Paleogene pseudoglyptodont xenarthrans from central Chile and Argentine Patagonia

A t american museum

Novitates

PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY
CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024
Number 3536, 18 pp., 4 figures October 19, 2006

Paleogene Pseudoglyptodont Xenarthrans from
Central Chile and Argentine Patagonia

MALCOLM C. McKENNA, 1 ANDRE R. WYSS, 2 AND JOHN J. FLYNN 3

ABSTRACT

Herein we describe a new, large-bodied species of Pseudoglyptodon, a close sloth ally, from
volcaniclastic deposits of the Abanico (= Coya-Machali) Formation of the central Chilean Andean
Main Range. This species, P. chilensis, is a rare element of the Tinguiririca Fauna, on which the
recently formalized Tinguirirican South American Land Mammal “Age” is founded, being known
from just two specimens. The holotype of P. chilensis, a partial skull and largely complete
mandibles (preserving seemingly complete upper and lower dentitions), is by far the best-preserved
specimen referable to Pseudoglyptodon known. As such, this material permits a more refined
phylogenetic placement of this enigmatic xenarthran than has been possible previously, with
Pseudoglyptodon representing the proximal outgroup to the clade including the most recent
common ancestor of Choelepus and Bradypus, plus all its descendants (i.e., crown clade sloths).

A fragmentary specimen from Argentina is removed from Glyptatelus and referred to
Pseudoglyptodon. Although this specimen is distinct from P. chilensis and other previously
recognized species of Pseudoglyptodon, it offers too meager a basis for formally establishing a new
name.

Finally, phylogenetic definitions of the names Phyllophaga and Tardigrada are proposed.
Historically these terms have been used largely interchangeably, but here we advocate linking the
latter to the crown clade.

1 Division of Paleontology, American Museum of Natural History (m4pmck@indra.com).

2 Corresponding Author, Department of Earth Science, University of California, Santa Barbara, CA 93106
(wyss@geol.ucsb.edu).

3 Division of Paleontology, American Museum of Natural History (jflynn@amnh.org).

Copyright © American Museum of Natural History 2006

ISSN 0003-0082

2

AMERICAN MUSEUM NOVITATES

NO. 3536

INTRODUCTION

South America continues to yield enlight¬
ening paleontological surprises. Here we
describe the first-known associated skull and
mandibles of Pseudoglyptodon Engelmann,
1987, from Eocene-Oligocene volcaniclastic
sediments of the Abanico (= Coya Machali)
Formation, Termas del Flaco, valley of the
Tinguiririca River, central Chile. This is
the first xenarthran from the Tinguiririca
Fauna (Wyss et al., 1994) to be described
in detail. The two Chilean specimens are
referred to a new, large-bodied species of
Pseudoglyptodon, an aberrant early sloth
relative, the type species of which, P. sallaen-
sis, is based on a lower jaw from Branisa
Focality V-12, lower part of the Salla
Beds, Deseadan South American Fand
Mammal “Age” (SAFMA) of Salla, Bolivia
(MacFadden et al., 1985). The name “Pseu¬
doglyptodon ” is intended to reflect the mor¬
phology of the cheek teeth of these edentates,
superficially resembling the cheek teeth of
glyptodontids in their trilobate external
form, but lacking the central figure (an
axial crest of osteodentine) typical of glypto¬
dontids. We also refer several teeth from
the Mustersan and Deseadan of Argentine
Patagonia—previously interpreted as glyptate-
line glyptodontids—to Pseudoglyptodon. The
new Chilean species, P. chilensis, is similar
to P. sallaensis in many features but is
about twice the size of the latter. A second
specimen probably referable to P. chilensis
is known from Termas del Flaco, but it
yields limited useful information. A third
specimen, referable on present evidence to
P. chilensis, was described by Florentino
Ameghino (1897) from the couches a
Pyrotherium (Deseadan in current terminolo¬
gy) of Patagonia, being placed in the poorly
known early glyptodont species Glyptatelus
tatusinus.

The Chilean specimens described here are
derived from concretionary nodules harvested
in place from volcaniclastic sediments of the
Abanico (= Coya Machali) Formation of the
central Andean Main Range. The Tinguiririca
Fauna forms the basis of the recently formal¬
ized Tinguirirican SAFMA (Flynn et al.,
2003); the age of the fossiliferous strata in
this area is constrained by 40 Ar/ 39 Ar radioiso¬

topic dates to roughly 31.5 Ma (Wyss et al.,
1993; Flynn et al., 2003)—early Oligocene
following the time scale of Swisher and
Prothero (1990). The skull preserves much of
the lower jaws and snout, but rearward from
the orbit the specimen is heavily damaged.
Nonetheless, both petrosal bones are in
position (although “floating” in the hard
matrix), as are parts of the right zygomatic
arch and mandibular condyle. All of the
comparatively few teeth of the animal are
present, but the mandibles are clenched
tightly to the skull. Separation of the mand¬
ibles from the skull has not been attempted;
instead, much of the dental pattern has
been elucidated through computed tomo¬
graphic (CT) scanning. The depositional
mechanism(s) accounting for the newly recog¬
nized prevalence of mammal remains in
post-Neocomian volcaniclastic strata of the
Andean Main Range remain(s) uncertain.
Specimens described here may have been
engulfed in a lahar or volcanic debris flow
and literally cooked to death, with the
thinner parts of the skull and jaws reduced
to cinders and only the more massive parts
remaining, more or less in their natural
positions. Postcranial elements were not re¬
covered, nor were any traces of osteoderms
that might have accompanied the skull.
Moreover, no obvious glyptodontid osteo¬
derms are known from any of the localities
in the Abanico Formation at Termas del
Flaco, even though such durable elements
would be expected to have withstood de¬
position. Well-preserved dasypodid osteo¬
derms occur in moderate abundance in
strata near Termas del Flaco, but these
are unlikely to pertain to Pseudoglyptodon.
This absence of glyptodontid osteoderms
might be argued to reflect the general
scarcity of this taxon in these deposits (with
only two specimens recovered) rather than
the taxon’s actual lack of osteoderms. We
would point out, however, that dasypodids,
which are known from equally few
dental remains in these strata, are nonetheless
fairly abundantly represented by osteo¬
derms—sometimes as large, articulated por¬
tions of the carapace. In short, if P. chilensis
possessed obviously glyptodontid dermal ar¬
mor, it seems highly unlikely that these would

2006

McKENNA ET AL.: PALEOGENE PSEUDOGLYPTODONT XENARTHRANS

3

have gone undetected, given the extensive
collecting efforts undertaken in the area to
date.

The dental pattern exhibited by
Pseudoglyptodon chilensis sheds light on a va¬
riety of issues concerning xenarthran fossils
and phylogeny. It is at once apparent that
the new Chilean animal is closely similar
in most respects but size to Engelmann’s
Pseudoglyptodon sallaensis from the De-
seadan assemblage of Salla, Bolivia, and
to teeth once referred to two species of
the early glyptodont Glyptatelus Ameghino,
1897, from the Mustersan and Deseadan of
Argentina. Various features of the skull
and mandible of Pseudoglyptodon are
clearly slothlike, however, as Engelmann
(1987) first appreciated. The newly revealed
occlusion of the caniniform teeth, wherein
the lower caniniform tooth occludes almost
directly opposite the upper caniniform tooth,
presages the “reversed occlusion” seen in
numerous sloths including Choloepus, and
the small number of cheek teeth recalls sloths
as well.

The new material from Chile clarifies
somewhat the problem of glyptateline rela¬
tionships by reinforcing the disassociation
of the type osteoderms from the teeth dubi¬
ously referred to this group by Ameghino
and accepted by various later commentators
(e.g., Hoffstetter, 1958: 573; Scillato-Yane,
1977: 250). We believe that the teeth described
by Ameghino as pertaining to two species
of Glyptatelus instead should be referred to
Pseudoglyptodon, an aberrant animal with
tardigrade affinities now known from more
informative material than was available to
Engelmann in 1987. Pseudoglyptodon may
have possessed osteoderms, of course, as
did many other xenarthrans (including oro-
phodonts and mylodonts among sloths),
but none is known as yet. The new cranial
and dental material emphasizes the morpho¬
logical diversity exhibited by sloths and
their closest allies as early as the Eocene/
Oligocene transition. Although the relation¬
ships of Pseudoglyptodon to sloths or other
xenarthrans remain less than “ironclad,”
the new information presented here adds
materially to the potential solution of this
question.

