figure 13 summarizes the known lithostratigraphic and estimated
temporal ranges in San Diego County of the bunodont artiodactyls
described above. Most taxa are known from only one or two locali-
ties (short vertical bars), although Antiacodon venustus and
Achaenodon sp. cf. A. robustus are documented from several locali-
ties in the conglomerate tongue and upper tongue of the Friars For-
mation. No bunodont artiodactyls are known from member B of the
Santiago Formation.

In addition to the taxa described here, Walsh (1996:86) listed a
record of “cf. Lophiohyus sp.” from the Friars Formation. The speci-
men in question is SDSNH 3612/47877, which includes left and right
dentary fragments with well-worn cheek teeth and partial anterior
dentitions, a well-worn M3, and associated limb and tarsal bones.
Walsh (1996) tentatively identified this specimen on the basis of its
possession of large diastemata between pl—2 and p2-3, which are
similar to those in the type specimen of Lophiohyus alticeps (AMNH
1518; see Sinclair, 1914, figs. 9-10). Although McKenna and Bell
(1997) retained Lophiohyus as a distinct genus, R. K. Stucky (pers.
comm., 1997; see also Stucky, 1998) notes that the presence and
magnitude of these diastemata are highly variable in species of
Helohyus from the Bridger Formation, and that Lophiohyus is defi-
nitely a junior synonym of Helohyus. Coombs and Coombs (1977a)
and West (1984) had previously reached the same conclusion. Fur-
ther consideration of SDSNH 47877 suggests that it is best regarded
as an oromerycid, possibly related to Camelodon Granger, 1910, and
perhaps even referable to Merycobunodon Golz, 1976. I plan to de-
scribe this taxon in a revision of some of the oromerycid artiodactyls
of the southern California Uintan.

PALEOENVIRONMENT

Black (1978:228) and Storer (1984a:144) tabulated the number
of bunodont and selenodont artiodactyl genera and species from sev-
eral Uintan assemblages of North America to infer the relative im-
portance of forest vs. savanna type vegetation in the depositional
area under consideration. The present report, with other papers pub-
lished since 1984, necessitates a revision of these tabulations. Table
5 shows the number of dentally bunodont, bunoselenodont, and sele-
nodont forms from several assemblages of early and late Uintan age
in western North America. Antiacodontines, leptochoerines, and

Bunodont Artiodactyls (Mammalia) from the Uintan (Middle Eocene) of San Diego County, California

oOo OO CO

upper member

a SLs 565950,
LO OKo 2

O
jaa)
jaa!
Gio OLeOnOLLO
< Oo

MEMBER C

Miramar Sandstone Mbr.

=
LL
oO
wD)
<
kK
Zz
<
NY

Mission Valley Formation

oP OP oC oP Lp
oO mo Mawar member
Selol ole

FRIARS
FORMATION
EARLIEST
UINTAN
MEMBER B

SANTIAGO

tongue

Figure 13. Known lithostratigraphic and estimated temporal ranges in San Diego County of the bunodont artiodactyl taxa discussed herein: (1), Parahyus sp.
(2), Antiacodon venustus; (3), Antiacodontinae, unident. gen. and sp.; (4), Achaenodon sp. cf. A. robustus; (5), Ibarus sp. nov.; (6), Zapochoerus memillini; (7),
Tapochoerus egressus. The Friars Formation and Poway Group occur in southwestern San Diego County, oat the Santiago Formation occurs in northwestern
San Diego County. See Wilson (1972) and Rasmussen et al. (1995) for discussion of members B and C of the Santiago Formation. Estimated numerical ages
of these units are based on information in Walsh (1996), Walsh et al. (1996), and Walsh and Gutzler (1999).

99

Stephen L. Walsh

TABLE 5. Numbers of dentally bunodont, bunoselendont, and selenodont artiodactyl genera and species in selected early and late Uintan

assemblages of North America.

