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 : 27-30

Paralouatta MNHNCu 76.1014, 76.1016,

and 76.1017

The ulnae in the hypodigm are highly incomplete ( fig. 13A–I). Even so, the size contrast between both MNHNCu 76.1014 and 76.1017 on the one hand and MNHNCu 76.1016 on the other is quite noticeable, the former being larger for given linear measurements by,20–25%. MNHNCu 76.1014 and 76.1017 are from opposite sides of the body, but in size they are very similar and quite possibly represent the same animal.

Although the posterior border of the ulna was evidently rather straight in its upper moiety, as in Xenothrix , too little is left of the shaft to ascertain whether the distal section was straight or bowed. As in Xenothrix , muscle scars around the coronoid process are well marked. Of interest is the fact that the supinator crest is displaced relatively posteriorly, as in Cebus , Lophocebus , and Theropithecus , but unlike Alouatta (or other large-bodied platyrrhines) in which the indistinct supinator crest lies closer to the interosseous crest ( fig. 14).

The preserved part of the shaft in MNHNCu 76.1016 and 1017 is anteroposteriorly as wide as, or wider than, in other largebodied platyrrhines such as Alouatta ( fig. 14D– F; table 7) and Brachyteles . Mediolaterally it is compressed into a knife edge, whereas in other large-bodied platyrrhines the shaft tends to be more rounded in this dimension. This feature is possibly correlated with the extensive fossa beneath the radial notch, which recalls a similar feature seen in Macaca and some other cercopithecids in the comparative set (e.g., Erythrocebus , Theropithecus ; fig. 14J–M). In these last taxa, the fossa is needed in order to give free passage to the bicipital tuberosity during movements of supination/pronation, because the tuberosity is located relatively higher on the shaft than in large-bodied platyrrhines. Xenothrix lacks this feature. In Lagothrix the ulna is compressed in this region, but it too lacks the fossa.

MNHNCu 76.1014 ( fig. 13A) provides evidence that the triceps process of Paralouatta was not especially large and was built as in other platyrrhines (i.e., the process was not posteromedially retroflexed as in some cercopithecids; cf. fig. 14L).

The proximal articular areas are incomplete to some degree in all Paralouatta specimens. The trochlear notch is moderately deep, intermediate between the conditions seen in suspensory platyrrhines and terrestrial cercopithecids. As in Xenothrix , the triceps process is not medially twisted or deviated. The disposition of the nonarticular areas within the trochlear notch are hard to interpret because of damage to joint surfaces. In MNHNCu 76.1016, however, on the lateral side there is evidence that the nonarticular surface penetrated rather far into the notch. It is clear that the radial articular facet was relatively ‘‘inset’’ on the lateral aspect of the ulna, as in Xenothrix (cf. figs. 12, 13). In this feature, both xenotrichins agree with other platyrrhines, and in particular with the largebodied taxa in which the elbow joints emphasize mobility (e.g., Alouatta , fig. 14F).

In Old World monkeys ( fig. 14I, J, M), even in highly arboreal Colobus , the radial articular facet is more ‘‘outset,’’ projecting at a sharp angle out of the plane of the shaft’s lateral aspect and thereby imparting a strong curve to the radial notch ( Fleagle, 1983). In the comparative set, projection reaches a maximum in Theropithecus , Erythrocebus , and Macaca , in which the radial notch is divided in two by an intervening nonarticular area (e.g., fig. 14K). The ‘‘outset’’ condition and presence of a subdivided radial facet are evident in many grounddwelling cercopithecids. In Trachypithecus pileatus and Colobus guereza there is only one radial facet, but in Cercocebus galeritas there are two, practically conjoined, whereas in Lophocebus albigena there are clearly two and they are quite separate ( fig. 14G). This variation implies that the subdivided radial notch is not completely diagnostic of preferred substrate. The fact that platyrrhines are all quite similar for the ‘‘inset’’ version of this feature suggests that it is not strongly correlated with any single locomotor style.

