Newly-discovered bones answer questions about fossil primate locomotion

Utah may seem like an odd place to search for primates, but you can find them if you know where to look. Although scrubby and arid today, between 46-42 million years ago what is now the northeastern part of the state was a lush forest which was home to a variety of peculiar fossil primates. Called omomyids, these relatives of living tarsiers are primarily known from teeth and associated bits and pieces of bone, but newly discovered postcranial remains may provide paleontologists with a better idea of how some of these ancient primates moved.

For most of their early evolution omomyids were relatively small, but by the time represented by the Uinta Formation larger varieties (estimated to be about a kilogram or more) had evolved. These forms, such as Macrotarsius and Ourayia, were considerably bigger than their ancestors, and given their larger size it has been questioned whether they would have moved in a different way due to their increased body size. While the remains of small omomyids have confirmed that early members of the group scampered about on all fours and leaped from branch to branch, larger forms may not have been as agile and so many have been doing something different. Unfortunately most of the relevant postcranial material known represents early species of omomyids, but, as described by Rachel Dunn in the Journal of Human Evolution, bones from larger-bodied species found in the Uinta Formation and California's Mission Valley Formation may help bring resolution to this issue.

On the basis of a comparison between numerous new fragments from the legs, ankles, and feet of the omomyids Chipetaia lamporea, Ourayia uintensis, and Mytonius hopsoni and the corresponding bones in living primates, Dunn found that these fossil primates probably moved through the trees like their ancestors did. The specimens of Chipetaia and Ourayia, especially, were most similar to those of other primates which frequently leap but lack the anatomical specializations of vertical clingers or leapers, or primates which move by hanging onto vertical supports and leaping to another one. In other words, they probably moved like bush babies or galagos, scampering through the trees on all fours but frequently leaping off to reach other branches.

Interestingly, however, the ankle of Mytonius may indicate that it was moving in a different way. While its remains were too fragmentary to be certain, the ankle of Mytonius appears to have been more flexible than that of the other large omomyids. This led Dunn to suggest that Mytonius may not have been leaping as frequently as the other primates, but this hypothesis will have to be tested against more complete remains.

Dunn, R. (2010). Additional postcranial remains of omomyid primates from the Uinta Formation, Utah and implications for the locomotor behavior of large-bodied omomyids Journal of Human Evolution DOI: 10.1016/j.jhevol.2010.02.010