Xenothrix AMNHM

Phee, R. D. E. M & Meldrum, Jeff, 2006, Postcranial Remains of the Extinct Monkeys of the Greater Antilles, with Evidence for Semiterrestriality in Paralouatta, American Museum Novitates 3516, pp. 1-66 : 15-17

Xenothrix AMNHM 268002,

268008 268005, and 268008

The humerus is the best represented long bone in the Xenothrix hypodigm (N 5 6; figs. 5, 6; table 4). In 1991, the only known examples of this element were immature or highly incomplete, and none seemed conclusively primate (as opposed to large rodent). Now that several humeri of the same type have been recovered, including one nearly intact specimen, allocation is no longer in doubt. AMNHM 268008, an adult right humerus lacking only the proximal articular surface ( fig. 5), is obviously primatelike and can be immediately distinguished from that of the Jamaican coney ( Geocapromys brownii ) and most other caviomorphs by the lack of a large, flaring deltopectoral crest. The less complete specimens ( fig. 6A–D) clearly represent the same species. Like AMNHM 268008, AMNHM 268002 was found at Drum Cave, but in another area (Molehole Passage). AMNHM 268005 is an unexpected addition to the hypodigm: it was found serendipitously while examining Jamaican faunal collections housed in the AMNH’s Department of Mammalogy. The specimen was retrieved from a small box containing an assortment of rodent bones and and a note reading ‘‘South Side New Cave, Jamaica —Karl Koopman, Max Hecht, 1950’’. All of the rest of the material in the box was found to belong to Geocapromys . As the location of the cave and its local name are not recorded in any of Koopman’s notes, the only course available is to accept the box’s labeling and identify the site as ‘‘New Cave’’, location currently unknown.

Considering these specimens as a group, among New World monkeys the humerus of Xenothrix mcgregori may be said to have roughly the size and proportions of the equivalent element of a small individual of Cebus apella ( fig. 11A–C). There are, however, notable differences. Compared to the brown capuchin, in X. mcgregori (1) the diaphysis is more robust overall and more concave posteriorly, (2) the deltopectoral eminence is larger and more projecting, (3) the crest for insertion of teres major m. is more projecting, (4) the medial epicondyle is smaller, and (5) the lateral border of the intertubercular sulcus is much less distinct. In many other regards they are similar, such as in (1) the relative size and conformation of proximal and distal articular surfaces, (2) the presence of the entepicondylar foramen, (3) the size and disposition of the tubercles, and (4) the moderate development of the brachialis flange. Several of these contrasts, which hold for other similar-sized platyrrhines such as Chiropotes satanas and Pithecia pithecia ( Fleagle and Meldrum, 1988) , are discussed further under Functional Considerations.

The proximal articular end, not well preserved on AMNHM 268008, is complete in AMNHM 268002 and 268005 ( fig. 6A–D; see also AMNHM 259901 in MacPhee and Fleagle, 1991). In proximal aspect, the articular end appears ovate rather than rounded, much as in Cebus , and in posterior aspect its distal margin appears rather tapered, as in Cacajao and Saimiri . It is also less hemispherical overall (cf. ratio A, table 4) than that of Lagothrix or Brachyteles . The new material also helps to clarify the height relationship between the proximal articular surface and the tubercles (the scapular articulation rises above the level of the tubercles, but only slightly; fig. 6C). The tubercles themselves are as described previously, being moderately prominent and separated by the broad intertubercular groove. These features of the proximal end of the humerus are correlated in other anthropoids with limited shoulder movement outside of the extension–flexion plane (cf. Rose, 1997). The deltopectoral eminence is broad, V-shaped, and somewhat laterally displaced, so that in anterior view the lateral margin of the shaft bows slightly outward ( fig. 5A), as in arboreal quadrupedal primates generally (cf. fig. 11). The specimen is robust for its length (ratios C and D, table 4), and there is no torsion of the head.

The distal articular surface ( fig. 5C) bears a relatively broad capitulum; it is quasispherical and distolaterally extensive, indicating a large range of extension and rotation at the elbow joint ( Meldrum et al., 1990). The capitulum and lateral epicondyle are only moderately rolled ventrally, relative to the long axis of the diaphysis, and the broad, shallow radial fossa is distinctly separated from the coronoid fossa ( fig. 5A). Quadrupedal primates in which the elbow is habitually held in a semiflexed position usually exhibit this complex of features ( Napier and Davis, 1959).

The lateral lip of the trochlea is indistinct in Xenothrix , and the medial lip is likewise only moderately prominent in both distal and anterior directions. A prominent medial lip is characteristic of arboreal quadrupeds such as Saimiri and Cebus , in which it functions to resist adducting torques generated by the digital and carpal flexors when the forearm is pronated during quadrupedal progression ( Jenkins, 1973; Fleagle and Simons 1978). Platyrrhines employing greater frequencies of suspensory and clinging positional behaviors, such as Ateles or Pithecia ( Fleagle and Meldrum, 1988) , exhibit a trochlea that is more cylindrical, with a less prominent medial lip.

The medial epicondyle is relatively short, tapered, and posteriorly (dorsally) directed (,54 °; see table 6). Platyrrhines with posteriorly directed medial epicondyles (e.g., Cebus , fig. 11A–C) are more quadrupedal in behavior than those in which the epicondyle projects directly medially. Functionally, posterior inclination redirects the line of pull of the digital and carpal flexors toward the dorsal trochlea ( Jolly, 1967; Jenkins, 1973).

The olecranon fossa is moderately deep but is unfenestrated, whereas it often is in Cebus apella ( fig. 11A). The depth and shape of the olecranon fossa are determined by the interaction of several factors, primarily the depth of the trochlear notch and the prominence of its posterior border.

As already noted, AMNHM 259906 ( fig. 6E) from Long Mile Cave is clearly subadult because there is evidence that the proximal epiphysis was unfused at the time of death ( MacPhee and Fleagle, 1991). However, the individual must have been close to adulthood because the distal epiphysis is completely fused in the other small humerus of similar size, AMNHM 259905 ( fig. 6F).

Because of the generally poor condition of the Long Mile specimens, it is difficult to define measurements that permit useful size comparisons with the single complete humerus, AMNHM 268008. However, one comparison of relevance can be managed by reference

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(B) aspects; AMNHM 268005 (L [rev.]) in anterior

(C) and proximal (D) aspects; and AMNHM

259906 (L) (E) and AMNHM 259905 (R) (F), both in posterior aspect. E and F after MacPhee and

Fleagle (1991). In B, dashed line indicates presumed original contour of damaged area. To same scale. to the entepicondylar foramen, permitting measurement of ‘‘partial shaft length’’ (edge of proximal epiphyseal line on diaphysis to proximal border of bridge of entepicondylar foramen). Comparing these lengths in AMNHM 259906 and 268008 (table 4) reveals that the partial shaft of the Long Mile humerus is approximately 23% shorter than the comparable region of the Drum Cave specimen, even though the former is presumably fairly close to adult size. This accords with one’s general impression that the Long Mile partial humeri are more gracile than the Drum or New Cave specimens.