Limb Diversity in Marsupial and Eutherian Mammals

Kelly and Sears (2011)  “Limb specialization in living marsupial and eutherian mammals: constraints on mammalian limb evolution” aimed to test the argument that the functional requirement of newborn marsupials to crawl to the teat is constraining the evolution of the marsupial forelimb, based on two core hypotheses: first, that marsupial forelimbs are less specialized than eutherian forelimbs; and second, that marsupials tend to have more specialized hind limbs, whereas eutherians follow the opposite pattern and tend to have more specialized forelimbs. The first hypothesis was formulated based on the fact that marsupial forelimbs show a small range of possible forms, are very similar among different functional groups, and are less morphologically different from the average mammal than eutherian forelimbs are.   The second hypothesis was based on the fact that marsupial young have highly developed forelimbs and shoulders at birth because they are born premature and need to crawl up to the mother’s teat immediately after birth, where they attach and finish developing. This specialized morphology is necessary at such an early time in their development that the theory is that it prevents variation, and thus reduces the likelihood that marsupial forelimbs will evolve and specialize. The hind legs are not used in this post-birth crawl, which leaves them free to diverge and specialize. Eutherians, on the other hand,  tend to have more specialized forelimbs and less specialized hind limbs. The proposed theory is that eutherian hind limbs are functionally important in locomotion, which constrains how much their morphology can vary, but the forelimbs are free to evolve and diversify—the opposite of limb specialization seen in marsupials.