The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)

Morphological variation in adapiform and omomyoid distal phalanges


Interdepartmental Doctoral Program in Anthropological Sciences, Stony Brook University

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The earliest fossil euprimates (adapiforms and omomyoids) possess distal phalanges that indicate the presence of flattened nails. However, adapiform distal phalanges are morphologically unique among fossil primates. They have been described as having a “nail-like” flattened tip combined with a “claw-like” proximal end. To evaluate and interpret the morphological affinities of these fossils, a sample of adapiform (n=12) and omomyoid (n=16) non-hallucal and non-pollical distal phalanges are compared to those of 21 living primate and 42 claw-bearing mammal species. A set of 13 measurements was taken from each specimen and converted into size-adjusted shape variables via division by the geometric mean. Data were explored and summarized using an array of principal component and discriminant function analyses.

Adapiform and omomyoid distal phalanges are shown to differ consistently and strongly from each other. The proximal ends of adapiform distal phalanges are indeed intermediate in form between claws and nails, while distally they closely resemble the phalanges of extant nail-bearing primates. The similarity to claws is caused by an elongated and flattened platform on the volar surface of the proximal end of the phalanx. A similar, though smaller and less-flattened, volar process is present in claw-bearing mammals; it is associated with bilateral nutrient foramina positioned near the process. The volar platforms of adapiforms are also characterized by paired, enlarged nutrient foramina. Omomyoids lack a discernable volar process, though nutrient foramina are variably present. Among living primates, adapiform distal phalanges are most similar to those of lemurids while omomyoids overlap in morphology with tarsiers.

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