Bunodont Bunoselenodont Selenodont
Assemblage gen./sp. gen./sp. gen./sp. Source
LATE UINTAN
Santiago Fm., mbr. C, CA 2/3 0/0 5/9 Golz (1976); Walsh (1996; this paper). Includes the
Laguna Riviera local fauna, which may be early
Duchesnean in age (Walsh, 1996).
Tapo Cyn. and Brea Cyn. local faunas, CA 1/1 0/0 4/8 Kelly (1990); Kelly and Whistler (1994).
Uinta C, UT 1/1 4/5 WAZ Prothero (1996).
Badwater Locs. 5, 6, and 7, WY 2/2 0/0 6/6 Black (1978); Stucky (1998).
Candelaria local fauna, TX 0/0 0/0 3/4 Wilson (1984; 1986); Runkel (1988).
Serendipity local fauna, TX 0/0 1/1 S/7 Wilson (1984; 1986); Runkel (1988).
Casa Blanca local fauna, TX 1/1 0/0 3/4 Westgate (1990).
Swift Current Creek local fauna, SASK. 2/2 1/1 3/3 Storer (1984a; 1996).
EARLY UINTAN
Friars Fm., CA 4-5/4-5 0/0 4/5 Golz (1976); Golz and Lillegraven (1977);
Walsh (1996; this paper).
Uinta B1 + B2, UT 1/2 3/4 6/8 Prothero (1996).
Middle unit of Washakie Fm., WY 2/3 1/1 1/1 McCarroll et al. (1996).
Sand Wash Basin, CO 1/1 2/2 2/2 Stucky et al. (1996); Stucky (1998).
Whistler Squat local fauna, TX 2/2 0/0 2/4 Wilson (1984; 1986); Runkel (1988); Stucky (1998).

helohyines are considered to be bunodont forms; “homacodontines”
are considered to be bunoselenodont forms; and agriochoerids,
oromerycids, protoceratids, hypertragulids, and camelids are consid-
ered to be selenodont forms.

As seen in Table 5, the Friars Formation contains more bunodont
forms than any other early Uintan assemblage, and contains subequal
numbers of bunodont and selenodont forms, but no bunoselenodont
forms. The artiodactyl composition of the Whistler Squat local fauna
is similar in this respect to that of the Friars Formation, but the over-
all diversity of Whistler Squat is lower. In general, early Uintan as-
semblages have subequal numbers of bunodont and bunoselenodont
forms. In contrast, late Uintan assemblages generally have slightly
fewer bunodont forms and a greater number of selenodont forms, as
expected from the observations of Black (1978) and Storer (1984a).
The large number of bunoselenodont forms known from the Uinta
Formation may be attributable to significant time-averaging, which
would result in a greater diversity of taxa than was present in this
area at any one time.

Given the relatively high proportions of bunodont taxa in the
Swift Current Creek local fauna, Storer (1984a) suggested that this
fossil assemblage accumulated in a local setting less savanna-like
than the paleoenvironments prevailing during the deposition of co-
eval Uintan assemblages in the Rocky Mountains, west Texas, and
southern California. However, the data summarized in Table 5 now
indicate that the diversity of bunodont forms in the Swift Current
Creek local fauna was similar to that in other late Uintan assem-
blages (e.g., member C of the Santiago Formation and Badwater lo-
calities 5, 6, and 7).

Stucky (1985:275-276) urged caution in using proportions of
bunodont and selenodont forms to infer paleoenvironment. Leaf fos-
sils from the Friars and Mission Valley formations exist in UCMP
and SDSNH collections (Lillegraven, 1979; Walsh, 1996) but have
not been formally studied. Fortunately, recent palynological work on
the San Diego Eocene allows us to attack the problem better.
Frederiksen (1989; 1991) documented a pollen assemblage from the
late Uintan Mission Valley Formation. Frederiksen (1991:568) stated:

This flora includes a high diversity of pollen types prob-
ably produced by trees (for example, Palmae, Bombacaceae-
Sterculiaceae—-Tiliaceae complex, Eucommiaceae, Faga-

ceae(?), Juglandaceae, Myrtaceae(?), Symplocaceae(?),
Ulmaceae); many additional pollen taxa most likely produced
by shrubs or trees; and a scarcity of pollen taxa obviously
produced by herbs.

