Meeting Abstract

22.2  Sunday, Jan. 4  Covariation, disparity and constraints in marsupial and eutherian limb evolution SEARS, K.E.; University of Illinois ksears@life.uiuc.edu

Marsupial newborns complete a life-or-death crawl to the teat, powered solely by their forelimbs. As a result, marsupial fore- and hind limbs have disparate functional requirements early in development. In contrast, eutherians develop largely in utero, and neither limb has early functional requirements. I hypothesize that the highly disparate functional requirements on marsupial fore- and hind limbs reduced their evolutionary covariation relative to eutherians. Reduced covariation is generally thought to increase evolvability, but I hypothesize that the increased modularity of marsupial limbs facilitated the specialization of the forelimb for the crawl, thereby constraining its further evolution. To test these hypotheses, I quantified the limb skeletons of a diverse selection of marsupial (n=72) and eutherian (n=126) species, and analyzed among-species covariance, correlation and disparity. All mammals exhibit higher evolutionary covariance among elements of individual limbs than between fore- and hind limbs. However, as hypothesized, marsupials display markedly less covariation between fore- and hind limbs than eutherians. A principal components analysis generated a forelimb morphospace (MS), which is consistent with the existence of a marsupial forelimb constraint. Although analogous marsupial and eutherian functional groups (FGs) (e.g., arboreal quadrupeds) fall in similar MS areas, the marsupial FGs are always significantly less divergent from the overall centroid than their eutherian counterparts. Therefore, the forelimbs of all marsupials are more similar, and relatively less specialized. Accordingly, the average correlation between marsupial FGs is higher than that between analogous marsupial and eutherian FGs, while the among eutherian FG average is lower.