The radius (MNHNCu 76.1015; fig. 15; table 8) attributed to Paralouatta consists only of the proximal end and a small portion of the shaft, and extensive interpretation or comparison is not warranted. Most of the articular circumference is chipped away, but there is no reason to think that it was not extensive, as in primates generally. The short neck and rather high positioning of the long, contoured bicipital tuberosity strongly recalls radii of some Old World monkeys, such as Lophocebus and Erythrocebus .

Functional Considerations

OLECRANON SURFACE OF TROCHLEAR NOTCH: In many primates, including Xenothrix , the upper or olecranon surface of the trochlear notch (OSTN) is saddle shaped, with medial and lateral wings flaring from it to a roughly equal extent. In large-bodied platyrrhines with wide but shallow olecranon fossae (e.g., Alouatta , fig. 14D–F), the medial and lateral surfaces of the OSTN have sharp, nonreflected margins without facets, because nowhere along the ulna’s range of motion do they impinge on the low borders of the humeral trochlea/olecranon fossa.

By contrast, in Erythrocebus ( fig. 14J) and Theropithecus ( fig. 14K–M), the lateral ‘‘wing’’ does not really exist as such. This side of the OSTN is molded instead into a flat, crescentic facet on the surface of the triceps process, and throughout the range of motion it remains in contact with the large, continuous lip formed by the lateral margins of the trochlea and the olecranon fossa. In Theropithecus , both lateral and medial wings of the OSTN are abbreviated to mesh with the posteriorly narrowing trochlea. In Erythrocebus only the lateral wing is abbreviated; its short external margin is reflected and turned upward to articulate with the lateral lip, which is, in effect, a continuation of the trochlear surface into the olecranon fossa. Together, these surfaces increase the articular area of the ulnohumeral joint, and by their conformation provide stability against laterally directed forces when the arm is extended.

Paralouatta appears to have had a somewhat intermediate arrangement. In MNHNCu 76.1016, which is well preserved in the relevant area, there is no lateral shelf of the OSTN; the articular area is flush with and continues onto the triceps process (asterisk, fig. 13D), more or less as in Theropithecus or Erythrocebus . In MNHNCu 76.1014 most of the relevant area is damaged, although it is possible to identify the distal part of the reflected facet ( fig. 13A). Unfortunately, the condylar portions of the humeri MNHNCu 76.1010 and 1035 are incomplete laterally to about the same degree ( figs. 7, 8), although it is possible to see that the well-defined lateral margin of the trochlea bears a short, striplike continuation of the articular surface onto the sidewall of the olecranon fossa. In Alouatta , by contrast, the indistinct lateral margins of the trochlea and the olecranon fossa are only weakly connected ( fig. 11E). These comments apply in equal measure to Ateles , Brachyteles , and Lagothrix and all other platyrrhines in the comparative set.

Ratio A in table 7 compares the height of the trochlear notch to its width across the OSTN section. Theropithecus and Erythrocebus have very narrow OSTN sections and correspondingly low ratios (0.65, 0.71). Highly arboreal New World monkeys, wheth- er predominantly quadrupedal or suspensory, have flaring medial and lateral OSTN wings and ratios above 0.80. (This is also true of Colobus guereza , data not shown). Paralouatta has a relatively low value (0.76) in the table, but only one specimen could be measured for both width and height and it is somewhat damaged. However, it is unlikely that in an

+

Paralouatta (C). On the lateral side there is instead a narrow facet (single asterisk in A and D), flush with the sidewall of the triceps process, which articulates with the continuous crest formed by the lateral margins of the trochlea and olecranon fossa. Double asterisks in D and F indicate position of large fossa on shaft distal to radial notch. To same scale.

209927 (A–C); Alouatta seniculus AMNHM 23377 (D–F).

intact specimen the ratio would have exceeded 0.80 significantly. Interestingly, Xenothrix has a very high ratio (1.01), higher than any other member of the comparative set except Ateles belzebuth (1.44).

OS COXAE