SYSTEMATICS

XENARTHRA

PHYLLOPHAGA OWEN, 1842,

AS MODIFIED BELOW

Pseudoglyptodon Engelmann, 1987: 217

Taxonomic Note: Confusingly, different
taxonomic names are currently used to refer to
the same minimally inclusive clade encom¬
passing the xenarthran mammals com¬
monly known as sloths: Tardigrada and
Phyllophaga. Here we propose phylogenetic
definitions (sensu de Queiroz and Gauthier,
1990) to remedy this ambiguity, tying each
name to a different clade. We define Phyl¬
lophaga (a name coined by Owen, 1842, but
generally disused until resurrected by
McKenna and Bell, 1997) as all xenarthrans
more closely related to Bradypus or Choloepus
than to myrmecophagids or dasypodids.
Consistent with familiar, present-day usage,
we tie the name Tardigrada to the crown
clade. Thus, Tardigrada is defined as the most
recent common ancestor of Bradypus and
Choloepus plus all of its descendants. The
distinction between these names is especially
relevant to the current study because—as
detailed below—new specimens from Chile
argue that Pseudoglyptodon is not a member
of Tardigrada (the crown clade), but
rather that it represents its nearest known
outgroup (and hence is a member of
Phyllophaga).

Type Species of Pseudoglyptodon : P. sal¬
laensis Engelmann, 1987: 217. Holotype of P.
sallaensis , PU 20552, collected from Branisa’s
locality V-12, lower Salla beds, Salla, Bolivia.

Other Material: Other instances of orig¬
inally described material or references to
the presently described material are listed
below.

Glyptatelus tatusinus : Ameghino, 1897: 507, in
part. The osteoderm, not the tooth, de¬
scribed by Ameghino (1897: 507, 1902: 48)
from the Deseadan of Argentine Patagonia
is selected here as the (lecto)type specimen
of G. tatusinus. We do this for the same
reason as that given by Simpson (1948: 93)
in selecting the lectotype of G. fractus.
Association of Ameghino’s two syntypes

4

AMERICAN MUSEUM NOVITATES

NO. 3536

of G. tatusinus is unproven and unlikely,
although possible. The tooth referred to G.
tatusinus by Ameghino (1897) is clearly
related to Pseudoglyptodon and on present
evidence refers to that taxon, whatever the
relationships of Pseudoglyptodon to other
xenarthrans might be. Glyptatelus was made
the type of the Glyptatelinae by Castellanos
(1932).

Glyptatelus fractus : Ameghino, 1902: 51 or 49,
in part. The osteoderm and tooth described
by Ameghino, said to be from the
Mustersan (couches a Astraponotus ) of
Argentine Patagonia, were discussed by
Simpson (1948: 93), who selected the
osteoderm as the (lecto)type specimen of
G. fractus. As with G. tatusinus, no associ¬
ation of the osteoderm with the tooth is
evident. The (lecto)type osteoderm is that of
an early glyptodont, but the referred tooth,
like that of “G. tatusinus”, is related to
Pseudoglyptodon sallaensis, the type species
of Pseudoglyptodon. On present evidence
this tooth is distinct from other recognized
species of Pseudoglyptodon, but the scant
material presently known does not yet
warrant recognition of a new species. The
most complete specimen of this unnamed
species of Pseudoglyptodon (to which
Ameghino’s referred specimen of G. fractus
pertains as well) is AMNH 29483 (see
below).

Undescribed glyptateline from Quebrada
Fiera, Mendoza Province, Argentina
(Scillato-Yane, 1988): This Deseadan taxon,
represented by MLP 79-XIII-18-9, is known
exclusively from osteoderms.
Pseudoglyptodon sp.: Wyss et al. (1990: fig. 4),
specimen SGO PV 2995. This specimen is
designated as the holotype of Pseudoglypto¬
don chilensis below.

Diagnosis of Pseudoglyptodon: Slothlike
xenarthran with a total of probably just
four teeth in each tooth row; first teeth
caniniform and massive, lower one with
triangular base, upper one with massive oval
base; caniniforms followed by just three open-
rooted molariform cheek teeth, each trilobed
and superficially glyptodontlike but without
the central figure of glyptodont cheek teeth;
skull short, with fused maxilla and premaxilla
and fused nasals although the latter are still

Fig. 1. AMNH 29483, mandibular fragment
bearing one complete cheek tooth and part of
a second, collected by G.G. Simpson from Cerro
Blanco, Chubut Province, Argentina. Identified by
Simpson (1948: 93) as “Glyptodont, incertae sedis,
perhaps Glyptatelus”, this specimen is here trans¬
ferred to a new, but unnamed, species of
Pseudoglyptodon. Reproduced from Simpson
(1948: 93, fig. 23). Scale, 2X.

separate from the maxillae; lacrimal possibly
fused to maxilla; large lacrimal foramen;
zygomatic arch apparently deep, with weak
anterior attachment to skull; lower jaw mas¬
sive, ventrally everted, with anterior “spout”
and underlying large foramen, fused symphy¬
sis, low coronoid process, and mandibular
condyle.

Pseudoglyptodon, unnamed species

Referred Specimens: AMNH 29483
(fig. 1), a mandibular fragment bearing one
complete cheek tooth and part of a second,
discussed and figured by Simpson (1948: 93,
fig. 23) as “Glyptodont, incertae sedis, perhaps
Glyptatelus” , and also in more general terms
by Hoffstetter (1958: 573, fig. 25). A cheek
tooth from the Mustersan (or later; see below)
of Patagonia referred by Ameghino (1902)
to Glyptatelus fractus is here referred to
Pseudoglyptodon, pertaining to the same un¬
named species as AMNH 29483.

Locality Information: Cerro Blanco,
Chubut Province, Argentine Patagonia.
Simpson (1948) provided no precise informa¬
tion about the provenance of AMNH 29483,
nor is the specimen mentioned in his un¬
published field notes. Nevertheless, the speci¬
men’s label reads, “Musters Formation, F5
beds, Cerro Blanco, Expedition ’30.” The
provenance of Ameghino’s specimen is un¬
certain.

2006

McKENNA ET AL.: PALEOGENE PSEUDOGLYPTODONT XENARTHRANS

5

Age: Mustersan, according to Simpson
(1948). The Mustersan, traditionally consid¬
ered medial Eocene in age, has recently been
suggested to be substantially younger, i.e., to
postdate 35-36 Ma (Kay et al., 1999).

Diagnosis: Cheek-tooth wall of hard den¬
tine thicker than in P. sallaensis and lobes less
angular. Differs from P. chilensis in smaller
size (the lower tooth row being —30% shorter
anteroposteriorly).

Comment: These small Pseudoglyptodon
cheek teeth are evidence of little but the
presence of the taxon in Mustersan deposits,
but this at least confirms other pre-Deseadan
records for early phyllophagans in South
America (Hoffstetter, 1958: 573).

Pseudoglyptodon chilensis, new species

Pseudoglyptodon sp.: Wyss et al. (1990: fig. 4).

Type Specimen: SGO PV 2995, damaged
skull and mandibles with seemingly complete
dentition.

