Paralouatta

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 : 18-25

Paralouatta MNHNCu 76.1010–

76.1024 76.1013,, and 76.1024 76.1035, and 76.1035

Although the humerus is also well represented in the Paralouatta postcranial collection (N 5 6; figs. 7–10; table 5), only two of the specimens are appreciably complete. MNHNCu 76.1010 is very nearly entire, although there are losses to the head, tubercles, and the lateral part of the distal epiphysis (capitulum and lateral epicondyle). MNHNCu 76.1035 is missing its shaft proximal to the level of the deltopectoral tuberosity as well as the lateral part of the distal epiphysis. MNHNCu 76.1011 is much smaller in all dimensions and is judged to be juvenile. The other accessioned specimens are more fragmentary, but in some cases they provide useful information on parts not represented in the two more complete specimens. All are figured here save for MNHNCu 76.1024, a badly damaged distal articular end.

The humerus of Paralouatta is not simply a relatively longer version of that of Xenothrix : the diaphysis is less posteriorly concave, and for its length it is more gracile (cf. ratios C and D, tables 4 and 5). It also displays marked contrasts in the conformation of the distal articular end (see Functional Considerations).

Although MNHNCu 76.1035 gives the impression of being considerably more robust than MNHNCu 76.1010, this is mainly due to the size of its medial and lateral supracondylar ridges. Midshaft circumference, which is taken above the termination of these lines, is very similar in the two specimens, as is minimum shaft width (table 5). The deltopectoral eminence ends in both examples in a distinctive ‘‘V’’ shape, presumably defined by the line of origin of the medial head of triceps m. Compared to MNHNCu 76.1010, the shaft of MNHNCu 76.1035 is relatively flatter above the level of the olecranon fossa, and the medial epicondyle is more prominent and less abraded.

The one striking contrast between these specimens that requires comment is the entepicondylar foramen, present in MNHNCu 76.1010 but not MNHNCu 76.1035. It is also clearly present in the diaphyseal specimen, MNHNCu 76.1011 ( fig. 9C, D), and a slight depression just above the shallow coronoid fossa in MNHNCu 76.1013 ( fig. 10A) hints at an additional occurrence. According to Fleagle (1983), the entepicondylar foramen is found in about half of the extinct platyrrhine taxa for which the humerus is known. Among living platyrrhines foramen presence can be variable even within species (e.g., Aotus , Callicebus , and Alouatta ; J. Meldrum, unpubl. obs.; consistently absent in Callithrix and Leontopithecus but not Saguinus ; Ford, 1986a, 1986b; Meldrum, 1990). In view of the overarching similarity of all of the humerus specimens from the monkey caves (and, not incidentally, the teeth; Horovitz and MacPhee, 1999), we conclude that, like several extant platyrrhine species, P. varonai was variable for entepicondylar foramen development, and that only one kind of primate is represented in the hypodigm.

Among humeri in the comparative set, MNHNCu 76.1010 is approximately as long as that of Alouatta seniculus and Trachypithecus pileatus ( fig. 11D–F, G–I). It resembles these taxa in having a comparatively straight shaft, but is more like Erythrocebus and Theropithecus ( fig. 11M–O, P–R) in exhibiting a ML narrow distal epiphysis. The overall appearance of the posterior aspect of the distal humerus is also somewhat cercopithecidlike in the lack of medial projection of the medial epicondyle, the proportions of the posterior extension of the trochlear surface, and the depth of the lateral margin of the olecranon fossa. The capitulum is wider (in anterior aspect) than the trochlea, although this is hard to measure accurately because the waist defining their respective limits is indistinct on these specimens ( fig. 10A). In Alouatta ( fig. 11E) and other large-bodied atelids, the posteromedial edge of the trochlea ends in a long medial lip that is aimed medially more than ventrally. In Trachypithecus , Lophocebus , Erythrocebus , and Theropithecus the homologous area is directed inferiorly ( fig. 11H, K, N, Q). In Paralouatta conditions are intermediate ( fig. 10A).

TABLE 5

Humerus: Paralouatta varonai, Dimensions (in mm) and Ratios

a,b

The olecranon fossa is notably deep in the two most complete specimens. The floor of the fossa is intact in MNHNCu 76.1010 but perforated in MNHNCu 76.1035 (which affects measurement). Whether the perforation is natural or due to postmortem breakage is difficult to say. Under high magnification, involved edges are not smooth and show a thin layer of cancellous tissue sandwiched between compact bone surfaces, suggesting that material has spalled off.