Frederiksen (1991) noted that it was “difficult to determine on
the basis of the pollen flora whether narrow gallery forests existed
{on the San Diego coastal plain during Mission Valley time] . . . or
whether the savanna was ‘traversed by streams whose floodplains
supported a relatively luxuriant mesic forest of many species’
(MacGinitie, 1969, p. 49).” Nevertheless, Frederiksen (1991:568)
noted that relatively little pollen/spore turnover occurred between
Bridgerian and late Uintan time in San Diego, as evidenced by mi-
crofloras from the Delmar Formation and Ardath Shale compared to
the microflora from the Mission Valley Formation. As the early
middle Eocene (probably Bridgerian) Torrey flora indicates a warm,
paratropical climate for San Diego (Myers, 1991), the late Uintan
climate and paleoenvironment was probably similar.

Frederiksen (1991) also recognized a “Middle Eocene Diversity
Decline” (MEDD) in pollen assemblages that took place in southern
California and the Gulf Coast beginning in the late middle Eocene.
Recognition of this event in California was based on pollen assem-
blages from middle Eocene strata of the San Diego area and middle
to late Eocene strata of the Transverse Ranges (Frederiksen, 1989).
The MEDD was estimated by Frederiksen (1991, fig. 3) to have be-
gun during the early Bartonian (42-43 Ma using the time scale of
Berggren et al., 1985, but 40-41 Ma using the time scale of Berggren
et al., 1995). The MEDD, therefore, probably began during the early
Duchesnean, soon after deposition of the pre-MEDD, late Uintan
Mission Valley Formation. According to Frederiksen (1991), the
MEDD reflected a major climatic cooling and drying event that af-
fected southern California and the Gulf Coast in different ways. The
general later Eocene climatic deterioration in North America has also
been discussed by Leopold et al. (1992). Frederiksen (1991:568)
stated:

In the Gulf Coast . . . the MEDD was immediately fol-
lowed by a burst of first appearances of angiosperm taxa in-
cluding many shrubs and herbs, among them the first grasses
(Gramineae). However, in southern California, the MEDD was
not followed by significant first appearances of taxa; that is why

Bunodont Artiodactyls (Mammalia) from the Uintan (Middle Eocene) of San Diego County, California 23

there is no indication of an increase in angiosperm pollen diver-
sity in the middle Bartonian in southern California as there was
on the Gulf Coast... No pollen definitely of Gramineae has
been found in the Eocene of southern California.

Despite the apparent lack of grasses in the Uintan environment of
southern California, a healthy diversity of selenodont artiodactyls
thrived in this province (Golz, 1976). As noted by Gregory (1971:69),
however, this should not be surprising, because the teeth of these
relatively low-crowned Uintan selenodonts “are more comparable to
those of [modern] browsing deer and chevrotain than to any grazers”
(see Webb, 1977:362).

Although the climate change that apparently caused the early
Duchesnean MEDD cannot be invoked as a causal mechanism to
explain the early Uintan—late Uintan mammalian faunal turnover in
southern California (Walsh, 1996), it has obvious implications for
the evolution of North American mammalian faunas during the
Duchesnean, and the establishment of the Chadronian—Whitneyan
White River Chronofauna (Tedford et al., 1987). In this fauna, the
only remaining bunodont artiodactyls indigenous to North America
were the leptochoerids, which were never diverse or numerically
abundant (Edwards, 1976).

PALEOBIOGEOGRAPHY

Golz (1976) examined the paleobiogeographic distribution of
Eocene artiodactyl taxa between the western interior and California

and noted that only one bunodont taxon (Japochoerus egressus) was
then known from the latter region. The specimens described here
show that while the California Uintan faunas contained several bun-
odont taxa, they were still depauperate relative to those of the west-
ern interior. Table 6 summarizes the currently known geographic dis-
tribution of middle Eocene bunodont and bunoselenodont genera in
California, the Rocky Mountains, Texas, and Saskatchewan.

The presence of Antiacodon venustus in the early Uintan of San
Diego constitutes another species-level taxonomic similarity between
the early Uintan fauna of San Diego and the late Bridgerian fauna of
the Rocky Mountains (Lillegraven, 1979; Walsh, 1996). The genus
Antiacodon has yet to be confidently recorded from any other early
Uintan assemblage in North America. Conversely, the antiacodontine
Auxontodon is known from the late Uintan of Utah and Wyoming
(Gazin, 1958; Storer, 1984a; Stucky, 1998), the late Uintan of
Saskatchewan (Storer, 1984a), possibly the late Uintan of trans-Pecos
Texas (Stucky, 1998:365), possibly the early Uintan of Wyoming
(Eaton, 1985), and possibly the Duchesnean of British Columbia
(Stucky, 1998:365), but has not been recorded from California.