Type Locality: The type and referred
Chilean specimens are from the Tinguiririca
River valley (—35°S) in the Cordillera
Principal of the Central Andes, approximately
7 km west of the Argentine border. They are
derived from a steep set of exposures north of
an unnamed pass (the latter of which is
identified by its 2738 m elevation on the
topographic sheet [Anonymous, 1985]), ap¬
proximately 3 km south of the summer resort
town of Termas del Flaco. Pseudoglyptodon
chilensis and its associated fauna occur in
35-50° westward-dipping volcaniclastic depos¬
its of various colors, dominantly brownish
red, interbedded with volcanic flows and
tuffs (fig. 2). Prior to discovery of fossil
mammals in the region (Novacek et al.,
1989) these deposits were mapped as pertain¬
ing to the Colimapu Formation of poorly
constrained Aptian-Albian age (e.g., Klohn,
1960). More recent detailed mapping and
associated geochronologic studies (Wyss
et al., 1993; Charrier et al., 1996) indicate
that the mammal-producing unit pertains
to the Abanico Formation (= Coya-Machali
Formation), a widespread and stratigraphi-
cally important unit in this region of the
Andes. Fossils occur most abundantly in

a massive, purplish brown, volcano-sedimen¬
tary horizon, near the apparent local base of
the formation. Owing to structural complex¬
ity, it has not been possible to establish the
relative stratigraphic position of the fossilifer-
ous horizon within the approximately 2-km-
thick Abanico Formation. A second, sub¬
stantially older fauna has been recovered from
volcaniclastic sediments of the Tinguiririca
Valley some 15 km west of those hosting
P. chilensis (Flynn et al., 1991; Wyss et al.,
1996), indicating that the Pseudoglytodon-
producing beds do not correspond to anything
approaching the lowest stratigraphic levels
in the formation. Still further to the west
(—20 km), but still at the same latitude (35°S),
thick exposures of the Abanico Formation
have yielded three stratigraphically super¬
posed fossil mammal faunas, the lowest of
which also clearly predates the Tinguirirican
SALMA (Wyss et al., 2004).

Age: Tinguirirican SALMA. The diverse
fauna co-occurring with Pseudoglyptodon at
Termas del Flaco allows unambiguous corre¬
lation with the SALMA sequence. The ab¬
sence of such diagnostic taxa as Pyrotherium,
primates, mesotheres (which is problematic,
because this group occurs in the Divisaderan),
Archaeohyrax, Plagiarthrus, hegetotheres, and
Morphippus (Marshall et al., 1983) indicates
a pre-Deseadan age for this Chilean fauna.
Co-occurrence of taxa known elsewhere only
from Mustersan and older deposits (notosty-
lopids, notopithecines, and polydolopids) with
taxa previously known only from younger
beds (a clade of notohippids diagnosed by
hypsodont incisors, interatheriine interatheres,
and rodents) identifies the fauna as represent¬
ing a biochronologic interval interposed be¬
tween the Deseadan and Mustersan SALMAs,
the Tinguirirican (Flynn et al., 2003). In this
connection, the Pseudoglyptodon- containing
fauna from Chile bears little resemblance
to the problematic Divisaderan assemblage
(known from a single locality some 250 km
to the northeast, in western Argentina).
Whatever the still uncertain relative temporal
relationship of the faunas from Termas del
Flaco and Divisadero Largo may be, the two
are undoubtedly distinct.

That the Tinguririca Fauna derives from
a thick volcanic and volcaniclastic sequence is

6

AMERICAN MUSEUM NOVITATES

NO. 3536

Fig. 2. View east of the fossiliferous outcrop (2-3 km south of Termas del Flaco, Chile, and immediately
northwest of an unnamed pass of 2738 m elevation [Anonymous, 1985]) from which the two known
specimens of Pseudoglyptodon chilensis were recovered. The view is roughly perpendicular to the strike of the

2006

McKENNA ET AL.: PALEOGENE PSEUDOGLYPTODONT XENARTHRANS

7

fortuitous from the standpoint of radioisoto¬
pic dating. Multiple single-crystal laser fusion
40 Ar/ 39 Ar dates (Wyss et al., 1993) and fewer
conventional 40 K/ 4(5 Ar analyses (Wyss et al.,
1990) constrain the absolute age of
Pseudoglyptodon chilensis. Dates from imme¬
diately above the fossiliferous horizon indicate
P. chilensis to be minimally ~31.5 Ma (early
Oligocene) in age (Flynn et al., 2003). Levels
immediately below the foss ilif erous horizon
(but within the same stratigraphic unit) have
been dated (Flynn et al., 2003) at a locality
producing a fauna indistinguishable from the
one associated with P. chilensis, but this
second locality has not yet produced P.
chilensis itself. Present evidence suggests,
albeit indirectly, that the Tinguirirican
SALMA likely extends no further back in
time than an additional 1-2 Ma (Flynn et al.,
2003), i.e., very near the Eo-Oligocene transi¬
tion.

Referred Specimens: A second specimen
from Termas del Flaco, SGO PV 2999,
consists of a badly damaged anterior end of
a right mandibular ramus and part of the
fused symphysis. Only alveoli and fragmen¬
tary tooth bases remain. One alveolus suggests
a trilobed cheek tooth like that of the type
specimen of P. chilensis. Unfortunately, the
referred specimen from Termas del Flaco
provides little useful information. A
Deseadan cheek tooth from Patagonia re¬
ferred by Florentino Ameghino (1897: 507) to
Glyptatelus tatusinus, not demonstrably asso¬
ciated with the (lecto)type specimen and not
a glyptodont in any case, may belong here
as well. It provides limited information but is
less certainly conspecific with P. chilensis than
is SGO PV 2999.

Diagnosis: Pseudoglyptodon chilensis dif¬
fers from P. sallaensis and the unnamed
species discussed above in the former’s much
larger size, thinner cheek-tooth wall of hard
dentine, and more sharply angular cheek¬
tooth lobes.

Description: Skulk The skull of
Pseudoglyptodon chilensis (fig. 3) is evidently
short, as suggested by the small number of
teeth, the position of the jaw articulation, and
the position in the matrix of the two petrosal
bones. The distance between the front of the
skull and the anterior edge of the orbit (as
judged by the position of the lacrimal fora¬
men) is truncated, a condition generally seen
in sloths but even more marked in glypto-
donts. The little of the orbit that may be
discerned occurs not far above the roots of the
upper cheek teeth, which, although almost
certainly hypselodont (no evidence of closed
roots is seen on the CT scans—which are
frontal sections), are not highly elongate
prisms requiring a deep maxilla. Because all
upper teeth appear to originate in the maxilla
and no suture is evident at the anterior end
of the maxillary wall of the rostrum, the
premaxilla was either lost postmortem or is
completely fused to the maxilla in the speci¬
men at hand. The two nasal bones are fused to
each other but not to the maxilla. They extend
posteriorly to at least a position over the
posterior end of the first molariform upper
cheek tooth, but damage obscures their full
posterior extent and whether they widened in
the rear. The nasals are thus quite long and
thin, contrasting with the short wide nasal
judged to be typical—and ambiguously syna-
pomorphic—of tardigrades (Gaudin, 2004—
his character 100); foreshortened nasals are
also typical of glyptodonts. In SGO PV 2995
the right nasal is ~5 mm wide at the midpoint
of its preserved portion, while the element was
at least 30 mm long and quite likely reached
twice that length originally. Striking features
of the otherwise already bizarre dentition of
P. chilensis are the massive upper and lower
“canines.” The oval, upper caniniform tooth
base is housed in a prominent bulge in the
maxillary bone on the side of the snout. The
snout is too damaged to provide information
about the anterior end of the palate, housing

west-dipping strata. Fossil mammal localities occur in the dark band of volcaniclastic sediments of the
Abanico (= Coya Machali) Formation in the mid-foreground (straddled by the top half of the circle). A
cluster of tents in the circle gives a sense of scale. The light-colored strata in the middle distance are of the
Banos del Flaco Formation (Neocomian), with the snow-covered rocks in the distance belonging to the Rio
Damas Formation (Kimmeridgian). The horizon approximates the border with Argentina.

