BOOTSTRAP COMPARISONS OF VERTEBRAL MORPHOLOGY OF PALEOCENE PLESIADAPIFORMS (MAMMALIA: ?PRIMATES): FUNCTIONAL IMPLICATIONS

Plesiadapiformes have traditionally been allied with primates. Alternatively, it has been suggested that they were more closely related to the extant flying lemur, Cynocephalus (Mammalia: Dermoptera), and that certain plesiadapiform taxa were mitten-gliders. New plesiadapiform skeletons from middle Clarkforkian limestones in the Clarks Fork Basin, Wyoming are the most complete known and have well documented dental associations that allow testing of previous hypotheses of positional behavior of these extinct mammals.

Mammalian vertebral morphology is diverse, reflecting a range of positional behaviors. Structure and proportions of the vertebral column of two plesiadapiforms, Ignacius and Carpolestes, are compared to those of a sample of extant taxa for which positional behaviors are known. Dimensions of vertebral centra, normalized to average height of the centra of the first six thoracic vertebrae, are plotted in serial diagrams for extant taxa and then compared to those of the fossil taxa. Total proportional difference between two taxa is quantified by calculating the differences at each vertebral position and summing those values. To assess the degree of proportional similarity along the vertebral column, we randomized the vertebral positions of one taxon in the comparison and recalculated the total difference. This process of randomization and recalculation was repeated 1000 times to produce a distribution of differences. The more similar the serial vertebral proportions of two animals, the smaller is the actual total difference between them, relative to the randomized distribution.

Our analysis indicates that Ignacius had a flexible vertebral column with proportions similar to extant scansorial tree shrews (Mammalia: Scandentia). Carpolestes had a less flexible spine similar to that of extant vertically clinging tarsiers (Mammalia: Primates). Cynocephalus is distinctly different from both plesiadapiforms in having a relatively rigid vertebral column that reflects suspensory and gliding behaviors. Analyses of the morphology of vertebral processes support these results. Structure and proportions indicative of dermopteran-like gliding are absent from both Ignacius and Carpolestes.