Morphological diversity in the postcranial skeleton of Casamayoran (?middle to late Eocene) Notoungulata and foot posture in notoungulates

2007·6.3 MB·English

by  Shockey, Bruce J

AMERICAN MUSEUM
Novitates
PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY
CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024
Number 3601, 26 pp., 7 figures, 1 table December 12, 2007
Morphological Diversity in the Postcranial Skeleton
of Casamayoran (?Middle to Late Eocene)
Notoungulata and Foot Posture in Notoungulates
BRUCE J. SHOCKEY' AND JOHN J. FLYNN”
ABSTRACT
Appendicular skeletons of isotemnid notoungulates are described from Canadén Vaca (Vacan
“subage’’, Casamayoran South American Land Mammal “Age’’, ?middle to late Eocene), Simpson
documented three of these, Thomashuxleya externa, Anisotemnus distentus, and Pleurostylodon
similis, some 70 years ago, in fashioning a composite isotemnid skeleton, but he did not emphasize
their differences from one another. We note variation, especially in the forelimb, that appears to be
functionally significant as well as phylogenetically informative, For example, the downwardly
curved olecranon, ventrally concave bowing of the ulnar shaft, and orthogonally directed
articulation of the elbow joint suggest an erect forelimb stance in Thomashuxleya externa, whereas
the forelimbs of Anisotenmmus distentus and Pleurostylodon similis show indications of a crouching
posture, including ventrally convex bowing of the ulnar shaft with a slight upward curvature of the
olecranon, and an elbow joint in which the antebrachium rotated obliquely relative to the humerus,
Articular facets on the proximal carpals suggest that the manus of Aniso/emnus was habitually
extended, indicating a plantigrade stance of the forelimb. Although none of these three taxa have
associated hindfoot material, all known Vacan notoungulate astragali have shallow trochlea, well-
developed and deep grooves for the flexor hallucis longus, which are separated from the trochlea
by a fossa that contains a superior astragalar foramen. An isolated notoungulate pes, not referred
to any of the three taxa above, appears to be pentadactyl, having a distinctive, divergent
tarsometatarsal joint for its hallux, It also has a shallow trochlea, an astragalar foramen, and
a flexor groove, indicating limited rotation of the upper ankle joint. Indeed, a survey of known
Casamayoran-aged notoungulate astragali indicates that most taxa had limited mobility at the
tibioastragalar joint, in stark contrast to post-Eocene faunas in which nearly all the ungulates had
greater rotation of the upper ankle joint and were subcursorial, as evidenced by their longer and
Department of Biology, Manhattan College, New York, NY 10471 (bruce.shockey(@manhattan.edu)
and Diyision of Paleontology, American Museum of Natural History.
* Division of Paleontology, American Museum of Natural History ([email protected]).
Copyright © American Museum of Natural History 2007 ISSN 0003-0082
AMERICAN MUSEUM NOVITATES NO. 3601
to
deeper trochlear articulation and loss of the astragalar foramen. We suggest that the change from
ambulatory- to subcursorial-dominated ungulate faunas across the Eocene-Oliogocene boundary
mirrors the changes from brachydont to hypsodont faunas over the same time. Decreased
temperatures and rainfall resulting in more open habitats may be related to both morphological
evolutionary patterns.
INTRODUCTION locomotor or taxonomic differences among
the three forms,
Little has changed since Simpson (1936a: 1)
The purpose of the current work is to
remarked, ““The skeleton of South American provide greater understanding of the post-
ungulates has been practically unknown for cranial skeletal morphology of these three
any stage before the Deseado’’ (Deseadan
isotemnids. Also, additional specimens of
South American Land Mammal “Age” Casamayoran isotemnids are described here
[SALMA], late Oligocene). His contribution for the first time, providing the first informa-
regarding the postcranium of an Eocene
tive record of the pes of any pre-Deseadan
(Casamayoran SALMA) “isotemnid” (order notoungulate. Description of this pes helps
Notoungulata) remains a nearly singular document the apparently plantigrade pedal
example of a detailed analysis of a pre- condition of an “archaic” (basal) notoungu-
Deseadan South American notoungulate skel- late and provides an opportunity to investi-
eton. gate the transition from plantigrady to digiti-
Although both the feasibility and merit of grady in this major lineage of extinct, endemic
Simpson’s accounts (1936a, 1967) of the South American ungulates.