AMERICAN MUSEUM NOVITATES

NO. 3536

Fig. 3. Lateral view of the holotype of Pseudoglyptodon chilensis, SGO PV 2995. Visible near the base the
coronoid process, appressed against the ventrolateral margin the mandibular ramus, is a thin fragment of
bone interpreted as a remnant the descending process of the jugal. Wedges of the anterior and posterior parts
of the base of the lower caniniform are visible immediately linguad of the upper caniniform, reflecting the
unusual side-to-side occlusion of these enlarged anterior teeth.

for the organ of Jacobson, septomaxillary
bone if any, or other anterior structures. The
narial opening was large, but apparently little
flared. Details of the orbit are lacking due to
damage, but the orbit was probably not large.
An apparent lacrimal bone occurs on the right
side, where it appears to be fused to the
maxilla. Its large lacrimal foramen lies anteri¬
or to the orbital rim. The anterior end of
a possibly deep, posteroventrally descending
wing of the anterior part of the zygomatic arch
arises between the lacrimal foramen and the
anterior end of the second of the three
molariform upper cheek teeth. Although the
maxillary part of the arch does not appear to
have been especially strong (judged from its
broken cross section), there is circumstantial
evidence of a strong descending process of the
jugal. A thin, triangular fragment of bone is
appressed against the dorsoexterior border of
the right mandibular ramus near the base of

the coronoid process, and outboard of the last
upper and lower cheek teeth. This element
(obviously not part of the mandible) sits at
a considerable distance from broken base of
the anterior root of the zyomatic arch.
Nevertheless, if this element is in anything
close to its life position, it can only represent
a distal portion of an elongated ventral
process of the jugal. The leading edge of
this element is seemingly smooth and un¬
broken, its orientation consistent with that
expected for a descending process of the jugal,
as seen in many sloths. The possibility that
this element represents a displaced element
from the skull roof or orbit cannot be
completely excluded, however. If this element
is indeed a portion of the zygomatic arch, it
resembles much more the condition seen in
tardigrades than in glyptodontids (wherein the
descending process is much more anteriorly
situated).

2006

McKENNA ET AL.: PALEOGENE PSEUDOGLYPTODONT XENARTHRANS

9

The zygomatic arch was probably not
continuous with the squamosal, but evidence
is weak. We have not seen the infraorbital
foramen, but it may be obscured by breakage
and unremoved matrix. No traces of the
frontals remain, unless one of several frag¬
ments of bone above the right lacrimal
foramen represents the anterolateral corner
of the right frontal. The parietals, squamosals,
occipitals, alisphenoids, basisphenoid, basioc-
cipital, vomer, pterygoids, and certainly any
possible mesethmoid are all now absent, but
the rear of the palate, presumably involving
the palatine bones, extends to the rear past
and around the posterior lobe of the last upper
molariform cheek tooth, forming an indented
torus of sorts that may incorporate a part of
the palatine as well as the maxilla. The palate
is unusually narrow between the cheek-tooth
rows (best seen on CT scan images). At the
gumline the two upper tooth rows are nearly
parallel centrally but diverge slightly anterior¬
ly (particularly from the first molariform
tooth forward) as well as posteriorly (partic¬
ularly m3).

The lower jaw is massive, especially in the
symphyseal area, which is fused but shows
traces of the suture on SGO PV 2995 but not
on SGO PV 2999. The horizontal rami bulge
laterally beginning below the second molari¬
form tooth, extending and becoming more
pronounced posteriorly. This results in a ~5-
mm-wide shelflike area lateral to the third
molariform tooth. The anterior end of the jaw
supports a short upturned “spout,” below
which lie one large and several smaller mental
foramina on each ramus. Immediately behind
the “spout” is the massive base of the lower
caniniform tooth, which is followed by the
three molariform lower cheek teeth (which are
set off from each other by short diastemata).
The lower cheek-tooth rows diverge poste¬
riorly, especially deep within the alveoli.
However, their occlusal surfaces meet those
of the upper cheek-tooth row with less
posterior divergence.

The ascending process of the mandible
arises from the side of the horizontal ramus
lateral to the last lower molariform cheek
tooth, slanting up at an angle of about 135° to
the plane of occlusion. The junction of the
ascending and horizontal rami of the mandible

occurs near the midpoint of the third lower
molariform tooth; this, coupled with the slow
rate of climb of the ascending process, results
in the cheek teeth being exposed in lateral view
(i.e., not covered by the ascending process),
save for the posterior third of the last lower
teeth and the posteroventral corner of last
upper teeth. Importantly, there is no evidence
of an external opening of the posterior
mandibular canal near the horizontal-ascend¬
ing ramus junction. Although the inferior
portion of the horizontal ramus is broken on
the right side of SGO PV 2995, enough is
preserved to demonstrate that no such fora¬
men was present. The occurrence of a foramen
in this region uniquely characterizes tardi-
grades among xenarthrans (Gaudin, 2004).

Owing to the shelf of bone lateral to the
third lower molariform mentioned previously,
the ascending process occupies a plane sub¬
stantially lateral to the cheek-tooth row. The
ascending process appears to be small, un¬
excavated either laterally or medially, evident¬
ly not projecting upward or rearward very far.
Breakage of the dorsal, posterior, and ventral
borders of the process, however, obscures its
original size and shape. A small, detached
knob of bone floating in the matrix near the
posteroventral corner of the preserved part of
the ascending ramus may be a remnant of the
right mandibular condyle. If so, the condyle is
positioned low, near the plane of occlusion,
just in front of and lateral to the right petrosal.
This contrasts with the primitive condition
seen in most sloths (except mylodontids and
Choloepus ) and dasypodids (glyptodontids
included), wherein the condyle is positioned
well dorsal to the tooth row (Gaudin, 2004).
Nothing can be said of the posteroventral
parts of the mandible. A trace of a robust
hyoid bone may possibly be represented by
a bone fragment in the matrix at the appro¬
priate position anterior to the right petrosal
and medial to the presumptive mandibular
condyle.

Dentition : The significance of SGO PV 2995
was revealed on the outcrop when its melon¬
sized encasing nodule was delicately cleaved
with a sledge hammer; just the surface of the
anterior end of the left mandible was visible
initially. Mechanical preparation revealed the
labial faces of the teeth. Because the mandibles

10

AMERICAN MUSEUM NOVITATES

NO. 3536

are tightly clenched, however, it has not been
possible to disengage the upper and lower
dentitions. Computerized tomography was
used to more fully elucidate the dental
morphology of SGO PV 2995. Twenty CT
cross sections, taken as parallel to the occlusal
plane as possible (CT scan nos. 563-11
through 563-30) were generated by Scientific
Measurement Systems, Inc., (Austin, Texas)
using a 420-kV 3-mA X-ray source. This stack
of slices ranges from the bases of the lower
cheek teeth to above the roots of the upper
molariform cheek teeth. Distance in the x
direction is 128.8 mm (i.e., preserved skull
length), and distance in the y direction is
61.8 mm (i.e., preserved skull width). Each
slice is 0.25 mm thick. Separation of the slices
is 2.5 mm. The following description is based
largely on this CT imagery. It must be
cautioned that although the CT scans roughly
parallel the occlusal plane, because the vertical
axes of the high-crowned teeth are not
consistently normal to this plane, the tooth
outlines seen on the scans are distorted by the
progressively more oblique angle at which
they were sectioned (fig. 4). This is particular¬
ly true for sections taken the greatest distance
from the occlusal plane, especially for the
posterior postcanines, whose apparent bucco-
lingual dimensions are exaggerated near the
tooth bases owing to canted and slightly
bowed vertical axes of these teeth.

The number of teeth in Pseudoglyptodon is
unusual, probably just four above and four
below on each side, all fairly closely spaced
with only short gaps between them. A more
substantial gap behind the upper caniniform
tooth on the specimen’s left side is likely
artifactual, as a large crack disrupts the
specimen in this region. Additionally, the
degree to which the upper and lower left
caniniforms are compressed into each other
anteroposteriorly suggests a small degree of
postmortem distortion in this region of the
specimen. It is uncertain whether teeth oc¬
curred anterior of the upper caniniforms,
because that region of the rostrum is missing.

The caniniform teeth of Pseudoglyptodon
may or may not be true canines. Grasse (1955)
regarded the anterior teeth in sloths to
represent the true canine of the upper tooth
row and the first premolar of the lower.

Nevertheless, as with other xenarthrans, until
detailed embryological work is carried out, the
homology of these teeth remains uncertain.
It seems plausible, however, that either the
upper or the lower caniniform tooth in
Pseudoglyptodon is not a true canine, because
the occlusion of these teeth, as in sloths, differs
from that seen in other mammals. This
conclusion assumes that an anterior premolar
can be more readily transformed into a canine
imposter, than the position of true canines can
be shifted anteriorly or posteriorly relative to
the opposing tooth. Regardless of whether
phyllophagan “canines” are Cl/pl, Cl/cl, or
some other permutation, tardigrades are
unique among xenarthrans in having upper
tooth rows extend anterior to the lower tooth
rows. On the damaged left side of SGO PV
2995 the lower caniniform appears to occlude
behind the upper, but on the better preserved
right side the upper and lower caniniforms sit
side by side. Thus, the upper and lower tooth
rows of Pseudoglyptodon terminate at nearly
the same level anteriorly. Pseudoglyptodon is
therefore alone among xenarthrans in this
regard, bearing an apomorphic resemblance to
the condition seen in tardigrades (where the
upper tooth row extends anterior to the
lower). Some artiodactyls convert an anterior
lower premolar into a caniniform tooth that
occludes behind the upper canine, but the
resemblance to sloths is not as close as that
seen in Pseudoglyptodon.

