We provide a complete description of one of the oldest plesiosaurians, Stratesaurus taylori from the earliest Hettangian of the United Kingdom. At least 25 apomorphies distinguish S. taylori from the sympatric Thalassiodracon hawkinsii, to which all three specimens of S. taylori were originally referred. Several features of the skull of S. taylori suggest specialization on small prey items, or sieve feeding. In particular, it has anteriorly inclined premaxillary and mesial maxillary teeth and an only weakly heterodont maxillary dentition. This indicates niche partitioning among sympatric small-bodied plesiosaurians: T. hawkinsii has a pronouncedly heterodont dentition. With a body length estimated around 2 m, S. taylori is one of the smallest plesiosaurians, comparable to T. hawkinsii. Our anatomical review of S. taylori suggests difficulty determining its precise phylogenetic affinities. This is consistent with a general lack of phylogenetic resolution among earliest Jurassic plesiosaurians, which may result from missing data on their Triassic ancestry. However, due to its plesiomorphic morphology and well-characterized anatomy, we recommend S. taylori as an ingroup representative of Plesiosauria for future cladistic analyses of Triassic sauropterygians.
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