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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