Pleurostylodon similis 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 : 13-17

Pleurostylodon similis Ameghino, 1901

Material: AMNH 28904, partial skeleton containing right M1; left forelimb including humerus, ulna, radius, and Mc IV, right Mc II– V, right trapezoid; and both right and left femur and tibia, right navicular, and right Mt I. AMNH 28904 was found at Cañadón Vaca (‘‘Vacan’’ ‘‘subage’’ of the Casamayoran) in 1931 and was included in Simpson’s (1936a, 1967) descriptions of the skeleton of Thomashuxleya .

Description: AMNH 28904 is significantly smaller and morphologically distinct from the elements referred to Anisotemnus and Thomashuxleya above. The M1 associated with the postcranial elements is referable to Pleurostylodon ; thus, we refer AMNH 28904 to P. similis based on the size and morphology of the M1 on the assumption that there is only one species of Pleurostylodon from Cañadón Vaca of that size range ( Simpson, 1967).

Damage to AMNH 28904 precludes a comparative analysis of the proximal end of the humerus, but the shaft and distal end are reasonably complete (fig. 2C). The deltopectoral crests are elevated, and the region of the lateral (deltoid insertion) side is greatly expanded. The medial crest (lesser tubercular) of the shaft is enlarged and expanded medially to a point greater than halfway down the shaft. The distal region is quite distinct from those referred to Anisotemnus and Thomashuxleya , being dorsoventrally compressed, whereas those of Anisotemnus and Thomashuxleya are deeper dorsoventrally (fig. 2). Some or much of this compression must have occurred after death, since the trochlea is too narrow to have articulated properly with the associated ulna. The medial epicondyle is relatively larger than in Anisotemnus and Thomashuxleya . Like Anisotemnus and Thomashuxleya , it has an entepicondylar foramen. The trochlea is shallow, and the medial trochlear flange is only slightly raised above the trochlear groove. The trochlea terminates posteriorly above the broad, shallow olecranon fossa. Solid bone fills the fossa, whereas this region was very thin or perforated in the olecranon fossae of Anisotemnus and Thomashuxleya .

The ulna (fig. 3C) is distinct from those in taxa described above (fig. 3A, B) in its smaller size and straighter olecranon, which lacks the medial curvature present in Anisotemnus and Thomashuxleya , as well as the downward curvature seen in Thomashuxleya . The coronoid process has the same relative height as in the other taxa, but the anconeal process is lower and has broader curvature. The excavation of the shaft is not as great as in Anisotemnus and is nearly lacking at the midshaft area, which is almost quadrate in cross section. The distal end does not taper, but rather becomes slightly enlarged in the dorsoventral dimension. The distal region is too damaged to document any of its characteristics.

The proximal radius (fig. 3F) is similar to that of the Barrancan Pleurostylodon modicus ( AMNH 28878) in having a capitular eminence, neither of which is as sharp and distinctive as that of Anisotemnus . The radius referred to P. similus is distinct, however, in that the proximal ulnar facet is conspicuously large and smooth and grades into a ridge connecting the articular circumference of the head to the bicipital tuberosity.

The manus (fig. 4B) is incomplete but includes Mc II– V and a trapezoid. One cannot conclusively say the hand was pentadactyl, since no Mc I was recovered, but an Mt I is preserved, suggesting a pentadactyl foot. The metacarpals are similar to those of Anisotemnus , but they are generally more gracile. Also, the magnum–Mc III joint appears to differ from that of Anisotemnus (and most notoungulates) since it lies on the same plane as the cuneiform–Mc IV joint, whereas the carpo-metacarpal joint of the third digit is oblique and more proximally positioned than that of digit IV.

Of foot elements, the navicular and Mt I are preserved in this specimen. The navicular is nearly identical to the one described below for the indeterminate isotemnid, except that it lacks the distinctive entocuneiform facet that forms the medial border of that element. The proximal portion of Mt I is larger, but it is otherwise identical to that of the small, indeterminate isotemnid described below.

