A new Amphicyon from the Deep River Miocene

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GEOLOGICAL SERIES

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FIELD MUSEUM OF NATURAL HISTORY

Volume VI Chicago, March 24, 1939 No. 23

A NEW AMPHICYON FROM
THE DEEP RIVER MIOCENE

By Paul O. McGrew

Assistant, Paleontology

In 1898 Mr. Elmer S. Riggs and Dr. Oliver C. Farrington obtained
a small collection of mammals from the Deep River beds of Montana.
MesogauliLS hallensis has been described from this collection (Riggs,
1899) and a new species of Amphicyon is described here.

The specimen of Amphicyon, consisting of the anterior portion
of skull, jaws, axis, and proximal end of an ulna, was collected by
Mr. Riggs on Rabbit Creek, about ten miles northwest of White
Sulphur Springs, Montana, and hence almost in the center of the
fossiliferous area indicated by Scott (1898). Merychippus, Dromo-
meryx, and an undetermined merycochoerid were obtained from
the same locality and apparently from the same horizon. This
association indicates upper Miocene or true Deep River age
(Douglass, 1903).

Amphicyon riggsi^ sp. nov.

Holotype.—FM. No. P12029. Anterior portion of skull with
complete dentition (except for M-), jaw, proximal portion of ulna,
and axis. C- and M^ not fully erupted.

Locality and horizon. — Deep River beds, about three miles above
mouth of Rabbit Creek, ten miles northwest of White Sulphur
Springs, Montana.

Diagnosis. — Small (see Measurements, p. 346); premolars
unreduced; upper premolars not widely spaced (relatively); lower
premolars closely spaced; protocone of P- prominent, situated
internal to and but little anterior to the paracone, directed lingually
and but slightly anteriorly; P^ with small posterior accessory cusp.

Discussion. — Upper dentition : The incisors are simple, relatively
large, and widely spaced. I-~- each bears a posterior basal shelf

1 For Mr. Elmer S. Riggs, who collected the specimen.
No. 440 341

Haiural History Survey
Library

342 Field Museum of Natural History — Geology, Vol. VI

but no accessory cuspules. I- is large, robust, and caniniform, with
a low, external, vertical ridge, and a greatly expanded posterior base.

The canine is large, long, and but slightly recurved. It bears a
rather sharp posterior ridge and a low anterior one.

The first premolar, lying 2 mm. behind the canine, is rather small,
and single-rooted. It is composed of a single median cusp which is
connected with the anterior and posterior borders of the tooth by
sharp antero-posterior crests. The transverse width is about two-
thirds of the antero-posterior length. P- is double-rooted and is
separated from P- by a short diastema 5 mm. in length. It has a
rather high, simple, median cusp with a crest running anteriorly and
posteriorly from it. There is a broad swelling along the postero-
internal base, making the tooth more than half as broad as it is long.
P- is one-third longer, antero-posteriorly, than P- and is separated
from it by a diastema of 3 mm. The posterior half is transversely
expanded.

The carnassial is large and rather massive. The paracone is the
highest cusp and lies about halfway between the anterior and
posterior borders of the tooth. A low ridge runs forward from the
paracone to the greatly reduced parastyle. The metacone forms a
trenchant crest about 7 mm. in length. The protocone is prominent
and extends inward and slightly anterior to a point about 7 mm.
from the lingual base of the paracone. It is situated well behind
the anterior edge of the tooth.

The first molar is quite Cams-like. It is roughly triangular in
general outUne with a relatively narrow inner half which is directed
slightly posteriorly. The paracone is large and higher than the
metacone. A low shelf, rather than a distinct cingulum, constitutes
the outer border of the tooth. The protocone forms the apex of a
V-shaped shelf extending inward from the bases of the paracone and
metacone. The protoconule and metaconule are barely discernible.
Lingual and posterior to the protocone is a very broad cingular shelf
which bears no distinct hypocone. M- is but little smaller than M^.
It is roughly oval in outline and its inner part is nearly as broad as
the outer. The paracone is but slightly larger and higher than the
metacone, and both cusps are much lower than the corresponding
ones on M-. The protocone and the ridges running antero-externally
and postero-externally from it are exceedingly low. The small
metaconule lies close to the base of the metacone. The protoconule
is essentially absent. The inner cingular shelf is much broader than
that of M- and forms a complete half-circle, lingual to the protocone.