Casamayoran isotemnid postcrania might be
questioned, his goal clearly was to document
OVERVIEW OF
the morphology of an idealized isotemnid
THE CASAMAYORAN SALMA
skeleton. To do so, he used the remains of
three individuals representing three different Carlos Ameghino discovered the beds that
taxa to describe a composite notoungulate would serve as the type locality for
skeleton. The taxa included Thomashuxleya Casamayoran during the austral summer of
externa, a second species that he did not 1894-1895, in Patagonia, along the Gulfo San
specify (but we refer to Anisofemnus distentus), Jorge, near Punta Casamayor (Simpson, 1984).
and a species of Pleurostylodon that later However, the distinctiveness of this fauna was
would be referred to P. simulis in Simpson's not recognized until 1899, as Carlos and his
final attempt to sort out the taxonomic status elder brother, Florentino, initially thought that
of Casamayoran istotemnids (Simpson, 1967). fossils from Punta Casamayor were of
Simpson figured the entire composite skeleton Deseadan age (“Couches a Pyrotherium” in
(Simpson, 1935, 1936a, 1967) but not in- their terminology [F. Ameghino, 1895]),
dividual elements, nor did he provide any Therefore, the first descriptions of a variety of
metric data for the elements (although he did Casamayoran fossils were included in F.
provide some unitless ratio indices), Because Ameghino’s (1897) second contribution re-
his goal was to describe an idealized, general- garding the Deseadan SALMA. Later, howev-
ized isotemnid skeleton, he did little in the way er, Carlos discovered Casamayoran fossils at
of differentiating the morphology of the three the Gran Barranca in their stratigraphic
taxa from one another, Instead, he empha- context, below and stratigraphically discordant
sized the similarity among them, noting that from those assigned to the Deseadan. The
the “structure is essentially the same in all superpositional context was now clear, so he
with differences apparently of not more than wrote his brother in 1899, “It turns out what we
generic value” (Simpson, 1936a: 2, 1967: 153). have been calling the Pyrotherium fauna
Although such a method may be useful in according to new observations is, in reality,
estimating the form of a hypothetical ancestor, the succession of two different faunas, separat-
the character analysis was not done within an ed by an enormous interval of time” (translated
explicit phylogenetic framework, and thus it from Spanish by Simpson, 1984: 71). Carlos
has the serious disadvantage of obscuring any proposed to name it the Notostylops fauna, in
2007 SHOCKEY AND FLYNN: MORPHOLOGICAL DIVERSITY IN NOTOUNGULATES 3
reference to a common taxon there. Thus, it ran into temporally distinct ‘“‘subages”, the
was designated as the Notostylopéen by the Barrancan and the somewhat older Vacan.
Ameghinos (F. Ameghino, 1906), but soon However, absolute ages of these or any other
thereafter was given the geographically based Casamayoran locality remained unresolved
and currently recognized name, Casamayoran until recently.
(Gaudry, 1906). Kay et al. (1999) reported radioisotopic age
Although F. Ameghino (1906) regarded the determinations, complemented by paleomag-
Casamayoran SALMA as Cretaceous in age, netic stratigraphy. that yielded an age estimate
Gaudry (1906) (and everyone else since) has for the Barrancan “subage” of 35,34 to
considered it to be Eocene, usually early 36.62 Ma, placing it in the late Eocene, some
Eocene (e.g.. Gaudry, 1906; Simpson, 1940; 15 to 20 milion years younger than previously
Flynn and Swisher, 1995). Recently, however. thought. Flynn et al. (2003) raised technical
work at the Gran Barranca suggests that the objections to this interpretation and suggested
Casamayoran SALMA, or at least the a more conservative minimum age estimate
Barracan part of it. may be as young as late of 38 Ma. Nevertheless. such a young age
Eocene (Kay et al., 1999). estimate for the Casamayoran implies that
George Gaylord Simpson and his small a huge temporal gap remains in the fossil
crew collected numerous Casamayoran fossils record of the early and middle Eocene of South
during the Scarritt Expeditions to Patagonia America, even if the Vacan “subage” of the
in the austral summers of late 1930-early 1931 Casamayoran is eventually determined to be
and 1933-1934. Most of these came from significantly older than the Barrancan. Most
two localities, Colhué Huapi, at the Gran strikingly, even though Barrancan and Vacan
Barranca, Chubut, Argentina, and Canadon assemblages are distinct. their differences are at
Vaca, which lies some 60 km northeast of the relatively low taxonomic levels, few major
Gran Barranca in badlands having drainage groups disappear or originate between them,
into the Rio Chico (Simpson, 1948: fig. 1. and the morphology of taxa within higher level
map). Much of the fauna from these two clades appears to change very little across this
localities has been documented in Simpson’s potentially long time span. The temporal extent
two-part work titled “The Beginning of of this relative stasis is unknown, since the
the Age of Mammals in South America” upper age limit of the Riochican (the SALMA