The lower caniniform tooth of Pseudo¬
glyptodon occludes with the sloping postero¬
medial wear facet of the upper caniniform
tooth, much as in Choloepus except that, in
Pseudoglyptodon, the lower tooth is more
medially placed relative to the upper. The
occlusal relationship of these teeth is best
exhibited on the right side of SGO PV 2995
(fig. 3), because postmortem deformation has
damaged the left pair of caniniforms. On the
right side, the upper and lower caniniforms
sit in a more normal orientation, directly side
by side, the medial surface of the upper tooth
occluding against the lateral side of the lower.

Judged from the CT scans, the tip of the
lower caniniform tooth was not accommodat¬
ed by an excavation in the palate. The three
molariform cheek teeth following the canini¬
form tooth on both the upper and lower tooth

2006

McKENNA ET AL.: PALEOGENE PSEUDOGLYPTODONT XENARTHRANS

11

Fig. 4. CT scans of the holotype of Pseudoglyptodon chilensis, SGO PV 2995, revealing aspects of this
taxon’s dental morphology (dorsal plane). Upper scan, no. 563.18, showing molariform 2 and molariform 3
near the gumline. Lower scan, no. 563.24, illustrating cross sections of the upper canine roots and the upper
postcanine dentition, also above the gumline. “M” and “m” signify upper and lower molariform teeth,
respectively, as the true positions of these teeth are unknown.

rows are anteroposteriorly elongate and tri-
lobed, being about as high as they are long. It
cannot be established whether any of the teeth
is deciduous, or had replaced a precursor.

The upper caniniform tooth is enormous
and is supported by a large oval base (not

really a root in the usual sense) that extends
high into the maxilla. At its dorsal extremity
the base of the upper caniniform tooth opens
widely rather than closing to a blunt tip. CT
sections of the unworn parts of the tooth are
narrower at the front than the rear, and the

12

AMERICAN MUSEUM NOVITATES

NO. 3536

labial wall of the tooth is relatively convex,
whereas the lingual wall is flatter. A rather flat
transverse wear facet has been created by
action with the lower caniniform tooth, from
the recurved anterior tip of the upper canini¬
form tooth diagonally upward until the facet
reaches the broad rear of the tooth’s base at
the gum line.

The lower caniniform tooth differs in shape
from its upper counterpart. CT scans show that
its massive open base is wide in front and nearly
flat on the anterior face within the alveolus. It
then narrows, followed by a narrower rear-
projecting lobe. The cross sections within the
alveolus thus have a “pinched,” triangular
shape. Near the gum line, the broad anterior
face of the lower caniniform tooth becomes
more rounded, and the posterior lobe becomes
even narrower. Above the gum line, the anterior
face is transversely worn by the action of the
upper caniniform tooth. The diagonal (antero-
ventral-posterodorsal) slope of the transverse
wear facet is guaranteed by the initial wear that
would have occurred when these curiously
shaped teeth first made contact.

As with the caniniforms, the homologies of
the molariform teeth in Pseudoglyptodon are
uncertain. All three molariform cheek teeth in
both the upper and lower dentition have
essentially the same trilobed external shape
of glyptodont teeth, in outline reminiscent of
a bat in flight seen from directly below. The
long axes of these teeth parallel the long axis
of the tooth row. As in glyptodontids, the
crowns of these teeth are worn nearly flat
except for the anterolabial lobe of the right
third lower molariform cheek tooth, which
projects somewhat between the second and
third upper molariform cheek teeth labially in
a manner reminiscent of the anterior ends of
the crowns of rear lower molariform cheek
teeth of Orophodon and Octodontotherium
(Hoffstetter, 1958: fig. 42). The various
molariform cheek teeth change slightly in
shape with wear, as seen in their various cross
sections, but they do not change significantly
in dimensions throughout the various levels of
each tooth. The base of each molariform
cheek tooth is open, as in most tooth-bearing
xenarthrans. Unlike glyptodont molariform
cheek teeth, there is no central figure in the
dentine of teeth of Pseudoglyptodon.

The first of the three upper molariform
cheek teeth is the smallest of the upper
postcaniniform series and is the narrowest
transversely. Its anterior lobe is flattened and
oriented normal to the tooth’s anteroposterior
axis deep within the alveolus. Near the occlusal
surface the fiat anterior surface faces more
linguad. The isthmus between the anterior and
medial lobes is narrower than in the two more
posterior upper cheek teeth. The medial lobe is
blunter and projects less than those of the
succeeding teeth, and the posterior wall of the
posterior lobe is more flattened. The indenta¬
tions demarcating the lobes of the molariform
teeth are less pronounced on the lingual walls of
the teeth than they are labially. Both the
anterior and posterior labial lobes diverge from
one another strongly, in contrast to those of the
succeeding teeth.

The second of the molariform upper cheek
teeth is more symmetric about its medial lobes
than the first, although the labial reentrant
between the anterior and medial lobes seems
to have a small secondary fold high above the
present occlusal surface (at least on the
specimen’s right side). The anterior wall of
the anterior lobe and the posterior wall of the
posterior lobe are gently convex. The medial
lobe is smaller and less acute than that of
the third molariform tooth. As on the first
molariform tooth, the anterior and posterior
lobes of the second diverge labially more than
lingually, contrasting with the orientation of
the lobes of the posterior tooth.

The third (and last) upper molariform cheek
tooth is the largest of the upper series. The
anterior wall of its anterior lobe is gently
convex and is not subdivided by an anterior
indentation, as is its lower counterpart. The
posterior lobe is broad, with a flattened,
posterolabially facing wall that is indented
slightly on the animal’s left tooth but not on
the right one. Both the anterior and posterior
lobes are more acute labially than lingually,
but the prominent medial lobe is acute both
lingually and labially, forming the widest part
of the tooth. Breakage of the maxilla posterior
to the last left molariform reveals that this
tooth is implanted such that its vertical axis
slopes labially from top to bottom.

The symphysis of SGO PV 2995 is well
enough preserved that the presence of any

2006

McKENNA ET AL.: PALEOGENE PSEUDOGLYPTODONT XENARTHRANS

13

lower teeth anterior of the caniniforms seems
unlikely. Confoundingly, SGO PV 2999 ex¬
hibits the broken stubs of two small teeth
floating in matrix above the symphyseal
region. Both consist of little more than
broken, ovoid cross sections 2-3 mm in di¬
ameter. Nevertheless, these tooth remnants
are positioned symmetrically (one on either
side of the symphysis, and about 1 cm apart
from one another), so it must be assumed that
they are preserved in life position. What are
these teeth? Two explanations seem credible.
The bone-matrix interface on SGO PV 2999
is indistinct anteriorly, making it difficult to
determine what these teeth were originally
attached to. Nevertheless, there is a distinct
rim of bone immediately lateral of the right
tooth—this rim is clearly the medial margin of
an alveolous for an enlarged anterior lower
tooth, probably the caniniform. The medial
position of this tooth relative to the mandib¬
ular alveolus suggests that both tooth rem¬
nants are likely the tips of the upper canini¬
forms (which were clenched), the remainder of
the upper dentition having been broken away.
Alternatively (but less likely), these tooth
remnants could represent small anterior teeth
of the lower dentition, elements which simply
are not preserved in SGO PV 2995.

The first lower molariform is the smallest
cheek tooth, and has the narrowest isthmuses
between lobes. The anterior lobe lies mainly
anterolabial to its isthmus, with the result that
there is little or no anteroposterior curvature
of the lingual wall of the tooth anterior to the
medial lobe. The medial lobe projects slightly
labially, but forms a larger and more acute
projection on the lingual side, limiting the
anterior end of a deep reentrant behind it. The
posterior lobe is more symmetrical than the
offset anterior one, and it is slightly indented
at the rear. It is the widest and most massive
lobe.

