Micromomyidae

Micromomyids are by far the smallest (30-40 g) plesiadapiforms for which postcrania are known. Postcranial specimens are late Clarkforkian to middle

Wasatchian in age. They represent three genera: Chalicomomys, a Chalicomomys-like new genus, and Tinimomys. Taking all of these specimens into consideration reveals the morphology and inter-element proportions of almost the entire skeleton (Figure 7). While no specific features seem to detract from the ability of these animals to use vertical postures in the man-

Figure 7. Reconstruction of a micromomyid on a large-diameter support. Features it shares with other plesiadapiforms described here support such a posture. Gray areas depict bones present in one specimen (UM 41870). Note that the posterior three vertebrae and a proximal ulna are not depicted in either Figure 2A or B. These were recently recovered from a block discovered to have broken off from the main limestone (block 821419 from SC-327) early in the preparation process. The association was confirmed by a connection between the broken ulnar shaft and the proximal end of the left ulna. Scale = 3 cm.

Figure 7. Reconstruction of a micromomyid on a large-diameter support. Features it shares with other plesiadapiforms described here support such a posture. Gray areas depict bones present in one specimen (UM 41870). Note that the posterior three vertebrae and a proximal ulna are not depicted in either Figure 2A or B. These were recently recovered from a block discovered to have broken off from the main limestone (block 821419 from SC-327) early in the preparation process. The association was confirmed by a connection between the broken ulnar shaft and the proximal end of the left ulna. Scale = 3 cm.

ner suggested by the general plesiadapiform morphology, they also exhibit a suite of unique features indicative of some behavioral peculiarities reflected in the morphology and relative length of the radius, the morphology of the innominate, the morphology and relative length of the tibia and fibula, and the morphology of the astragalus.

The radius is unique in having a large, raised area for the origination of the pronator teres muscle on the lateral aspect of the shaft just proximal to its midpoint; a shaft that is mediolaterally expanded starting at the level of the pronator teres muscle (a tuberosity) and continuing to the distal end (providing room for origination of powerful digital flexors and the pronator quadratus muscle, respectively); and a distal articular surface that is deeply cupped, elliptical, and has a distinct dorsal ridge that causes this surface to face palmarly. The form of the distal end is most comparable to that of sloths, dermopterans, and Ptilocercus in which it presumably reflects use of suspensory postures wherein the palmar-flexed hand is "hooked" over relatively small-diameter, sub-horizontal supports. Bats have a similar dorsal ridge and palmar-facing articulation, but the shape of the articular surface itself is much different in being almost sloth-like in micromomyids. Taken together, the morphology of the radius seems to indicate sustained use of vertical and underbranch clinging. During underbranch clinging, a strong pronator teres muscle would resist supinatory torque (Miller et al., 1964) produced by gravity, tending to pull the hands out of plane with and away from the substrate.

The fibula and UAJ in micromomyids are substantially different from those of other plesiadapiforms. At such a small body size, micromomyids experienced an arboreal milieu presenting relatively larger diameter supports, and in part these morphological differences seem to reflect that. More specifically, micromomyids appear to have been capable of stronger flexion of the digits and foot, and stronger resistance to pedal inversion than other plesiadapi-forms. These inferred functional implications of the ankle morphology are similar to and consistent with those from the forelimb morphology. The proximal end of the fibula flares anteroposteriorly and is blade-like, unlike that known for any other plesiadapiform. The shaft then gradually tapers distally until it obtains a circular cross section. This proximal, blade-like expansion of the shaft provides a large area for origination of pedal plantar-flexor muscles and the pedal evertor muscle, peroneus longus.

The astragalus of micromomyids differs from that of other plesiadapiforms in having, on the body, a relatively high medial ridge that reduces the degree of natural inversion of the foot; on the tibial facet, a deeper groove that limits the UAJ to sagittal flexibility; and an enormous groove for the tendons of the pedal plantar-flexor muscles (flexor tibialis and fibularis) on its plantar aspect as would be expected from the large area for origination of these muscles on the tibia and fibula. The calcaneum of micromomyids differs from that of other plesiadapiforms [except Phenacolemur praecox and some other paromomyids (Beard, 1989; Szalay and Drawhorn, 1980)] in having a longer tuberosity, giving the gastrocnemius and soleus muscles more leverage, and in having a more distally and laterally extended peroneal tuberosity, giving the tendon of peroneous longus an even more transverse line of action and making it a more devoted pedal evertor.

Although leaping between vertical supports is not out of the realm of possibility for micromomyids, given the idiosyncrasies described thus far, pronograde postures and any sort of bounding were probably infrequent. Such obligate arboreality is also probably reflected in the innominate. Unlike in Ignacius and bounding taxa generally, the ilium is extremely long and rod-like (Sargis, 2002c), the ischium is relatively short and rod-like; and the ischiopubic symph-ysis is short and caudally shifted relative to the acetabulum, similar to that of dermopterans (Sargis, 2002c) and lorises, both of which often use suspensory postures and neither of which use pronograde bounding. We note that such features also characterize Ptilocercus (Sargis, 2002b,c) and primitive eutherians such as Ukhaatherium (Horovitz, 2000, 2003), and may be more reflective of the primitive condition (see Szalay et al., 1975) than a behavioral specialization.

The major differences between micromomyids and other plesiadapiforms reflect the ability of micromomyids to more powerfully flex the digits and manus, to plantarflex the pes, to resist supination and inversion, and to less effectively use pronograde postures. Such adaptations suggest more time spent on the undersides of branches (Bloch et al., 2003), where they would be out of sight of aerial predators.

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