Indeterminate Isotemnid Taxon (Possibly Near P. similis )

Material: AMNH 28690, left hindlimb bones including proximal femur, much of the tibia and fibula, tarsals (lacking only the distal portion of the calcaneum), Mt I, Mt II, and another distal metatarsus (Mt III?). Collected by G.G. Simpson near ‘‘Oficina del Diablo’’, Cañadón Vaca (‘‘Vacan’’ ‘‘subage’’ of the Casamayoran); AMNH 142462, left calcaneum, also from Cañadón Vaca.

Note: AMNH 28690 has postcranial elements smaller than homologous elements of Pleurostylodon similis described above (e.g., the total length of the tibia in this indeterminate taxon is about 90% that of AMNH 28635 [table 1]). It may represent a small individual of P. similis or a different, but unknown, related species. It is probably too large and morphologically distinct to represent any of the notostylopids or oldfieldthomasiids from Cañadón Vaca. The morphologically distinctive (and likely apomorphic) Mt I is close in form to that of P. similis , suggesting a close (or conspecific) affinity.

Description: Only the proximal end of the femur was recovered. This is similar to AMNH 28635 except that it is smaller and the third trochanter is a little more distally placed. Its greater trochanter is just slightly higher than the head, which is well defined by the constriction behind it. The head is directed about 45 ° from the long axis of the shaft. The lesser trochanter projects from the shaft, beginning about 1 cm below the head and continuing distally for about 2 cm. On the lateral surface, the base of the third trochanter projects near the level where the lesser terminated; however, the body of the third trochanter itself is broken.

A nearly complete pes is preserved (fig. 6). Although Mt I and II are the only complete metatarsals preserved, the morphology is sufficient to confidently infer that the foot was pentadactyl and had serially arranged tarsals that would have been most often positioned in a plantigrade stance during life.

The astragalus is nearly identical to that described and figured by Ameghino (1904: fig. 29) and referred only to the Isotemnidae (‘‘ Isotemnidae indeterminado’’, p. 259). It differs from that referred to Pleurostylodon biconus (5 P. modicus) by Ameghino (1904: fig. 30) in the relatively smaller neck and head and relatively larger body. The body of the AMNH 28690 astragalus has a shallow trochlea that is slightly asymmetric, with the lateral ridge being a little higher than the medial. The smooth articular surface of the trochlea extends over the medial surface, demarcating the articulation with the tibial malleolus. A conspicuous superior astragalar foramen is present at the mid-proximodorsal region of the trochlea. Although the foramen is distinct from the smooth articular surface of the trochlea, it is engulfed on either side by dorsal extensions of the articular region. The articular surface for the medial tibia extends beyond the foramen such that it grades into the medial ridge of the distinctive groove for the flexor hallucis longus. The lateral trochlear ridge terminates in a manner that leaves a gap between it and the lateral wall of the flexor groove. The superior astragalar foramen extends through the body, exiting within the proximal region of the astragalar sulcus (the deep groove of the plantar surface, between the sustentacular and ectal facets). The long axes of these calcaneal facets nearly parallel one another. The plane of the ectal facet has a somewhat oblique orientation, such that its surface can be clearly viewed from both plantar and lateral perspectives.

The neck of the astragalus is constricted behind the head, but it is not very long. On the dorsal surface of the neck lies an oblique ridge that serves as a tibial stop. The head is subspherical, but appears oval-shaped in distal view, with the lateral dimension being about 45% greater than the dorsoplantar.

The astragalus of AMNH 28690 is similar to that of the oldfieldthomasiid Colbertia magellanica from the late Paleocene of Itaboraí described by Cifelli (1983). It differs in its larger size, slightly more transversely convex trochlea, deeper and longer digital flexor groove, and blunter medial process. The region of the head of Colbertia that Cifelli (1983) identified as the facet for the collateral ligament does not function as such in this Vacan specimen. Instead, it articulates with the proximally extended medial process of the navicular (see fig. 6A). This process is likely a fused ‘‘tibiale’’ as seen at the medial head of the astragalus of Arctocyon ferox (5 Claenodon corrugatus in the description of Matthew [1937]).