The second lower molariform tooth is larger
than the first and bears a large posterola-
bially-anterolingually oriented anterior lobe,
the anterolabial wall of which is nearly flat.
The lingual part of the lobe is larger and less
acute than the labial part. The posterior lobe
is even larger than the anterior one but is
oriented somewhat posterolingually-anterola-
bially. It too is larger and less acute lingually

than labially. The medial lobe is smaller and,
like the other lobes, more pronounced lingual¬
ly than labially.

The third molariform, the largest lower
cheek tooth, is distinguished by a very large
and transverse anterior lobe that is markedly
indented anteriorly, resulting in a large,
rounded lingual sublobe and a smaller, some¬
what more acute labial one. The medial lobe is
also very large, transverse, and acute on both
sides of the tooth, but it is especially promi¬
nent lingually. The posterior lobe is somewhat
asymmetrically placed, lying mainly postero-
labially, somewhat the mirror image of the
anterior lobe of the first molariform tooth. As
in the other lower molariform cheek teeth, it is
more acute labially than lingually. Breakage of
the dentary posterior to molariform 3 on the
left side of SGO PV 2995 shows this tooth to
be inclined lingually. It also shows clearly that
the root of this tooth is quite short compared
with the second molariform of P. sallensis. In
the latter, the second molariform reaches the
base of the mandible, and the tooth is nearly
3 cm high (i.e., the height is triple the length).
In the Chilean form the molariforms are
subequal in height and width.

Petrosals : Although both petrosals are pre¬
served (indeed, they constitute nearly the
entirety of the preserved portion of the skull
posterior to the upper dentition), neither
reveals much anatomical or phylogenetically
informative data. The left petrosal consists of
a badly damaged, featureless lump. The right
petrosal preserves perfectly ordinary-looking
oval and round windows, and an unremark¬
able promontorium.

PSEUDOGL YPTODON
PHYLOGENETICS

Is Pseudoglyptodon a sloth? This of course
depends on one’s definition of “sloth”.
Pseudoglyptodon clearly falls phylogenetically
outside the minimally inclusive clade of which
Bradypus and Choloepus are a part, i.e., it is
the nearest outgroup to what has traditionally
been termed sloths. Is it preferable to amend
the definition of “sloth” such that it is
applicable to Pseudoglyptodon as well, or
should a different name be defined for the
minimally inclusive clade of xenarthrans of

14

AMERICAN MUSEUM NOVITATES

NO. 3536

which Pseudoglyptodon is a member? As
mentioned earlier, the recognition of two
new species of Pseudoglyptodon herein pre¬
sents an excellent opportunity to rectify a long¬
standing nomenclatural problem, the existence
of two names (Tardigrada and Phyllophaga)
that have been employed nearly interchange¬
ably in xenarthran systematics in reference to
the same group. We have opted to attach the
former to the crown clade, using the name
Phyllophaga to refer to all xenarthrans more
closely related to sloths (Tardigrada) than to
anteaters or armadillos. Following this usage,
we are confident that Pseudoglyptodon is
a member of the Phyllophaga, but it is almost
certainly not a tardigrade.

A number of features argue for the out¬
group placement of Pseudoglyptodon relative
to Tardigrada. In common with tardigrades,
Pseudoglyptodon apomorphically possesses
a short, deep skull and robust mandibles.
Anteriorly the mandibles of Pseudoglyptodon
bear a spoutlike structure and a large foramen
like that of tardigrades. There is no posterior
opening of the mandibular canal in
Pseudoglyptodon, contrasting with the condi¬
tion in tardigrades. There are likely only four
upper and four lower teeth on each side in
Pseudoglyptodon, of which the most anterior
are caniniform. This represents a reduction of
the dental formula from the five upper teeth/
four lower teeth considered ancestral for
tardigrades (Gaudin, 2004). The enlarged
caniniform teeth of Pseudoglyptodon occlude
in a manner approaching the condition seen
in some tardigrades; in sloths having canini¬
form teeth, the upper one occludes anterior
to the lower one (the reverse of the situation
seen in most mammals). By contrast, in
Pseudoglyptodon the caniniforms occlude
nearly side by side. The cheek teeth of
Pseudoglyptodon are hypselodont and tri-
lobed, reminiscent of glyptodont teeth in these
respects, but lacking the central dentine figure
of glyptodontids. In our estimation, the closest
approach to the dental pattern of
Pseudoglyptodon among tardigrades is seen
in various mylodonts, a group generally
recognized as having diverged early in the
history of sloths. Among these, orophodontids
may be singled out. The bilobed cheek teeth of
Octodontobradys puruensis from the Mio-

Pliocene of western Brazil (Santos et al.,
1993), may hint that this pattern was primitive
for sloths. This had been suggested earlier
by incomplete orophodont dentitions contain¬
ing several sequential bilobed teeth. Thus,
the cheek-tooth outline exhibited by
Pseudoglyptodon may represent: (1) a still
more primitive condition (wherein sloths
may have been characterized ancestrally by
trilobed cheek teeth behind simpler canini¬
forms); (2) an autapomorphic modification to
three lobes from the bilobed condition seen
in some early, Octodontobradys- like oropho-
donts; or (3) a completely independent deri¬
vation from an unlobed ancestral condition.

Beyond the unusual outline of the cheek
teeth in Pseudoglyptodon, the departure of this
taxon’s dental formula (4/4) from the pattern
typical of tardigrades (5/4) should also be
emphasized. Pseudoglyptodon retains fully
functional caniniforms, meaning that the re¬
duction of the upper dental count was likely
achieved through the loss of the first or the
last molariform. Thus, it seems inescapable
that this and potentially other aspects of
Pseudoglyptodon' s dental anatomy do not
typify phyllophagans ancestrally (nor any
other group of xenarthrans to which this
taxon is potentially related). Given the
early age of Pseudoglyptodon, its high
degree of aberrant dental specialization is
unexpected.

In a superb recent assessment of tardigrade
relationships, Gaudin (2004) identified 22
nonauditory cranial features as unambiguous¬
ly diagnostic of the group. Of these, currently
available specimens of Pseudoglyptodon per¬
mit scoring of only the following (using
Gaudin’s character/character state numbering
scheme). Gaudin’s analysis did not include
glyptodonts, however, so we caution that
several of these features are not unique among
xenarthrans.

37(3): Depth of mandible >22.5%, <25% of
maximum mandibular length. This number
is difficult to estimate in Pseudoglyptodon
due to breakage, but the maximum man¬
dibular length is —10 cm. The mandible
of P. chilensis is 2.5 cm deep at a minimum,
and probably approached 3 cm in life. Thus,
mandibular depth in Pseudoglyptodon meets

2006

McKENNA ET AL.: PALEOGENE PSEUDOGLYPTODONT XENARTHRANS

15

or exceeds the primitive sloth condition,
potentially matching the most extreme
deepening seen among tardigrades, e.g.,
Acratocnus, Megatherium, and Octomy-
lodon (Gaudin, 2004). Glyptodont mand¬
ibles are also deep, which is interpreted here
as homoplasy.

61(1): Fused mandibular symphysis. Fusion
of the symphysis in Pseudoglyptodon argues
that this feature is diagnostic of Phyllophaga.
A fused symphysis occurs also in glypto-
donts, presumably homoplastically.

74(1): Posterior external opening of mandib¬
ular canal. Absent in Pseudoglyptodon.
85(3): Length of snout (preorbital length
measured to tip of nasal) <25%, >15% of
basonasal length. In Pseudoglyptodon the
basonasal length is —12 cm. The preorbital
length is less securely known given
breakage of the anterior rostral region in
SGO PV 2995; we estimate it to be between

3 and 4 cm. Although there is considerable
variability in the length of the snout in
tardigrades, the condition in Pseudo¬
glyptodon clearly more closely resembles
the rostral form typically seen in that group
than it does the form in cingulates (except
glyptodonts) or myrmecophagids.

142(2): Lacrimal foramen large, diameter
>2.5%, <3% of basonasal length (BNL).
The diameter of the lacrimal foramen in
Pseudoglyptodon is ~3 mm, i.e., roughly
2.5% of BNL.