(Simpson, 1948, 1967). that precedes the Vacan “‘subage™) remains
Whereas Simpson (1948, 1967) described unknown, The Riochican has been shown to be
the individual taxa of the Casamayoran, faunistically more similar to the Vacan “sub-
Cifelli (1985) provided an analysis of the age”, than the Vacan is to the Barracan
overall faunal composition. He found the “subages” (Simpson coefficients were 47 for
Casamayoran faunas of the Gran Barranca the Riochican/Vacan compared to 29 for
and Canadon Vaca to be “strikingly dissimilar” the Vacan/Barranean; Cifelli. 1985), The
(Cifelli, 1985: 16). OF the 41 species that had Riochican SALMA has long been regarded as
sutlicient morphological and stratigraphic data being Paleocene (Patterson and Pascual, 1972:
to be included in the analysis, only 4 were Flynn and Swisher 1995; Marshall et al.. 1997):
considered to be common to both regions (this however, it has only been radiometrically
excluded one Cafiadon Vaca locality that constrained from below, such that it is only
Cifelli [1985] judged to be the same age as that known that it must be younger than 63 Ma
of the Casamayoran fauna of the Gran (Marshall et al., 1997), If it turns out that the
Barranca). The generic and specific indices of Riochican is indeed Paleocene, then the relative
faunal similarity between the Gran Barranca stasis from the Riochican through the Barracan
Casamayoran and Canadon Vaca were consid- “subage”’ may have extended over a period
erably lower than indices of faunal similarity greater than 15 million years, An alternative
derived from different North American land hypothesis is that the Riochican is actually of
mammal faunas assigned to the same North Eocene age, and thus the Jength of stasis and
American Land Mammal “Age”. Thus, Cifelli the pre-Barrancan depositional hiatus would
(1985) proposed to subdivide the Casamayo- be shorter than supposed.
4 AMERICAN MUSEUM NOVITATES NO, 3601
Whereas a late Eocene age for at least the lates (although tt is striking that he never
later part of the Casamayoran opens a pre- recognized Notoungulata itself as a distinct
Barrancan gap in the record, it simultaneously group). Of these, the Isotemnidae are the focus
crowds the currently undated Mustersan and of the current study due to the availability of
Divisidaran SALMAs between it and the postcranial specimens and the apparently basal
Tinguirirican SALMA, whose upper limit 1s position of isotemnids within Notoungulata
radioisotopically constrained at 31.5 Ma (e.g., as a hypothetical ancestor to advanced
(Flynn et al., 2003), This implies that rapid Toxodontia; Simpson, 1967; Cifelli, 1993).
changes occurred in South American faunas As anatomically generalized and early
from the late Eocene to the early Oligocene appearing notoungulates, isotemnids have
(Kay et al., 1999; Flynn et al., 2003). These always been considered to be basal to at least
changes are conspicuous both taxonomically some groups of more morphologically ad-
and in the dental evolution that occurred vanced groups of notoungulates. For example,
across diverse clades. Ameghino (1897) thought that isotemnids
Whereas Mustersan and older faunas are were ancestral to homalodotheriids and leon-
dominated by herbivores having low crowned tiniids. He placed some species of the larger
(brachydont) teeth, Tinguirirican and younger genera (Thomashuxleya, Anisotemnus, and
faunas are dominated by high crowned * Praasmodus” [= Periphragnis|, now generally
{hypsodont and/or hypselodont) taxa (Wyss recognized as being larger bodied isotemnids)
et al,, 1994: Kay et al., 1999; Flynn et al.,, in the Homalodotheriidae, as he thought these
2003). The upward revision of the ages of the genera to have been ancestral to the San-
Casamayoran, Mustersan, and Divisideran, tacrucian taxon Homalodotherium (Ameghino,
and the well-constrained younger limit of the 1906). Simpson (e.g.. 1936a) initially sup-
age of the Tinguirirican, implies a_ rapid ported a special relationship among the
change from browsing-dominated faunas to isotemnids and homalodotheriids by recogniz-
grazing faunas. This change occurred during ing the “Entelonychia”, a group that included
the late Eocene to early Oligocene cooling and notostylopids, isotemnids, and homalodother-
drying (Prothero, 1994), which may be related iids. Later (Simpson, 1967), he noted that the
to the development of circum-Antarctic cur- generalized morphology of isotemnids did not
rents that began as a result of the separation exclude them from being ancestral to noto-
of South America from the Antarctic conli- hippids, leontiniids, and toxodontids. He
nent and the subsequent deepening of the redefined the family Isotemnidae, “mostly by
Drake Passage (Kennett, 1977; Kay et al., primitive characters, a procedure which is not
1999; Lawver and Gahagan, 2003; Flynn et wholly satisfactory but to which there seems
al., 2003). Available data imply a history of to be no good alternative in this case”
late Eocene and older faunas being composed (Simpson, 1967: 118). He did recognize the
mostly of browsers, while by the early accessory cuspule of the molar trigonids as
Oligocene (e.g., Tinguirirican SALMA) gra- a possible derived character for isotemnids.
