Placodonts were unusual benthic-feeding marine reptiles known only from the Triassic of Europe, the Middle East and southern China. Best known for their wide, often armoured bodies, dense bones and the flattened crushing teeth for which they were named, they have variously been characterized as reptilian analogues of walruses or rays or as turtle-mimics; clearly they would have differed substantially from their pelagic, flippered kin, the plesiosaurs. Placodont teeth were discovered as early as 1809 and the first skull was found in 1824. Though these fossils were initially thought to belong to unusual fish, in 1858 the British anatomist Richard Owen argued that they represented a new group of reptiles related to plesiosaurs. Though this idea was challenged several times in the following decades, there is agreement today that placodonts were highly modified, basal members of the Sauropterygia, the marine reptile group that includes nothosaurs, plesiosaurs and their relatives (Fig. 1A). Experts agree that sauropterygians are part of the major reptile group called the Diapsida, but there is less agreement on where within Diapsida sauropterygian affinities lie.
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