THE DIAPSID REPTILE, PACHYSTROPHEUS-RHAETICUS, A PROBABLE CHORISTODERE FROM THE RHAETIAN OF EUROPE

The enigmatic Upper Triassic reptile, Pachystropheus rhaeticus, displa ys characters suggestive of choristoderan affinity and, as such, is po tentially the oldest known choristoderan reptile. Examination of the k nown skeletal elements indicates that the choristoderan lineage remain ed morphologically conservative throughout its recorded history. The o ccurrence of Pachystropheus fossils in marginal marine bone beds, howe ver, may reflect a previously unrecognized shift of habitat for the Ch oristodera, from the paralic environments occupied by early representa tives, to the more typical freshwater, often fluvial, deposits contain ing later forms.