Functional Considerations

RELATIVE MEDIAL EPICONDYLE SIZE AND ORIENTATION: In both Xenothrix and Paralouatta the medial epicondyle is posteriorly directed or retroflexed, as may be clearly seen in the least damaged specimens (cf. fig. 5C, 10C). Fleagle and Simons (1978: 705) pointed out that medial rotatory torques on the radiohumeral and ulnohumeral joints are increased if the medial epicondyle projects directly medially (as in Alouatta and Lagothrix ) rather than posteriorly (as in Cebus and Old World monkeys). They explain retroflexion of the medial epicondyle as an adaptation ‘‘enhancing the actions of the pronators and flexor muscles when the elbow is already pronated as in pronograde quadrupedalism.’’

Only the distal epiphysis MNHNCu 76.1013 retains an unabraded medial epicondyle ( fig. 10); in the two more complete humeri the epiphysis is damaged. Nevertheless, it is clear in all three specimens that the epicondyle’s mass is directed posteriorly, quite unlike the situation in the majority of platyrrhines (including all New World species of large BM) in which the epicondyle juts almost directly medially. The size of the medial epicondyle relative to the maximum (biepicondylar) width of the distal end of the bone can be expressed as a ratio (MEPR) by dividing maximum distal width by adjusted distal width (i.e., by deducting the medial epicondyle’s contribution). The higher the ratio, the less the medial epicondyle contributes to total width. In the case of our comparative set, MEPR is of limited value because it does not provide good discrimination even between suspensory and terrestrial species (table 6). The ratio is affected by the size of other features, especially the lateral epicondyle. Among measured taxa, xenotrichines display some of the highest values because of the limited contribution of the medial epicondyle to distal width. However, Brachyteles and Lagothrix are not far behind, despite the fact that they have relatively large, flaring medial epicondyles.

Posterior deviation (retroflexion) of the medial epicondyle (MEPD; table 6) is a more subjective measurement, due to the difficulty of deciding where the center of the epicondyle is located in distal aspect. There is variation within locomotor groupings, although some interesting consistencies can be identified. Large-bodied Old World species that spend significant periods on the ground tend to have very large deviations (.80 °), up to an extreme of,100 ° in the case of Theropithecus . Smallerbodied forms, however, do not show so great a degree of deviation. Gebo and Sargis (1994) did not mention medial epicondyle retroflexion as an informative character in their ecomorphological evaluations of Cercopithecus mitis and C. lhoesti .

New World suspensory species display the opposite trend in MEPD, from 29 ° in Brachyteles arachnoides to 0 ° in Ateles belzebuth (although both values are based on single specimens). Small arboreal quadrupeds ( Aotus infulatus , Callicebus moloch ) also show little medial epicondyle deviation, 9–22 °. The largest posterior deviation among living platyrrhines—51 ° —occurs in Cebus apella (range, 46–54 °). This converges on the single value for Xenothrix (AMNHM 268008, 54 °), but does not overlap with Paralouatta (65–68 °, based on the two least abraded examples).

In summary, with respect to medial epicondyle retroflexion, although Xenothrix and Paralouatta do not differ radically from generalized platyrrhine arboreal quadrupeds like Cebus , at the same time they also show some similarity to cercopithecids who spend considerable time on the ground or in lower tree branches.

TROCHLEAR ORGANIZATION: As noted, the capitulum is separated from the trochlea by an indistinct waist, the lateral margin of the trochlea, in Xenothrix and Paralouatta ( figs. 5, 10). According to Gebo and Sargis (1994), the lateral margin is very pronounced in highly arboreal Cercopithecus mitis but not in more terrestrial C. lhoesti . However, in their figures the contrast appears less marked than their description indicates. In any case, this margin is very strongly modeled in Theropithecus ( fig. 11M–O) and is therefore by itself not a strong correlate of arboreality. A more significant functional indicator is the short ML width of the trochlea in Paralouatta , which is relatively much shorter than in Xenothrix (ratio B in tables 4, 5) and also large-bodied atelids such as Alouatta ( fig. 11D, F). Cercopithecus lhoesti and C. mitis contrast in the same manner, as Gebo and Sargis (1994) noted. The narrow trochlea is important functionally because it tends to constrain the motion of the forearm on the arm to fore/ aft movements, with little or no ulnar deviation (cf. Theropithecus , fig. 11M, O).

ULNA AND RADIUS