THE EARLIEST POSSIBLE CHORISTODERE (DIAPSIDA) AND GAPS IN THE FOSSIL RECORD OF SEMIAQUATIC REPTILES

New fossil material of the problematic reptile species Pachystropheus rhaeticus from the uppermost Triassic (Rhaetian) of England, and re-ex amination of other specimens, demonstrate that it may be the earliest known example of the Choristodera, a poorly understood group of semi-a quatic diapsid reptiles. As a result, the fossil record of choristoder es is extended back in time by approximately 45 million years and a si gnificant gap in the fossil record of these reptiles is highlighted. F ailure of the record to reveal the evolutionary histories of those rep tilian taxa that are most prone to fossilization (i.e. semi-aquatic an d aquatic forms) emphasizes the need for rigorous, character-based stu dies in phylogenetic analysis of tetrapod lineages, rather than relian ce upon stratigraphical position as an indicator of phylogenetic branc hing sequence. Furthermore, cladistically-based predictions of pre-Jur assic choristoderes are themselves seemingly reinforced by this discov ery.