The distal region of the calcaneum of AMNH 28690 is missing, but most of the element is well preserved and indistinguishable from AMNH 142464. In general appearance the calcaneum is relatively long, having a distal border that is oblique in dorsal view, with the lateral side extending farther than the medial, which grades into a broad sustentaculum. The sustentacular facet is teardrop-shaped, the apex of which is directed toward and forms a sharp border with the cuboid facet. A groove on the plantar side of the sustentaculum indicates the passage of the tendon of the flexor hallucis longus. An oblique sulcus separates the sustentacular facet from the ectal facet, the latter of which lies obliquely above a dorsal prominence rising above the tuber, midway along the proximodistal line. The ectal facet is convex, as it wraps around the lateral surface of the prominence. Neither specimen shows clear evidence of a fibular facet, but both have some abrasion where limited articulation may have occurred. The distal region of the fibula suggests a weak articulation with the calcaneum. Just proximal to the middorsal calcaneal prominence is an elongated fossa, which lies dorsal to a crest that grades into the broad peroneal shelf. A blunt peroneal process, defined by a tendonal groove, forms the distolateral border of the calcaneum. The cuboid facet is teardrop-shaped with its apex directed to the medial side such that it underlies the apex of the sustentacular facet, which lies at a near right angle to the cuboid facet.

The calcaneum is similar to that of the oldfieldthomasiid Colbertia ( Cifelli, 1983) . Differences include its larger size, the small or absent fibular facet (that of Colbertia is well defined, and covers both the distal and proximal sides of the prominence), and the peroneal shelf being larger, giving the distal astragalus a broader appearance.

The cuboid is unremarkable other than that its distal articular facet is much broader than those of the cuneiforms, being nearly twice the width of the ectocuneiform. This strongly suggests that it articulated with two metatarsals: Mt IV and V.

The navicular has a deep, circular concavity that serves as the socket of the ball-and-socket joint of the astragalonavicular joint. It has a well-developed medial process, which is most distinctive in having an elongated facet for the entocuneiform.

The entocuneiform is the largest and most remarkable of the three cuneiforms. Its anteroposterior dimension is nearly twice that of the ectocuneiform. Its medial surface articulates with both the mesocuneiform and the navicular. The facet for Mt I is oblique to the metatarsal facets of the other cuneiforms and the cuboid, resulting in a diverging hallux. In medial oblique view this facet is concave, but it is somewhat convex in its other dimension. The tip of the distodorsal apex is broken and missing.

TABLE 1 Measurements of Elements of the Appendicular Skeleton of Selected Isotemnids 1

TABLE 1 (Continued)

Mt I is fairly short, but its proximal surface shows that it was not a vestigial element. It has an elongated articular surface, convex in the anteroposterior dimension and transversely concave, with the relative degrees of these curvatures clearly indicating that most of the action at the tarsometatarsal joint was in the parasagittal plane.

The proximal Mt II has the typically grooved proximal facet for the mesocuneiform. Its articulation with the ectocuneiform appears on a small lateral process of the Mt II that physically separated the shaft of Mt II from Mt III (fig. 6A), but it does not overlap any portion of the ectocuneiform.

None of the remaining metacarpals was recovered, except for a distal region of one of these (either Mt III or IV). However, the large size of the distal articular surface of the cuboid indicates that it must have supported more than a single metatarsal, indicating the probable presence of an Mt V, and therefore the pentadactyl state of the foot.

Compared to the pes of Arctocyon , that of the indeterminate isotemnid is smaller, has relatively and absolutely shorter and thinner metatarsals, a smaller mesocuneiform, a larger entocuneiform, and no separate tibiale, but it has an enlarged process of the navicular that may represent fusion of a tibiale to the navicular. The astragalus of the isotemnid is less cylindrical about the trochlea (flatter) and has a distinctive medial process lacking in Arctocyon , an oblique crest (‘‘tibial stop’’) on the dorsal side of the neck, a more distinctive flexor groove, and a medial groove from the superior astragalar foramen to the proximomedial surface of the element. The calcaneum of the indeterminate isotemnid is narrower (mediolaterally compressed) and has a less robust peroneal process and a more oblique articulation with the cuboid.