Yet to be collected from the early Uintan of San Diego are the
characteristic Bridger Formation taxa Microsus, Helohyus, and
Homacodon. West (1982) reported all three of these genera from the
early Uintan of Texas (Whistler Squat local fauna), but Stucky
(1998:367) concluded that the alleged Microsus tooth (TMM 41372-
245; West, 1982: fig. 6B) pertains to Hylomeryx sp. In addition, TMM
41443-470 (the ml or 2 identified by West, 1982: fig. 6C as Helohyus
sp.) is clearly a macrotarsiine primate. Stucky (1998:367) recognized

TABLE 6. Geographic distribution of middle Eocene bunodont and bunoselenodont genera in
four provincial areas of western North America. Compiled from West (1982), Wilson (1986),
Storer (1984a), Runkel (1988), Stucky (1998), and this paper.

California Rocky Mtns. Texas Saskatchewan

Duchesnean — Brachyhyops Brachyhyops os
= Pentacemylus = =
= leptochoerid, gen. nov. — —
—_— —_— Texodon a
— Apriculus — —
— Pentacemylus — =
— Mytonomeryx _ _
_ Mesomeryx — —
Late Uintan — Bunomeryx _ =
—_— Hylomeryx — —_

— Auxontodon Auxontodon? Auxontodon
Tapochoerus — — —

Ibarus _ — Ibarus

= — Laredochoerus —
— = Texodon =
— Bunomeryx — —
— Mesomeryx — —_
— Hylomeryx Hylomeryx —
Early Uintan Achaenodon Achaenodon — —_
Parahyus Parahyus Parahyus —_
= helohyid, gen. nov. —_— —_—
= — Helohyus =
Antiacodon —_— — —

“D.” woltonensis —

— Antiacodon — —
— Neodiacodexis — —
Later — “D.” woltonensis — —
Bridgerian — Microsus — =
_ Homacodon — —

—_— Helohyus —_— =

24 Stephen L. Walsh

this fact, but inadvertently implied that the tooth in question was TMM
41443-531, which is undoubtedly an m3 (West, 1982: fig. 6D). Never-
theless, Helohyus lentus was recorded from the late Bridgerian or early
Uintan part of the Canoe Formation of Texas by Wilson (1967) and
Runkel (1988), and West’s (1982) record of Lophiohyus sp. from the
Whistler Squat local fauna was referred to H. milleri by Stucky
(1998:369). Finally, West’s (1982: fig. 6D) assignment of TMM
41443-531 to Homacodon cf. H. vagans is questionable (note the ex-
tremely small hypoconulid lobe), and indeed Stucky (1998:367) did
not list Homacodon from Locality SB43A (Whistler Squat local fauna).
As a result of these revised identifications, the Whistler Squat local
fauna contains fewer “Bridgerian holdover taxa” than previously sup-
posed.

Achaenodon is known from the early Uintan of San Diego and
the Rocky Mountains but has yet to be recorded from the early Uintan
of Texas. However, fossils of this genus are sufficiently uncommon
that its apparent absence in the latter region may still be ascribed to
collecting bias (early Uintan rocks are unknown in Saskatchewan).
Note that the middle Bridgerian to early Uintan temporal range of
Achaenodon shown by Stucky (1998: fig. 23.6) is a drafting error
(Stucky, pers. comm.). This genus is known only from the early
Uintan. Parahyus is now known from the early Uintan of San Diego,
Wyoming, and Texas.

Walsh’s (1996, table 2) record of Ibarus sp. cf. I. ignotus from
late Uintan localities in San Diego constitutes the first record of the
Leptochoerinae from the California Eocene. /barus was previously
known only from the late Uintan of Saskatchewan (Storer, 1984a),
and the San Diego specimens pertain to a new species of the genus
(Walsh, ms). According to R. K. Stucky (pers. comm.), the San Di-
ego specimens are also similar to those of “Diacodexis” woltonensis,
which was recorded by Stucky (1998:363) from both the Bridger
Formation and the early Uintan Whistler Squat local fauna of trans-
Pecos Texas. The late Uintan leptochoerid from Texas (Laredo-
choerus) is markedly different from Ibarus (see Westgate, 1994), and
the poorly known early Duchesnean leptochoerid from Badwater,
Wyoming, is also distinct (Black, 1978; Stucky, 1998:364). Storer
(1996:248) also recorded an indeterminate leptochoerid from the
Duchesnean Lac Pelletier Lower Fauna of Saskatchewan.

