Meeting Abstract
Recently, we used a combination of 3D x-ray reconstruction of moving morphology (XROMM) and comparative analysis of fossil and extant anatomy to argue that the unique mammalian food processing system originally required independent rolling of unfused hemimandibles. Moreover, the original function of the therian tribosphenic molar was to grind food in a reverse mortar-and-pestle arrangement by which the talonid “mortar” moved transversely across the protocone “pestle.” This transverse motion was enacted primarily by long-axis jaw rotation. Primitive therian mammals including opossums (Monodelphis domestica) retain the ancestral mode of chewing, including the mortar-and-pestle rotational grinding stroke. Here we consider the experimental and comparative data further and show that jaw roll is broadly conserved across mammals, and that the rotational grinding stroke can be inferred to have been present at the therian ancestor — probably, in fact, well down the therian stem. Fusion of the jaw symphysis has occurred repeatedly in omnivorous and herbivorous therian clades and is associated with low-crowned teeth and grinding by transverse motion of the mandible. It is also associated with reduction of the angular process, which we argue serves to provide greater leverage for jaw-rolling musculature. Finally, we suggest that there is greater continuity in molar structure and function on the stem of Theria than has previously been appreciated.
