Abstract
New fossil material of the problematic reptile species Pachystropheus rhaeticus from the uppermost Triassic (Rhaetian) of England, and re-examination of other specimens, demonstrate that it may be the earliest known example of the Choristodera, a poorly understood group of semi-aquatic diapsid reptiles. As a result, the fossil record of choristoderes is extended back in time by approximately 45 million years and a significant gap in the fossil record of these reptiles is highlighted. Failure of the record to reveal the evolutionary histories of those reptilian taxa that are most prone to fossilization (i.e. semi-aquatic and aquatic forms) emphasizes the need for rigorous, character-based studies in phylogenetic analysis of tetrapod lineages, rather than reliance upon stratigraphical position as an indicator of phylogenetic branching sequence. Furthermore, cladistically-based predictions of pre-Jurassic choristoderes are themselves seemingly reinforced by this discovery.
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