550.D

A New Amphicyon

343

M- is wanting in this specimen and the maxillary is broken in such a
way that it is impossible to determine whether or not it was ever

wjr"- .^-^

Fig. 95. Amphicyon riggsi sp. nov. F.M. No. P12029, holotype.
and ventral views of facial region of skull, with upper dentition. X H-

Lateral

present. I assume that M- was present in this species, however,
since M^ is very strongly developed.

Lower dentition: The lower incisors, canines, and Py are lost.
Pif_x are similar in most respects to P-"-, but are less reduced.
Pu is about one-half as broad as it is long and has rather strong
crests running anteriorly and posteriorly from the median cusp.
P^ and P^ are successively larger and on both the inner basal swellings
are limited to the posterior half of the tooth. Posterior cingula are
absent on P^j and P^, but a prominent cingulum is present on P^. A

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A New Amphicyon 345

low accessory cuspule is present on the posterior slope of the median
cusp of Pi^. P7 is separated very slightly from P^ (by 1.5 mm.) and
is in contact with P-j. The latter, in turn, is in contact with My.

The first molar is rather massive and its cusps are high. The
paraconid part of the blade is little more than half as high as the
protoconid and is 5 mm. long, measured from the anterior base of
the protoconid. The protoconid is high and is situated immediately
posterior to the paraconid. The metaconid arises from the postero-
internal slope of the protoconid; it is higher than the paraconid but
lower than the protoconid. The posterior slope of the protoconid-
metaconid forms an angle of about 35° with the transverse axis of
the tooth. The relatively high talonid comprises about one-third
of the length of the tooth. The hypoconid lies directly behind the
protoconid; it is nearly as high as the paraconid and much stronger
and higher than the entoconid. The inner slope of the hjrpoconid
and the external slope of the entoconid unite to form an antero-
posterior valley. This lies near the inner border of the tooth and is
enclosed posteriorly by a low ridge connecting the posterior slopes
of the hypoconid and entoconid. M^ is large and massive, its width
at the trigonid being about two-thirds of the length of the tooth. The
protoconid and metaconid are equal in size and height, the latter
cusp being postero-lingual to the former. Anteriorly the tooth is
bounded by a low crescentic ridge which is connected externally to
the protoconid and internally to the metaconid. The paraconid is
absent. The antero-posterior length of the talonid is less than one-
half that of the trigonid. The hjrpoconid is more medial in position
than that of My, is trenchant, and relatively very strong. The
entoconid is low and forms the lingual border of the talonid. The
hypoconal and entoconal ridges meet posteriorly to form a crescentic
posterior border. M^ is only partially erupted but the crown is
visible. It is more than half as long as M.^. The protoconid and
metaconid are subequal. A ridge runs posteriorly from the proto-
conid to unite with the hypoconid. A serrate ridge extends trans-
versely across the posterior border of the tooth.

Skull : The skull is large and massive, with high maxillaries. Only
the facial region is preserved and it is so crushed that a detailed
description might be misleading; hence it will not be described.

Ramus: The ramus of A. riggsi is relatively slender, distinctly
more so than most species of the genus. In keeping with the un-
reduced premolars the horizontal ramus is as deep under P^ as it is
imder M^. The symphysis terminates posteriorly beneath P^. A

346 Field Museum of Natural History — Geology, Vol. VI

small mental foramen is present under the anterior portion of Pij
and a larger one under the anterior portion of P^. Beneath Mt^ and
M^ the ventral border of the ramus bends rather sharply upward.
The ascending ramus is relatively low and heavy. The masseteric

Fig. 97. Amphicyon riggsi sp. nov. F.M. No. P12029, holotype. Lateral
and anterior views of axis. X 3i-

fossa is deep, contains but few rugosities and extends forward to a
point below M^.

