Archicebus achilles and Teilhardina belgica are among the earliest Eocene primates so knowledge of their paleobiology is crucial to our understanding of early primate evolution. Since body mass is often a key to evaluating other important aspects of paleobiology determination of the likely body mass of these early primate taxa is a significant task for paleoprimatologists. These particular taxa pose several interesting problems for body mass estimation. First, they are at the far small end of body size distribution of living primates. Secondly, they are not nested within any family of living primates but lie at or near the base of the tarsiiform radiation. Thirdly, their nearest living relatives (tarsiers) are highly derived dentally, cranially, and postcranially. All of these raise the question of how to choose an appropriate reference group. In addition Archicebus, although represented by many skeletal elements, is a sample of one while Teilhardina is represented by more individuals but fewer different skeletal elements. Using samples of extant strepsirhines, tarsiers, and anthropoids to construct bivariate and multiple regression models we investigated the effects of choice of reference population (different size ranges and different phylogenetic groups) on estimates of body mass in these fossil taxa. We conclude that even the best statistical estimates have wide confidence intervals which need to be taken into account if body mass is used to predict other aspects of the fossil taxon’s behavior and ecology.
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The phyletic link of living tarsiers to fossil primates has been a difficult bridge to cross. Although Tarsiidae has been linked to fossil tarsiiforms such as omomyids and microchoerids, as well as to anthropoids, no consensus of opinion has been reached. Here we add several new postcranial elements for one of the most primitive of all tarsiiforms, Teilhardina belgica from Dormaal, Belgium. We compare this new material to that of living and fossil tarsiers as well as to other Eocene fossil primates. Besides the previously known tarsals for Teilhardina, we have been able to add a distal humerus, a proximal ulna, a second metacarpal, a proximal and a distal femur, tibiae, additional tarsals, first metatarsals, and several proximal and middle phalanges. Although most of these postcranial elements compare best with other omomyids, and therefore do not resolve the phyletic relationship of omomyids relative to tarsiers, the fingers and toes of Teilhardina are quite elongated, a similarity to living tarsiers. Middle phalangeal lengths of the diminuitive Teilhardina are comparable in length to much larger species of Tarsius suggesting relatively even longer digits. The digit features of Teilhardina and Tarsius are unusual for primates in general and may in fact represent an ancestral state although hands and feet of other fossil tarsiiforms are needed to test this hypothesis.
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