zers dominated temperate South American However, the diagnostic value of this fea-
faunal assemblages, Hypsodont mammals of ture may be limited, since it is absent in
post-Eocene faunas of South America in- some isotemnids (e.g., Periphragnis and
cluded diverse taxa. such as argyrolagid Disiylophorus) and present in some taxa
marsupials, xenarthrans, and some rodents: generally considered to be oldfieldthomasiids
however, most of the hypsodont taxa were (e.g.. Colbertia and Maxschlosseria) (Cifelli,
notoungulates. 1993).
The notion that isotemnids were the nearest
outgroup to leontintids, notohippids, and
CASAMAYORAN NOTOUNGULATA
toxodontids. as well as homalodotheriids,
In his initial description of Casamayoran received some support in the phylogenetic
fossils (initially thinking that they were of analysis of Cifelli (1993), although the alter-
Deseadan age), Ameghino (1897) first recog- native possibility that isotemnids and homa-
nized several of the major groups of notoungu- lodotheriids together formed a monophyletic
2007 SHOCKEY AND FLYNN: MORPHOLOGICAL DIVERSITY IN NOTOUNGULATES 5
group distinct from all other notoungulates studies. Indeed, few postcranial data have
was supported by the unmodified strict been included in previous phylogenetic anal-
consensus tree. Cifelli’s (1993) work remains yses of notoungulates of any age, with the
the only phylogenetic analysis of the interfa- interfamilial analysis of Cifelli (1993) being
milial relationships among notoungulates, exceptional in this regard. Also, knowledge of
Characters used in that analysis were mostly the postcranial skeleton frequently yields
craniodental, but one manual and five tarsal functionally significant information.
characters were included.
Whereas most elements of the postcranial
MATERIALS AND METHODS
skeleton of pre-Deseadan notoungulates re-
main poorly known, isolated tarsal elements
The basis of this study is the collection of
have been reasonably well documented
fossils in the AMNH obtained by G.G. Simpson
(Ameghino, 1904: Cifelli, 1983; Bergqvist,
and crew at the Casamayoran SALMA locality
1996). These, however, have lacked clear Canhadon Vaca during the First Scarritt Ex-
associations with dental material, the usual
pedition of 1930-1931. Comparative materials
basis for diagnosing fossil mammal species. To
included some specimens from the Casa-
establish associations between these elements,
mayoran horizon at Colhué Huapi (the Gran
Cifelli (1983) and Bergqvist (1996) attempted
Barranca), also collected by Simpson, and
to correlate the size of proximal tarsals
elements from Mustersan SALMA assemblages,
with dentitions from discrete localities of the including casts of specimens collected by S.
middle to late Paleocene, Itaborai, Brasil. Roth, the originals of which are curated in the
These attempts yielded credible associations, Museo de la Plata collections.
although specimens that directly associate
Traditional family names are generally used,
dental and postcranial material in single
with the caveat that the interfamilial relation-
individuals would provide the only definitive
ships among notoungulates are not well un-
taxonomic link between these elements.
derstood or rigorously tested, and some almost
In contrast, Ameghino (1904) did not surely represent basal, paraphyletic taxa (e.g.,
indicate his method of assigning the tarsal Isotemnidae). However, until these detailed
elements to a particular taxon. Simpson phylogenetic analyses are performed, it is
regarded Ameghino’s specific determinations pointless to add to the confusion by inventing
as “totally unreliable and the generic hardly provisional and likely unstable nomenclature.
less so” (Simpson, 1967: 193-194), but dis- To facilitate discrimination between taxa,
coveries of associated postcranials with cranial individual taxa (and even specimens) are
material provide some vindication for described separately. However, the figures
Ameghino’s associations (e.g., Scott, 1912a and tables are organized by skeletal elements
for Nesodon and Adinotherium: Chaffee, 1952 to aid comparisons of morphological features
for Rhynchippus pumilus; Sydow, 1988 and between the elements.