MEASUREMENTS
{In millimeters)*
ji iz la. c P-L P^ P^ P^ M^ M^

A-p 7.0 9.0 13 15 7 12 15.5 26.5 22.0 19

Tr 4.8 5.9 10 9 5 7 7.8 17.8 30.2 29

P2 P3 P5 Mt M^ M3

A-p 10.9 13.8 18.8 31.0 21.6 14.0

Tr 6.0 6.5 9.0 14.3 16.0 5.5

Depth of ramus under P^ 37

Depth of ramus under M^ 38

Distance from inferior base of angular process to top of
coronoid process 83

*All measurements maximum diameters.

Axis: A nearly complete axis associated with the skull and jaw
and unquestionably from the same individual offers some interesting
comparisons with Daphaenodon and Canis. The axis of Daphaenodon
is modified in much the same manner as that of A. riggsi, but in the
latter, divergence from the Canis type is carried further (cf. fig. 98).

The axis of A. riggsi as a whole is short antero-posteriorly, and
high. The hypophysial keel agrees with that of Daphaenodon and
Daphaenus in being low, inconspicuous, and grading posteriorly into
the convex body of the centrum.^

1 Peterson (1910, pp. 216, 217) indicated that there is a strong hypophysial
keel on the axis of Daphaenodon. Actually this is not the case, as the keel is low

A New Amphicyon

347

The anterior ventral face of the centrum is nearly horizontal
antero-posteriorly, and posteriorly it is rather deeply concave,
agreeing in this character with Daphaenodon. The anterior articular
surfaces are more expanded dorsally than those of Cants, again
approaching those of Daphaenodon. The odontoid process agrees
with that of Daphaenodon and Daphaenus and differs greatly from

Fig. 98. Axis vertebrae, a, Canis; h, Daphaenodon (from Peterson) ; c, Am-
phicyon riggsi. X %•

that of Canis. It is short, broad, heavy, and directed upward. The
transverse processes are like those of Daphaenodon in being short,
heavy, and directed more ventrally than in Canis. There are
no inferior branches on the transverse processes as described by
Peterson for Daphaenodon. The anterior opening of the vertebrar-
terial canal is much more ventral in position than in Canis and a
little more so than in Daphaenodon. The neural canal and laminae

and inconspicuous as it is on Daphaenus and A. riggsi. The posterior "broader
area" on the axis of Daphaenodon, as described by Peterson, is formed by two
diverging, low ridges extending posteriorly from the keel. Mr. John Burke kindly
sent the axis from the splendid skeleton of Daphaenodon in the Carnegie Museum
to me for comparison.

348 Field Museum of Natural History — Geology, Vol. VI

are short antero-posteriorly, as in Daphaenodon. The small portion
of the neural spine preserved is insufficient for detailed comparison.

Ulna: The ulna, at least in its proximal end, differs greatly from
that of Canis and closely approaches that of the bears; many of its
modifications are foreshadowed in Daphaenodon. The ulna of
A. riggsi is much more massive than that of Canis. The sigmoid
notch is proportionately larger and more twisted than in Canis and
its articular surface is greatly expanded dorso-medially, as in the
bears. The ventral portion of the sigmoid notch extends more
anteriorly than in Canis or Daphaenodon, reaching a point about
15 mm. anterior to the proximal part of the notch. The posterior
part of the articular surface for the radius is expanded, as in the
ursids, terminating posteriorly on a rather large process. Similarly,
the anterior part of the articular surface for the radius is expanded
and projects much farther forward than in Canis. There is a wide,
medial groove posterior to the sigmoid notch extending from below
the radial articulation surface to well upon the olecranon. Most
of the olecranon is lacking from the present specimen, but it was
probably short. The shaft js expanded antero-posteriorly and com-
pressed laterally.

COMPARISONS

Most species of Amphicyon are based upon such fragmentary
specimens that it is difficult to make adequate comparisons of them
with A. riggsi. The new form differs from most American and Old
World species by its smaller size and larger and less spaced premolars.

A. ingens Matthew (1924), A. sinapius^ Matthew (1902), and
A. reinheimeri Cook (1926) are much larger than A. riggsi and have
more reduced and widely spaced premolars.

A. frendens Matthew (1924) is again much larger and further
differs from A. riggsi in having lost the entoconid of M^^.

Some species are about equal in size to A. riggsi and although
for the most part they are based on fragmental specimens, there
seem to be sufficient differences in tooth structure between them and
the new species to establish its claim to distinction.

