Systematics and biology of some desmodont bivalves from the Australian Permian

The Australian Permian bivalves Megadesmus, Astartila, Pyramtis, and Myonia have previously been grouped in either the Family Pachydomidae (Fischer, 1887) or the Family Edmondiidae (King, 1850). Because of similarities in their musculature (in addition to those in dentition, micro‐ornament, and to some extent, shape) they can now be included in the same family as the deep‐burrowing bivalve Vacunella (previously Chaenomya). Vacunella is closely related to the living genus Pholadomya which has existed at least since the Mesozoic, and the two can be placed in the same family — Pholadomyidae (Fleming, 1828). Because of these similarities between Pholadomya, Vacunella, and the “Pachydomidae” it seems likely that the middle and upper Palaeozoic edmondioid bivalves gave rise to the Mesozoic and Cainozoic pholadomyoids, and that these two groups together constitute a major division of the Class Bivalvia — a division that may have been relatively distinct since Ordovician time.