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 : 25-27

Xenothrix AMNHM 268010

The ulna from Somerville Cave, AMNHM 268010 ( fig. 12; table 7), agrees in detail with AMNHM 259907, the immature specimen from Long Mile Cave described by MacPhee and Fleagle (1991: their fig. 14). Also, the trochlear notch of AMNHM 268010 articulates reasonably well with the complete humerus AMNHM 268008, which helps to corroborate the view that they belong to the same species. Because breakage occurs in approximately the same place in both ulnae, the new find adds little to what is already known. However, because AMNHM 268010 is from an adult and is less abraded than earlier finds it provides a better sense of the size and disposition of proximal joint surfaces and muscle markings. (In fig. 12, note prominent scars for supinator, brachialis, and flexor digitorum superficialis mm.)

It is difficult to estimate the degree of shaft curvature in incomplete ulnae. In the case of Xenothrix AMNHM 268010, the remaining proximal section of the posterior border ( fig. 12B) is straight, but distally there is the beginning of a slight curvature, probably much as in Cebus apella ( fig. 14B). It appears that the epiphysis of the triceps process of Xenothrix was small and tablike: in the new fossil there is no visible metaphyseal ‘‘line’’ in the cancellous bone fortuitously exposed by breakage on the medial side of the olecranon region ( fig. 12B). Posteromedial slant of the process, which is characteristic of both arboreal and terrestrial Old World monkeys (to differing degrees) is not indicated ( fig. 14I, J, M; table 7). A deep trochlear notch, with a sharply projecting coronoid as seen in Xenothrix , is characteristic of primates that emphasize quadrupedalism ( Knussman, 1967) (cf. Cebus apella , fig. 14B).

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The nonarticular areas on the trochlear surface are well marked in AMNHM 268010 ( fig. 12C): They extend onto the central part of the notch medially and especially laterally, but do not extend across the notch’s whole face. The radial notch is ‘‘inset’’ on the lateral aspect of the bone, being almost flush with it and extending onto and even slightly under the coronoid process. This feature, which has an interesting distribution, is discussed further below.

The radius of Xenothrix is not yet known.