Developmental constraint on the evolution of marsupial forelimb morphology

Compared with the placental mammals, marsupials are born at an almost embryonic stage, but nearly all of these neonates immediately climb or crawl to one of their mother's teats using precociously developed forelimbs. Marsupial adults also exhibit limited forelimb shape diversity relative to the members of their sister group. That the functional requirements of this natal climb have imposed a developmental constraint on marsupial forelimb evolution represents a compelling and widely accepted hypothesis, yet its resulting predictions for the comparative patterns of mammal limb shape diversity have never been tested. In order to perform such tests we conducted extensive taxonomic sampling of mammal limb morphology (including fossil specimens), and then examined these data using morphometric methods, non-parametric analyses of anatomical disparity, and phylogenetic comparative analyses of evolutionary rates. Our results strongly support the constraint hypothesis, and indicate that the highly significant differences between marsupial and placental forelimb shape diversity has been strongly influenced by different rates of morphological evolution among the distal forelimb elements in these two important mammal lineages.