145(2): Jugal with large ascending and des¬
cending processes. Although the ascending
process is not preserved in Pseudoglyptodon,
this taxon appears to have been marked by
a strong descending process.

In addition, there are a number of derived
features seen in Pseudoglyptodon that are
optimized as ambiguously synapomorphic
for Tardigrada by Gaudin (2004, his node 6).

2(2): Dental formula: 5 upper teeth, 4 lower
teeth. Pseudoglyptodon should actually be
scored with Gaudin’s character state 3 (i.e.,

4 uppers, 4 lowers), a condition occurring
elsewhere among xenarthrans only in

Mylodon. Nevertheless, the dental formula
seen in Pseudoglyptodon (4 uppers, 4 lowers)
closely approaches that typical of tardi¬
grades ancestrally.

7(1): Hypsodont cheek teeth. Pseudoglyptodon
is clearly high crowned, and moreover is
hypselodont. P. chilensis shows that early
members of Pseudoglyptodon are substan¬
tially less hypsodont than the slightly
younger P. sallaensis. Glyptodonts also
have hypso- and hypselodont cheek teeth,
presumably independently derived.

9(2): Modified orthodentine core of teeth,
large, typically well vascularized. Although
the degree of vascularization has not
been assessed in Pseudoglyptodon, the
teeth are nonetheless quite large for a xenar-
thran.

11(2): Outer layer of cementum forms thick
layer around outside of teeth. There is no
obvious evidence of cementum on the teeth
in Pseudoglyptodon.

18(1): Upper tooth row extends anterior to
lower. This condition is just barely met in
Pseudoglyptodon, judging from the less
distorted right side of SGO PV 2995.

20(1): Wear surface on Cl/cl oblique.

36(6): Trilobate m3. Among xenarthrans,
Pseudoglyptodon is remotely comparable
only to glyptodonts in this respect.

100(0): Length and width of nasals.
Tardigrades are typified (ambiguously) by
short wide nasals, the ratio of maximum
length to width measured at midpoint <3.
In Pseudoglyptodon this ratio is >6, prob¬
ably approaching 12, being difficult to
estimate due to anterior and posterior
damage to the nasals. Slightly less
elongated nasals (length:width ratio >4)
uniquely (and apomorphically) typifies
Scelidotheriinae among tardigrades. Pseu¬
doglyptodon is quite unusual in this respect,
probably reflecting the condition marking
pilosans primitively. Glyptodonts are also
characterized by short wide nasals.

153(1): Descending process of jugal present
and hooking posteriorly. The bone frag¬
ment floating in isolation just lateral to the

16

AMERICAN MUSEUM NOVITATES

NO. 3536

base of the coronoid process in SGO PV
2995 (see description) indicates that a des¬
cending process with this orientation was
likely present in Pseudoglyptodon.

CONCLUSIONS

New material referable to Pseudoglyptodon
from the Andean Main Range of Chile offers
tantalizing new evidence about character
evolution in the early history of Phyllophaga.
The Chilean taxon exhibits several diagnostic
features previously seen only in sloths; never¬
theless, its retention of at least one primitive
attribute argues for its divergence prior to the
appearance of the common ancestor that gave
rise to Bradypus and Choloepus plus all its
descendants. SGO PV 2995, the holotype of P.
chilensis, is regrettable for all the phylogenet-
ically important information it might have
revealed were it not for the violent, and likely
hot, conditions prevailing during the skull’s
volcanically associated deposition. The thinner
areas of the specimen (most of the back end of
the skull and basicranium, posteroventral re¬
gion of the mandible, and zygomatic arch) were
likely incinerated during burial.

The remains of pseudoglyptodontids are
extremely rare: of the hundreds of mammal
specimens collected at Tinguiririca, only two
are referable to Pseudoglyptodon. Further¬
more, we have recovered no specimens refer¬
able to the group from the dozens of other
Cenozoic mammal localities recently uncov¬
ered across a ~500-km-long swath of the
Andean Main Range. Given the scarcity of
reported specimens, the group was uncommon
in higher (Patagonia, Argentina) and lower
(Salla, Bolivia) latitudes as well.

Pseudoglyptodontids complicate what
would otherwise be a fairly straightforward
picture of dental evolution in the early di¬
versification of xenarthrans (teeth reduced to
simple, peglike structures in early xenarthrans,
with the number of these primitively simple
teeth greatly reduced in phyllophagans).
Inasmuch as the nearest known outgroup to
tardigrades is characterized by trilobed cheek
teeth, a number of more complex scenarios
must now be entertained. Was, as Engelmann
(1987) suggested, the ancestral tardigrade
dentition marked by lobate postcanines? If

it was, given the trilobate cheek teeth of
glyptodontids, are such teeth primitive for
dasypodids and/or xenarthrans as a whole,
with subsequent loss several times indepen¬
dently? Given the poorly resolved phylogenet¬
ic placement of glyptodontids relative to other
cingulates, the possible transformation history
of the dentition in early xenarthrans is
currently not readily optimized. Unless glyp¬
todontids can convincingly be shown to di¬
verge basal to all other cingulates, the trilobed
cheek teeth in Pseudoglyptodon are likely
convergent upon those in glyptodontids
(which lack the reduced number of teeth
characterizing phyllophagans). Thus, it is also
possible that simple ovoid teeth characterized
phyllophagans (and tardigrades) ancestrally,
with Pseudoglyptodon, orophodonts, and glyp¬
todontids each developing more complex
cheek teeth independently.

Finally, in view of the many derived
resemblances between Pseudoglyptodon and
glypodonts, a word about the possible re¬
lationship of these two groups is in order.
Might Pseudoglyptodon be an early-diverging,
peculiarly specialized glyptodont, rather than
a phyllophagan? It is conceivable, after all,
that the resemblances noted between Pseudo¬
glyptodon and glyptodonts reflect a unique
common ancestry—in which case there is
either a great deal of convergence between
this glyptodontoid clade and tardigrades, or
the unusual features common to Pseudo¬
glyptodon, glyptodontids, and tardigrades
represent ancestral conditions for Xenarthra.
Acceptance of an exclusive Pseudoglyptodon-
glyptodont relationship would imply that
Pseudoglyptodon diverged from other mem¬
bers of the clade prior to the origin of the
central dentine figure, and potentially before
the appearance of osteoderms (assuming
Pseudoglyptodon truly lacked them). While
intriguing, pending a clearer understanding
of the phylogenetic placement of glyptodonts
relative to other cingulates, we regard this
alternative as currently less viable than
the hypothesized tardigrade affinities of
Pseudoglyptodon favored above.

ABBREVIATIONS

AMNH American Museum of Natural

History

2006

McKENNA ET AL.: PALEOGENE PSEUDOGLYPTODONT XENARTHRANS

17

MLP Museo de La Plata

PU Princeton University

SGO PV Museo Nacional de Historia Natural,

Santiago, Chile, vertebrate paleontol¬
ogy collections

ACKNOWLEDGMENTS

We thank the National Geographic Society
for support of the initial field season, the
National Science Foundation, and the Eppley
Foundation. Reynaldo Charrier freely shared
his geologic expertise and has enthusiastically
embraced mammal fossils for deciphering
Andean geochronology. We thank the reverse
fault in the upper reaches of the Rio
Tinguiririca, for without it there would be no
Termas del Flaco, without which there would
have been no economic rationale for a road so
deep into the Andean Main Range, without
which the paleontological riches of the
Abanico Formation might not have come to
light for another century. Our work has had
the long-term backing of the Museo Nacional
de Historia Natural and the Consejo de
Monumentos Nacionales, Santiago, Chile.
Daniel Frassinetti and Maria Eliana Ramirez
have been pivotal in this regard. Gaston
Mancilla provided access to land producing
part of the Tinguiririca Fauna. Susan Bell
provided invaluable assistance with locality
and specimen data. Chester Tarka produced
the photograph making up figure 3. Timothy
Rowe was generous beyond the call of duty in
facilitating the CT scanning, without which
our description would be far less complete.
The comments of George Engelmann and
those of a highly insightful anonymous re¬
viewer improved the manuscript substantially.