Shockey et al., 2007 for Trachytherus), The following institutional abbreviations
Presumably, Carlos Ameghino made many are used throughout this paper: AMNH,
of the initial taxonomic identifications in the American Museum of Natural History;
field, but since he did not use sophisticated MLP, Museo de La Plata, La Plata, Ar-
collecting techniques he typically was unable gentina.
to recover broken long bones, but he generally
collected only the durable teeth and tarsals of
individual specimens. Regardless of the qual- SYSTEMATIC PALEONTOLOGY AND
ity of the initial identifications, the Ameghinos COMPARATIVE ANATOMY
were able to document Casamayoran astragall
representing at least 22 distinct operational FAMILY ISOTEMNIDAE AMEGHINO, 1897
taxonomic units (Ameghino, 1904).
Anisotemnus distentus (Ameghino, 1901)
Improving our knowledge of postcranial
skeletons of early notoungulates can greatly Originally, Ameghino described this as a
enhance phylogenetic and _ paleoecological species of Isoltemnus, but he later (Ameghino,
6 AMERICAN MUSEUM NOVITATES NO, 361
1902) decided it pertained elsewhere, creating worn left p4 or ml; AMNH 28906, partial
the new generic name <Anisotemnus (“not skeleton including the right scapula, right
Tsotemnus’’) for it. Simpson (1967) recognized and left humerus, ulna. radius, and manus,
A, distentus as a valid taxon and provided as well as much of the axial skeleton; AMNH
a generic diagnosis, but he confessed that the 28647, distal left radius, scaphoid, lunar,
genus “did not seem to differ in any readily trapezoid, Mec I-V (distal end of Mc V
diagnostic way from Thomashuxleya...or missing), two indeterminate phalanges, and
Pleurostylodon™ (Simpson, 1967; 137). Never- some sesamoids.
theless. its intermediate size between species Description: The scapula of Asisotemnuys
referred to Thomashuxleya and those to (fig. 1) is similar to that of Thomashuxleya
Pleurostvlodon makes A. distentus readily dis- externa in general form and in details of the
tinguishable, since it is the only Casamayoran acromion. The scapular blade is ovoid, rather
isotemnid of this size. than triangular, with the lateral border being
In his initial description of the partial fairly straight. whereas the anterior border is
skeleton here referred to Anisotemnus, strongly convex. The supraspinous fossa is
Simpson (1938) thought that it might repre- much larger than the infraspinous fossa, with
sent a small species of Thomashuxleva. One of about twice the surface area. The spine is
the few editorial changes Simpson (1967) badly damaged, but the region near the
made relative to his 1938 account of the glenoid is preserved well enough to show that
idealized or composite isotemnid skeleton was it was well elevated above the blade. The
his suggestion that his “specimen B’ (AMNH acromial angle is developed into a distinct
28906) might be referable to Anisoremmus. process, giving the acromion a forked appear-
Anisotemnus, however. had nat been re- ance similar to that seen in Santacrucian
ported previously from Cafiadon Vaca, interathertids (Sinclair, 1909). The acromion,
Simpson (1967) did not include any Canhadon however, extends farther beyond the gle-
Vaca specimens in the hypodigm of Ani- noid border in Anisotemnus than in the
sotemnus, and Cifelli (1985) did not list this Santacrucian interatheriids. The spine is too
taxon from Canadon Vaca in his faunal damaged to determine if there were any
analysis. We note, however. two lower molars other metacromia (e.g., as occurs in the
from Canadon Vaca in the AMNH collection Santacrucian toxodontids Nesodon and
that are referable to Anisotemnus. AMNH Adinotherium, which have two [see Scott,
28648 is an m3 having mesial-distal and 1912a: pls. XXIL1, XXVIL8]). The glenoid is
transverse dimensions of 27.9 and 14.5 mm, teardrop-shaped, with the apex directed to-
respectively. These are smaller than any of the ward the well-developed and recurved cora-
m3s of Thomashuxleya and larger than those coid process.