A. idoneus Matthew (1924) is about the same size as A. riggsi
but the protocone of P- in the former species is much less prominent,
the parastyle is much more external and the metacone of M^ is less
strongly developed.

' A. amnicola Cook (1909) is a synonym of this species, according to Matthew
(1924).

A New Amphicyon

349

A. pontoni Simpson (1930) differs from A. riggsi in having a non-
trenchant talonid on M^ and in having a strong cuspate entoconid.

Certainly the nearest known American ally of A. riggsi is A.
americanus Wortman (1901).* Although the two species are of
alniost the same size, A. riggsi may be distinguished by the following
characters: (1) premolars less reduced and less widely spaced; (2)

Fig. 99. Amphicyon riggsi sp. nov. F.M. No. P12029, holotype. Lateral,
anterior, and medial views of proximal portion of left ulna. X %•

protocone of P- more posterior and directed more nearly at a right

angle to the antero-posterior axis; (3) inner cingulum of M- extending

more anteriorly; (4) inner portion of M- relatively much broader

antero-posteriorly and inner cingulum more expanded.

There are several Old World species of Amphicyon (e.g. A. shah-

hazi, A. palaeindicus, A. aurelianensis) which appear on the basis of

fragmentary specimens to be close to A. riggsi. The geographic

separation seems to warrant specific differentiation and when more

^ Wortman's figure is somewhat inaccurate, and not natural size as indicated,
each dimension being smaller than that of the specimen. A better figure of this
specimen may be found in Scott (1937, p. 579). Dr. G. Edward Lewis kindly
supplied information regarding the holotype of this species.

350 Field Museum of Natural History — Geology, Vol. VI

complete material of the Old World species is available this taxonomic
division will probably be supported by structural characters.

Knowledge of this new species of Amphicyon does not call for
further speculation on the relationships and phylogeny of the am-
phicyonine dogs. It is sufficient to say that the small size, unreduced
premolars, and essential lack of diastemata of A. riggsi indicate
that the species is primitive among the amphicyons, and that in
these primitive characters it approaches Daphaenodon. This lends
support to the opinion of Matthew (1924), Peterson (1910), and
others that Amphicyon is a direct descendant of Daphaenodon. This
conclusion is further supported by the close approach of the axis and
ulna of A. riggsi to those of Daphaenodon.

LITERATURE CITED
Cook, H. J.

1909. Some New Carnivora from the Lower Miocene Beds of Western Ne-
braska. Nebr. Geol. Surv., 3, pp. 259-272, text figs. 1-6.

1926. A New Gigantic Fossil Dog from Colorado. Proc. Colo. Mus. Nat.
Hist., 6, pp. 29-31, pi. 1.

Douglass, E.

1903. New Vertebrates from the Montana Tertiary. Ann. Carnegie Mus., 2,
pp. 145-199, text figs. 1-37.

Matthew, W. D.
1902. New Canidae from the Miocene of Colorado. Bull. Amer. Mus. Nat.

Hist., 16, pp. 281-290, text figs. 1-4.
1924. Third Contribution to the Snake Creek Fauna. Bull. Amer. Mus.

Nat. Hist., 50, pp. 59-210, text figs. 1-63.

Peterson, O. A.

1910. Description of New Carnivores from the Miocene of Western Nebraska.
Mem. Carnegie Mus., 4, pp. 205-278, text figs. 1-69, pis. 74-84.

RiGGS, E. S.

1899. The Mylagaulidae: An Extinct Family of Sciuromorph Rodents. Field
Columbian Mus., Geol. Ser., 1, pp. 182-187, 2 text figs.

SCOTT, W. B.

1898. The Mammalia of the Deep River Beds. Trans. Amer. Phil. Soc, 18,

pp. 55-185, pis. 1-6.
1937. A History of Land Mammals in the Western Hemisphere. Macmillan

Co., New York.

Simpson, G. G.

1930. Tertiary Land Mammals of Florida. Bull. Amer. Mus. Nat. Hist., 59,
pp. 149-211, text figs. 1-31.

WORTMAN, J. L.

1901. A New American Species of Amphicyon. Amer. Jour. Sci., (4), 11, pp.
200-204, 1 text fig.