REFERENCES

Ameghino, F. 1897. Mammiferes cretaces de
l’Argentine. (Deuxieme contribution a la con-
naissance de la faune mammalogique des
couches a Pyrotherium.) Boletin del Instituto
Geografico Argentino, Buenos Aires, 18:
406^129, 431-521 (also issued as separate, 117

pp.).

Ameghino, F. 1902. Notices preliminaries sur des
mammiferes nouveaux des terrains cretaces de
Patagonie. Boletin de la Academia Nacional de
Ciencias de Cordoba 17: 5-70 (3-68 in the
separates), 3 unnumbered text figures.

Anonymous 1985. Termas del Flaco Quadrangle,
1 : 50,000 topographic sheet. Instituto
Geographico Militar de Chile 3445-7015.

Castellanos, A. 1932. Nuevos generos de glypto-
dontes en relacion con su filogenia. Physis
(Revista de la Sociedad Argentina de Ciencias
Naturales) 11: 92-100.

Charrier, R., A.R. Wyss, J.J. Flynn, C.C. Swisher
III, M.A. Norell, F. Zapatta, M.C. McKenna,
and M.J. Novacek. 1996. New evidence for late
Mesozoic-early Cenozoic evolution of the
Chilean Andes in the upper Tinguiririca
Valley (35°S), central Chile. Journal of South
American Earth Sciences 9(5/6): 393-422.

de Queiroz, K., and J. Gauthier. 1990. Phylogeny as
a central principle in taxonomy: phylogenetic
definitions of taxon names. Systematic Zoology
39: 307-322.

Engelmann, G.F. 1987. A new Deseadan sloth
(Mammalia: Xenarthra) from Salla, Bolivia,
and its implications for the primitive condition
of the dentition in edentates. Journal of
Vertebrate Paleontology 7(2): 217-223, figs. 1,2.

Flynn, J.J., M.A. Norell, C.C. Swisher III, and A.R.
Wyss. 1991. Pre-Deseadan, post-Mustersan
mammals from central Chile: an update.
Journal of Vertebrate Paleontology 11
(suppl. to 3): 29A.

Flynn, J.J., A.R. Wyss, D.A. Croft, and R.
Charrier. 2003. The Tinguiririca fauna, Chile:
biochronology, biogeography, paleoecology,
and a new earliest Oligocene South American
Land Mammal ‘Age’. Palaeogeography, Pa-
laeoclimatology, Palaeoecology 195: 229-259.

Gaudin, T.J. 2004. Phylogenetic relationships among
sloths (Mammalia, Xenarthra, Tardigrada): the
craniodental evidence. Zoological Journal of the
Linnean Society 140(2): 255-305.

Grasse, P.-P. 1955. Ordre des Edentes. In P.-P.
Grasse (editor), Traite de Zoologie, vol. 17,
Mammiferes: 1182-1266. Paris: Masson.

Hoffstetter, R. 1958. Xenarthra. In J. Piveteau
(director), Traite de Paleontologie, tome VI,
vol. 2: 535-636, figs. 1-64. Paris: Masson.

Kay, R.F., R.H. Madden, M.G. Vucetich, A.A.
Carlini, M.M. Mazzoni, G.H. Re, M. Heizler,
and H. Sandeman. 1999. Revised geochronol¬
ogy of the Casamayoran South American Land
Mammal Age: climatic and biotic implications.
Proceedings of the National Academy of
Sciences (U.S.) 96(23): 13235-13240.

Klohn, C. 1960. Geologia de la Cordillera de los
Andes de Chile Central; provincias de
Santiago, O’Higgins, Colchagua y Curico.
Instituto de Investigaciones Geoleogicas.
Boletin 8: 1-95.

MacFadden, B.J., K.E. Campbell, Jr., R.L. Cifelli,
O. Siles, N.M. Johnson, C.W. Naeser, and P.K.

18

AMERICAN MUSEUM NOVITATES

NO. 3536

Zeitler. 1985. Magnetic polarity stratigraphy
and mammalian fauna of the Deseadan (late
Oligocene-early Miocene) Salla Beds of north¬
ern Bolivia. Journal of Geology 93: 223-
250.

Marshall, L.G., R. Hoffstetter, and R. Pascual.
1983. Mammals and stratigraphy: geochronol¬
ogy of the continental mammal-bearing
Tertiary of South America. Palaeovertebrata,
Memoire Extraordinaire 1983: 1-93.

McKenna, M.C., and S.K. Bell. 1997. Classification
of mammals above the species level. New York:
Columbia University Press.

Novacek, M.J., A.R. Wyss, D. Frassinetti, and P.
Salinas. 1989. A new ?Eocene mammal fauna
from the Andean Main Range. Journal of
Vertebrate Paleontology 9(suppl. to 3): 34A.

Santos, C.R. dos., A. Raney, and J. Ferigolo. 1993.
Octodontobradyinae, uma nova subfamilia de
Orophodontidae (Edentata, Tardigrada) do
Mioceno Superior-Plioceno do Estado do
Amazonas, Brasil. Ameghiniana 30(3): 255-
264.

Scillato-Yane, G.J. 1977. Sur quelques Glypto-
dontidae nouveaux (Mammalia, Edentata) du
Deseadien (Oligocene inferieur) de Patagonie
(Argentine). Bulletin du Museum National
d’Histoire Naturelle, 3e ser., 487, Sept.-Oct.,
1977, Sciences de la Terre 64: 249-262, pis. 1, 2.

Scillato-Yane, G.J. 1988. Algunos Cingulata
(Mammalia, Edentata) del Oligoceno de
Quebrada Fiera (Mendoza, Argentina). Abstracts
V. Jomadas Argentinas de Paleontologia de
Vertebrados, 16-19 May, 1988:26-27.

Simpson, G.G. 1948. The beginning of the age of
mammals in South America. Bulletin of the

American Museum of Natural History 91(1):
1-232, figs. 1-80, pis. 1-19, tables 1-82.
Swisher, C.C., III, and D.R. Prothero. 1990. Single¬
crystal 40 Ar/ 39 Ar dating of the Eocene-
Oligocene transition in North America.
Science 249: 760-762, 1 fig., 1 table.

Wyss, A.R., R. Charrier, D.A. Croft, and J.J.
Flynn. 2004. Paleontologial reconnaissance of
the central Andean Main Range by helicopter:
additional new Cenozoic mammal faunas from
Chile. Journal of Vertebrate Paleontology
24(suppl. to 3): 133A.

Wyss, A.R., R. Charrier, and J.J. Flynn. 1996.
Fossils mammals as a tool in Andean stratig¬
raphy: dwindling evidence of Late Cretaceous
volcanism in the South Central Main Range.
Paleobios 17(2-1): 13-27.

Wyss, A.R., J.J. Flynn, M.A. Norell, C.C. Swisher
III, R. Charrier, M.J. Novacek, and M.C.
McKenna. 1993. South America’s earliest
rodent and recognition of a new interval of
mammalian evolution. Nature 365(6445):
434-137, 2 figs.

Wyss, A.R., J.J. Flynn, M.A. Norell, C.C. Swisher
III, M.J. Novacek, M.C. McKenna, and R.
Charrier. 1994. Paleogene mammals from the
Andes of central Chile: a preliminary taxonom¬
ic, biostratigraphic, and geochronologic assess¬
ment. American Museum Novitates 3098: 1-31.
Wyss, A.R., M.A. Norell, J.J. Flynn, M.J.
Novacek, R. Charrier, M.C. McKenna, C.C.
Swisher III, D. Frasinetti, P. Salinas, and J.
Meng. 1990. A new early Tertiary mammal
fauna from central Chile: implications for
Andean stratigraphy and tectonics. Journal of
Vertebrate Paleontology 10(4): 518-522, figs.
1-4.

Complete lists of all issues of the Novitates and the Bulletin are available at World Wide
Web site http://library.amnh.org/pubs. Inquire about ordering printed copies via e-mail from
scipubs@amnh.org or via standard mail from: American Museum of Natural History,
Library—Scientific Publications, Central Park West at 79th St., New York, NY 10024. TEL:
(212) 769-5545. FAX: (212) 769-5009.

@ This paper meets the requirements of ANSI/NISO Z39.48-1992 (Permanence of Paper).