Meeting Abstract
It is now well known that numerous mammalian taxa including moles, pandas and elephants have sizeable mineralized structures on the medial side of the manus and/or pes referred to as the prepollex or prehallux (“predigits” collectively), among other names. It is often assumed that these are enlarged sesamoid bones, and it has indeed been demonstrated in several cases that described “predigits” are homologous to radial or tibial sesamoids in their presumed plesiomorphic non-enlarged state. We compiled published data and original observations to reconstruct the evolutionary history of radial/tibial sesamoids, specifically whether “predigits” always evolved from these precursors, and how many times they did. We applied maximum parsimony and likelihood character evolution reconstruction methods to these data on a time-calibrated evolutionary tree of Mammaliaformes. We found that there are many more unrecognized origins of “predigits” in other lineages, and that both radial and tibial sesamoid bones have evolved convergently multiple times within crown group Mammalia. These results prompted us to speculate that the bony sesamoids themselves evolved from an ancestral precursor made of soft tissue such as ligament, tendon, cartilage, and/or fibrocartilage, or a series of those states. More studies of the ontogeny and anatomy as well as the distribution of these features in mammals and their fossil outgroups are needed to refine and test these inferences of homology and evolution.
