Limb specialization in living marsupial and eutherian mammals : constraints on mammalian limb evolution

Abstract

Evolution of the marsupial forelimb complex is argued to be constrained by the functional requirements of the newborn's crawl to the teat. This constraint is often used to explain why marsupials are limited in their diversity relative to eutherians. However, despite the constraint's importance to mammalian evolution, one of its fundamental corollaries, that marsupials exhibit less forelimb specialization than eutherians, has not been tested explicitly. We used morphometric analyses of mammalian limbs to test 2 hypotheses concerning their specialization: marsupials have less-specialized forelimbs than eutherians, and marsupials tend to specialize their hind limbs rather than forelimbs and eutherians their forelimbs rather than hind limbs. In support of the 1st, marsupial forelimbs are more similar among functional groups, display smaller morphospaces, and have lower morphologic divergence from the average mammalian state than eutherian forelimbs. In support of the 2nd, marsupial hind limbs are more dissimilar than forelimbs and display higher morphological divergence, but the opposite is true for eutherians. Results suggest fundamental differences in the mechanistic underpinnings of limb specialization in marsupials and eutherians and support a constraint on the marsupial forelimb complex, and possibly the mammalian hind limb.