Limb Evo-Devo

Virtual reconstruction of a CT scanned brushtail possum neonate, bones are white . The strongly developed forelimb, contrasting with a distinct developmental lag of the hind limb, is typical for all marsupials.

Marsupials are born at a highly immature stage (rice-grain to jelly-bean size) without exception and nearly all actively move into their mother’s pouch. This early climbing ability is strongly impressed on marsupial skeletal development from the very earliest stages up to bone formation . One side-effect of the marsupial climb is a distinct lack of forelimb diversity – which makes sense, since a flippered or hoofed neonate would hardly make it to the pouch. The unique marsupial fauna of Australia makes it possible to research the correspondence between morphometric relationships within and among limbs, limb development, and limb evolution in marsupials and monotremes.

Virtual reconstruction of a hatchling echidna skeleton. Note the near-absence of the upper arm and absence of upper leg bones



Talking about monotremes: My CT scanning study has shown that monotremes (platypuses and echidnas) are truly strange when it comes to skeletal development. Unlike nearly any other land vertebrate, the monotreme upper arm and leg bones only start forming after the lower limb bones. Not only is this against the “rules” of proximo-distal limb development, but is to date known from only one other species of mammal – the European mole. The interesting thing is that both monotremes and moles have arms that are adapted to digging, with  particularly bizarre-looking upper arm bones. Food for thought about the interaction between limb development and adaptation…