SEARS, Karen: Adult morphological variation in mammalian limbs: Evidence for evolutionary constraint?

Marsupials compose only six percent of living mammals and occupy a limited set of ecological niches. Traditionally, it has been assumed that low marsupial diversity is due their unique mode of reproduction constraining their evolution. Immediately after birth, marsupials, using only their precociously developed forelimbs, crawl a distance one hundred times their body length to the teat where they attach and complete their development. The extremely large and fused scapula complex of the marsupial neonate provides the skeletal support and muscle attachment areas necessary for the “crawl from down under”. The primary goal of this study is to test the hypothesis that the obligate commitments of the marsupial scapula and forelimb in propelling the newborn have constrained their evolution. To test this hypothesis, I measured limb and girdle skeletal elements. I then calculated and compared marsupial and placental morphological disparity. Documentation of disparity will either support the existence of a constraint if placentals are more disparate than marsupials, or rule it out if placentals and marsupials have an equal range of morphologies or if marsupials are more morphologically disparate than placentals. I found marsupials to be less disparate than placentals in every aspect of limb morphology. In addition, marsupial and placental disparity patterns differ in some interesting ways that support the hypothesis, one of which being that marsupial scapulae are less disparate than marsupial pelves, but placental scapulae are more disparate than placental pelves. While it is impossible at this point to rule out other factors, these results support the hypothesis that marsupial forelimb evolution was constrained.