Limb Diversity in Marsupial and Eutherian Mammals

The Kelly and Sears (2011) paper (“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 marsupial forelimbs. The paper was based on two core hypotheses: first, that marsupial forelimbs are less specialized than eutherian (placental mammal) forelimbs; and second, that marsupials tend to have more specialized hind limbs whereas eutherians tend to have more specialized forelimbs. The first hypothesis was 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 are born with highly developed forelimbs and shoulders, as they are born premature and need to crawl up to the mother’s teat immediately after birth so they can attach and finish developing. Because specialized morphology is necessary at such an early time in marsupial development, the theory is that it prevents variation in the development of the forelimbs and thus reduces the likelihood that they will evolve and specialize. However, marsupials do not use their hind limbs in this post-birth crawl, which leaves them free to diverge and specialize as they develop. Eutherians, on the other hand, tend to have forelimbs that are more specialized than their hind limbs. The proposed theory for this is that eutherian hind limbs are functionally important to locomotion and this constrains how much their morphology can vary, but the forelimbs are not used for locomotion and this leaves them free to evolve and diversify—the opposite of the pattern of limb specialization seen in marsupials.