Anisotemnus distentus ( Ameghino, 1901 )

SHOCKEY, BRUCE J. & FLYNN, JOHN J., 2007, Morphological Diversity in the Postcranial Skeleton of Casamayoran (? Middle to Late Eocene) Notoungulata and Foot Posture in Notoungulates, American Museum Novitates 3601, pp. 1-28 : 5-10

Anisotemnus distentus ( Ameghino, 1901)

Originally, Ameghino described this as a species of Isotemnus , but he later ( Ameghino, 1902) decided it pertained elsewhere, creating the new generic name Anisotemnus (‘‘not Isotemnus ’’) for it. Simpson (1967) recognized A. distentus as a valid taxon and provided a generic diagnosis, but he confessed that the genus ‘‘did not seem to differ in any readily diagnostic way from Thomashuxleya …or Pleurostylodon ’’ ( Simpson, 1967: 137). Nevertheless, its intermediate size between species referred to Thomashuxleya and those to Pleurostylodon makes A. distentus readily distinguishable, since it is the only Casamayoran isotemnid of this size.

In his initial description of the partial skeleton here referred to Anisotemnus, Simpson (1938) thought that it might represent a small species of Thomashuxleya . One of the few editorial changes Simpson (1967) made relative to his 1938 account of the idealized or composite isotemnid skeleton was his suggestion that his ‘‘specimen B’’ (AMNH 28906) might be referable to Anisotemnus .

Anisotemnus , however, had not been reported previously from Cañadón Vaca. Simpson (1967) did not include any Cañadón Vaca specimens in the hypodigm of Anisotemnus , and Cifelli (1985) did not list this taxon from Cañadón Vaca in his faunal analysis. We note, however, two lower molars from Cañadón Vaca in the AMNH collection that are referable to Anisotemnus . AMNH 28648 is an m3 having mesial-distal and transverse dimensions of 27.9 and 14.5 mm, respectively. These are smaller than any of the m3s of Thomashuxleya and larger than those of Pleurostylodon , but they are of similar dimensions to Anisotemnus specimens reported by Simpon (1967) and data obtained in the present study. Given this dental evidence of the occurrence of Anisotemnus at Cañadón Vaca, combined with the presence of an isotemnid skeleton intermediate in size between that of Thomashuxleya externa and Pleurostylodon similis , we refer this suite of materials below to Anisotemnus distentus .

Material: All specimens referred here to Anisotemnus are from Cañadón Vaca (‘‘Vacan’’ ‘‘subage’’ of the Casamayoran) and were collected by G.G. Simpson and crew in the austral summer of 1930–1931: AMNH 28648, left jaw fragment with erupted but unworn m3; AMNH 28664, heavily worn left p4 or m1; AMNH 28906, partial skeleton including the right scapula, right and left humerus, ulna, radius, and manus, as well as much of the axial skeleton; AMNH 28647, distal left radius, scaphoid, lunar, trapezoid, Mc I– V (distal end of Mc V missing), two indeterminate phalanges, and some sesamoids.

Description: The scapula of Anisotemnus (fig. 1) is similar to that of Thomashuxleya externa in general form and in details of the acromion. The scapular blade is ovoid, rather than triangular, with the lateral border being fairly straight, whereas the anterior border is strongly convex. The supraspinous fossa is much larger than the infraspinous fossa, with about twice the surface area. The spine is badly damaged, but the region near the glenoid is preserved well enough to show that it was well elevated above the blade. The acromial angle is developed into a distinct process, giving the acromion a forked appearance similar to that seen in Santacrucian interatheriids ( Sinclair, 1909). The acromion, however, extends farther beyond the glenoid border in Anisotemnus than in the Santacrucian interatheriids. The spine is too damaged to determine if there were any other metacromia (e.g., as occurs in the Santacrucian toxodontids Nesodon and Adinotherium , which have two [see Scott, 1912a: pls. XXII.1, XXVII.8]). The glenoid is teardrop-shaped, with the apex directed toward the well-developed and recurved coracoid process.

The right humerus of Anisotemnus is extremely well preserved (fig. 2). The greater tubercle is broad but not exceedingly high. The greater tubercle and the large, bulbous lesser tubercle together form a well-defined bicipital groove. The crest of the lesser tubercle forms a sharply defined, raised posteromedial lip of the shaft that terminates at a raised tuberosity, the likely site of the teres major muscle insertion. Lateral (deltoid) and medial (pectoral) crests unite at a point more than halfway down the shaft. The supinator crest is bladelike and conspicuous. An entepicondylar foramen pierces the moderately developed medial epicondylar process. A common radial-ulnar fossa occurs just proximal to the capitulum.