of Pleurostyledon, but they are of similar The right humerus of Anixotenmus is ex-
dimensions to Anisofemmus specimens re- tremely well preserved (fig. 2). The greater
ported by Simpon (1967) and data obtained tubercle is broad but not exceedingly high.
in the present study. Given this dental The greater tubercle and the large, bulbous
evidence of the occurrence of Anisoteninus at lesser tubercle together form a well-defined
Canhadon Vaca, combined with the presence of bicipital groove. The crest of the lesser
an isotemnid skeleton intermediate in size tubercle forms a sharply defined, raised
between that of Thomashuxleya externa and posteromedial lip of the shaft that terminates
Pleurastyloden similis, we refer this suite of at a raised tuberosity, the likely site of the
materials below to Anisotemnus distenius. teres major muscle insertion. Lateral (deltoid)
Material: All specimens referred here to and medial (pectoral) crests unite at a point
Anisotemnus are from Canadon Vaca more than halfway down the shaft. The
(“Vacan” “subage” of the Casamayoran) supinator crest is bladelike and conspicuous.
and were collected by G.G, Simpson and crew An entepicondylar foramen pierces the mod-
m the austral summer of 1930-1931: AMNH erately developed medial epicondylar process.
28648, left jaw fragment with erupted A common radial-ulnar fossa occurs just
but unworn m3; AMNH 28664, heavily proximal to the capitulum.
2007 SHOCKEY AND FLYNN: MORPHOLOGICAL DIVERSITY IN NOTOUNGULATES 7
sup spn f
inf spn f
acr ang
Fig. 1. Scapula of Anisotemnus distentus, AMNH 28906. Left, lateral view; right, distal view.
Abbreviations: acr, acromion; acr ang, acromial angle; cor p, coracoid process; gln, glenoid fossa; inf spn
f, infraspinous fossa; sup spn f, supraspinous fossa.
The elbow articular region of the humerus on the distomedial surface, formed by what
has a distinctive, subspherical capitulum appears to be a pronator crest. The distal
having a long axis that lies in a dorsoventral region of the ulna is damaged, except the
direction, suggesting parasagittal rather than styloid process, which is preserved in one of
rotary movements. The trochlear groove is the ulnae.
deep, being bordered by a large, discoid As Simpson (1936a) noted, the radial shaft
medial flange. Whereas only a medial crest is curved, although curvature of the right side
buttresses the ulna on the anterior side, both may be exaggerated by postmortem damage.
medial and lateral crests form a deep groove In addition to being less circular than the
for the ulna posteriorly. The olecranon fossa is proximal radius of Homalodotherium, as
deep; however, the perforation appears to be Simpson (1936a) observed, a conspicuous
due to breakage. Nevertheless, little bone capitular eminence (fig. 3D) defines a notch
separated the olecranon fossa from the ante- on the dorsal (pronated) side of the proximal
brachial fossa of the anterior surface. radius that, in anatomical position, wraps
As in the antebrachia of all known notoun- around the dorsoventrally oriented capitulum
gulates, the ulna and radius are separate of the humerus. This form provides both
elements (fig. 3). The ulnar shaft is strongly precision of movement and a mechanical re-
excavated on the lateral side, such that the straint against extreme supination. In addition
midshaft region forms a “C”’ in cross section. to the articular surface on the ventral side of
The olecranon is fairly long and somewhat i1n- the proximal end of the radius (the ulnar
turned, although not to the extent seen in the facet), there is a smooth surface of the
Deseadan mesotheriid Trachytherus (Shockey dorsolateral circumradial region suggestive of
et al., 2007). The olecranon process is exca- an articular facet. This facet has been directly
vated on the medial side, forming a cavity that observed in various notoungulates in which
presumably was occupied by digital and wrist there is an articulation of the proximal radius
flexor muscles in life. The trochlear notch with an elbow sesamoid, such as in the basal
forms a broad crescent, terminating in a mod- toxodontid Nesodon (Scott, 1912a) and the
estly tall coronoid process, which appears mesothertid typotheres Trachytherus and
somewhat asymmetric due to the large size Plesiotypotherium (Shockey et al., 2007). The
of the radial notch. The shaft does not taper radial tuberosity is somewhat elongated,
distally, but instead there is some broadening commencing near the proximal ulnar facet
8 AMERICAN MUSEUM NOVITATES NO. 3601
Fig. 2. Isotemnid humeri from Cafiadén Vaca, Casamayoran SALMA (Vacan “subage’’). Anterior
(above) and distal (below) view of humeri of A, Ansiotemnus distentus, AMNH 28906; B, Thomashuxleya
extena, AMNH 28653; and C, Plexotemnus similis, AMNH 28904. Scale bar applies to all. Panels A and B
show right humeri and panel C shows a left humerus (but digitally reversed for ease of comparison).