Still unknown from the Uintan rocks of California are several
“homacodont” taxa common in the Uinta Formation, i.e., Meso-
meryx, Bunomeryx, Hylomeryx, Mytonomeryx, and Pentacemylus.
Their absence in California may be real rather than merely apparent,
because collecting bias can now be ruled out with more confidence
than was possible at the time of Golz’s (1976) study. Why these taxa
should have failed to disperse to the West Coast at some time during
the Uintan is mysterious, as several species of Protoreodon,
Leptoreodon, and Protylopus had no trouble doing so. Golz’s
(1976:24) suggestion that the numerous California species of the lat-
ter genera “offered too much competition to any possible invasion by
other selenodont genera or by homacodontine dichobunids” remains
possible. An alternative explanation would be that the homa-
codontines of the western interior were adapted to vegetational types
that did not exist in southern California. Unfortunately, Uintan
megafloras and palynological assemblages of the Rocky Mountains
are poorly known. MacGinitie (1974:64-65) briefly described floral
assemblages from Uintan strata of the Tepee Trail and Washakie for-
mations of Wyoming and noted the presence (although not the abun-
dance) of Gramineae in the Uintan part of the Washakie Formation.
Given the absence of grasses in the Uintan of California (Frederiksen,
1991), this may constitute a hint of vegetational differences between
the two areas that could control the distribution of artiodactyl taxa.

The poorly represented genus Texodon is known only from the
Uintan of Texas (West, 1982; Runkel, 1988; Stucky, 1998), and the
poorly represented genus Apriculus is known only from the late
Uintan of Badwater, Wyoming (Gazin, 1956; Coombs and Coombs,

1977b; Stucky, 1998).

Duchesnean faunas are relatively sparse, and the only known
bunodont and bunoselenodont taxa of this age are the unnamed
leptochoerid genus from Wyoming (Black, 1978; Stucky, 1998:364),
the indeterminate leptochoerid from Saskatchewan noted by Storer
(1996), Pentacemylus (a late Uintan holdover), and Brachyhyops,
the oldest known North American entelodont (e.g., Wilson, 1971).
Emry (1981:568) and McKenna and Bell (1997:412) regarded
Dyscritochoerus Gazin, 1956, as a junior synonym of Brachyhyops,
but Stucky (1998:370) agreed with Gazin that the type and only
known specimen (CM 11912; originally described as Helohyus(?)
sp. by Peterson, 1934) represented a distinct genus. Unfortunately,
this specimen has apparently been lost for at least 44 years (Gazin,
1956:26; Stucky, 1998:370), so the potential synonymy of Dyscrito-
choerus and Brachyhyops, while reasonable, is inconclusive (Wil-
son, 1971:12). The latter name is used in Table 6 for simplicity.

ACKNOWLEDGMENTS

I thank S. E. Foss, J. A. Lillegraven, S.G. Lucas, M. C. McKenna,
R. K. Stucky, and J. Sudre for discussions and information; J. D.
Archibald for access to a measuring microscope; and T. A. Deméré,
J. E. Storer, and R. K. Stucky for reviewing the manuscript. Impor-
tant San Diego County specimens were collected by G. Calvano,
R. A. Cerutti, R. L. Clark, B. O. Riney, and P. J. Sena, and skillfully
prepared by Cerutti, Clark, and Riney. I thank J. P. Alexander
(AMNB), H. N. Bryant (SMNH), M. Cassiliano (UW), R. J. Emry
and R. Purdy (USNM), P. Holroyd (UCMP), P. Mayer (MPM),
A. Tabrum (CM), S. A. McLeod (LACM), P. Murphey (UCM), C. R.
Schaff (MCZ), R. K. Stucky (DMNH), and M. A. Turner (YPM) for
loans of specimens and/or casts. Collection and curation of the San
Diego County fossils described herein was made possible by the
California Department of Transportation, Gatlin Development, D.R.
Horton, McMillin Communities, Monarch Communities of Califor-
nia, and Shea Homes.

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