The elbow articular region of the humerus has a distinctive, subspherical capitulum having a long axis that lies in a dorsoventral direction, suggesting parasagittal rather than rotary movements. The trochlear groove is deep, being bordered by a large, discoid medial flange. Whereas only a medial crest buttresses the ulna on the anterior side, both medial and lateral crests form a deep groove for the ulna posteriorly. The olecranon fossa is deep; however, the perforation appears to be due to breakage. Nevertheless, little bone separated the olecranon fossa from the antebrachial fossa of the anterior surface.

As in the antebrachia of all known notoungulates, the ulna and radius are separate elements (fig. 3). The ulnar shaft is strongly excavated on the lateral side, such that the midshaft region forms a ‘‘C’’ in cross section. The olecranon is fairly long and somewhat inturned, although not to the extent seen in the Deseadan mesotheriid Trachytherus ( Shockey et al., 2007) . The olecranon process is excavated on the medial side, forming a cavity that presumably was occupied by digital and wrist flexor muscles in life. The trochlear notch forms a broad crescent, terminating in a modestly tall coronoid process, which appears somewhat asymmetric due to the large size of the radial notch. The shaft does not taper distally, but instead there is some broadening on the distomedial surface, formed by what appears to be a pronator crest. The distal region of the ulna is damaged, except the styloid process, which is preserved in one of the ulnae.

As Simpson (1936a) noted, the radial shaft is curved, although curvature of the right side may be exaggerated by postmortem damage. In addition to being less circular than the proximal radius of Homalodotherium , as Simpson (1936a) observed, a conspicuous capitular eminence (fig. 3D) defines a notch on the dorsal (pronated) side of the proximal radius that, in anatomical position, wraps around the dorsoventrally oriented capitulum of the humerus. This form provides both precision of movement and a mechanical restraint against extreme supination. In addition to the articular surface on the ventral side of the proximal end of the radius (the ulnar facet), there is a smooth surface of the dorsolateral circumradial region suggestive of an articular facet. This facet has been directly observed in various notoungulates in which there is an articulation of the proximal radius with an elbow sesamoid, such as in the basal toxodontid Nesodon ( Scott, 1912a) and the mesotheriid typotheres Trachytherus and Plesiotypotherium ( Shockey et al., 2007) . The radial tuberosity is somewhat elongated, commencing near the proximal ulnar facet and extending about 3 cm down the shaft, with its ridge forming a double crescent. The width of the radial shaft increases distally where a bladelike extension on the dorso-ulnar side overlies a triangular articulation for the distal ulna. The diaphysis and epiphysis are fused, but their border is still evident at the distal radius. The epiphysis supports a single, longitudinal dorsal tubercle that terminates at the radiocarpal joint near the articulation for the scaphoid and lunar.

Simpson’s (1936a) description of the isotemnid manus was based on AMNH 28906 ( Anisotemnus distentus ), since the manus of his ‘‘specimen A’’ ( Thomashuxleya ) was badly damaged. Details are not repeated here, but its major features are summarized. Also, whereas Simpson compared this manus to that of Homalodolotherium, we compare it to that of a putative sister taxon of Notoungulata , Arctocyon ( AMNH 16543), since that of Anisotemnus appears to be the most morphologically primitive and earliest occurring hand known from a notoungulate (fig. 4A). The manus of Anisotemnus may be summarized as being pentadactyl, having a divergent, but not opposing, Mt I, and with the other metacarpals not compacted.

The manus of Anisotemnus is similar to that of the North American Paleocene ungulate Artocyon ferox (5 Claenodon ferox in Matthew, 1937) in that both are pentadactyl, have relatively short metacarpals, and have a similar arrangement of the carpals. In each, a single distal carpal supports a single metacarpal, except for the unciform, which articulates with both the Mt IV and V in the same plane, having no gross separation of the Mt IV or V facets of the unciform. None of the metacarpals is compacted. In terms of general size and form, the manus of Anisotemnus is a little larger and more robust than that of Arctocyon , which is more gracile, especially in terms of its elongated proximal phalanges, relative to the short, stout ones of Anisotemnus . Neither specimen of Anisotemnus has a separate centrale, but the process of the scaphoid that contacts the magnum likely represents the fusion of this element to the scaphoid, as occurs in some specimens of Arctocyon , but is a separate element in others ( Matthew, 1937). The lunar in both taxa has a smooth articular surface for the radius that extends well over the dorsal face of the element, suggesting extreme and frequent wrist extension, as would occur in a plantigrade manus. Simpon (1936a) reported that the ungual phalanges of the Casamayoran Anisotemnus specimen were fissured; however, we have not found material to confirm this. Fragmentary terminal phalanges, however, do appear dorsoventrally compressed, unlike the transversely compressed, clawlike phalanges of Arctocyon ( Matthew, 1937) . As compared in more detail below, the manus of Anisotemnus is more similar to that of the Mustersan Periphragnis than to that of the Vacan Pleurostylodon similis (see fig. 5A, B).