Abbreviations are: cap, capitulum; Ac, deltoid crest; ent f, entepicondylar foramen gt, greater tubercle; It,
lesser tubercle; m flg, medial flange; pc, pectoral crest; tt, teres tubercle.
and extending about 3 cm down the shaft, Simpson’s (1936a) description of the iso-
with its ridge forming a double crescent. The temnid manus was based on AMNH 28906
width of the radial shaft increases distally (Anisotemnus distentus), since the manus of his
where a bladelike extension on the dorso-ulnar “specimen A” (Thomashuxleya) was badly
side overlies a triangular articulation for the damaged. Details are not repeated here, but
distal ulna. The diaphysis and epiphysis are its major features are summarized. Also,
fused, but their border is still evident at the whereas Simpson compared this manus to
distal radius. The epiphysis supports a single, that of Homalodolotherium, we compare it
longitudinal dorsal tubercle that terminates at to that of a putative sister taxon of
the radiocarpal joint near the articulation for Notoungulata, Arctocyon (AMNH_ 16543),
the scaphoid and lunar. since that of Anisotemnus appears to be the
2007 SHOCKEY AND FLYNN: MORPHOLOGICAL DIVERSITY IN NOTOUNGULATES 9
Fig. 3. Isotemnid antebrachia, ulnae A, B, and C; radu D, E, and F. A and D are of Anisotemnus
distentus, AMNH 28906; B and E are of Thomashuxleya externa, AMNH 28653; C and F are of
Pleurostylodon similis, AMNH 28904. All are shown as being from the night side, but the elements of
Pleurostylodon (C and F) and the radius of Anisotemnus (D) are from the left, but digitally reversed to appear
as right. Abbreviations are: ap, anconeal process; cp, coronoid process; and ce, capitular eminence. The
olecranon process of Pleurostylodon (C) is reconstructed.
10 AMERICAN MUSEUM NOVITATES NO. 3601
3cm
Fig. 4. Isotemnid hands: A, left manus of Anisotemnus distentus, AMNH 28906 and B, Pleurostylodon
similis, AMNH 28904 shown as left, but reversed).
most morphologically primitive and earliest terms of its elongated proximal phalanges,
occurring hand known from a notoungulate relative to the short, stout ones of Aniso-
(fig. 4A). The manus of Anisofemnus may be temnus. Neither specimen of Anisotemnus has
summarized as being pentadactyl, having a di- a separate centrale, but the process of the
vergent, but not opposing, Mt I, and with the scaphoid that contacts the magnum likely
other metacarpals not compacted. represents the fusion of this element to the
The manus of Anisotemnus is similar to that scaphoid, as occurs in some specimens of
of the North American Paleocene ungulate Arctocyon, but 1s a separate element in others
Artocyon ferox (= Claenodon ferox in (Matthew, 1937). The lunar in both taxa has
Matthew, 1937) in that both are pentadactyl, a smooth articular surface for the radius that
have relatively short metacarpals, and have extends well over the dorsal face of the
a similar arrangement of the carpals. In each, element, suggesting extreme and frequent
a single distal carpal supports a single meta- wrist extension, as would occur in a planti-
carpal, except for the unciform, which articu- grade manus. Simpon (1936a) reported that
lates with both the Mt IV and V in the same the ungual phalanges of the Casamayoran
plane, having no gross separation of the Mt IV Anisotemnus specimen were fissured; however,
or V facets of the unciform. None of the we have not found material to confirm this.
metacarpals is compacted. In terms of general Fragmentary terminal phalanges, however, do
size and form, the manus of Anisotemnus is appear dorsoventrally compressed, unlike the
a little larger and more robust than that of transversely compressed, clawlike phalanges
Arctocyon, which 1s more gracile, especially in of Arctocyon (Matthew, 1937). As compared