Meeting Abstract

19.1  Saturday, Jan. 4 10:15  The radiation of a living fossil: Jaw morphology of tree squirrels ZELDITCH, M.L*; LI, J.; TRAN, L.A.P.; SWIDERSKI, D.L.; Univ. of Michigan, Ann Arbor zelditch@umich.edu

The dynamics of adaptive radiations have captured much attention recently, but equally interesting are groups famed for their morphological conservatism, i.e., living fossils. Some living fossils are diverse and geographically widespread, such as the largely New World tree squirrels (Sciurini) which closely resemble the first known squirrel in postcranial morphology and jaw proportions. In this analysis, we examine jaw size and shape disparity of 90 species, 75 of which had sequence available for a phylogenetic reconstruction. We compare the evolutionary dynamics of three clades, Sciurini, its sister group, the Indomalayan flying squirrels (Pteromyini) and Indomalayan tree squirrels (Callosciurinae). As anticipated, Sciurini is least disparate in both size and shape; its subclade disparity is consistently high relative to the total disparity. In contrast, the within-subclade disparity of Pteromyini is initially no greater than expected from Brownian motion, but it increases dramatically, exceeding the total for the group because of the relatively large distances between close relatives in comparison to the average distances across all species. Size apparently evolves by Brownian motion in all three clades but shape is better explained by an Ornstein-Uhlenbeck model, with Sciurini being most constrained. The low disparity of Sciurini is thus not due to low rates of jaw shape evolution; in both evolutionary rate and average branch length, Sciurini does not differ from Pteromyini. Rather, Sciurini more densely fills a smaller morphospace, rarely expanding its morphological range and repeatedly converging, while its lineages accumulate and its geographic range expands from boreal to tropical forests.