You’ve heard of plesiosaurs (and probably the short-necked plesiosaurs known vernacularly as pliosaurs). But unless you’re a palaeontologist or zoology uber-nerd, you might well not have heard of placodonts, pachypleurosaurs, nothosauroids and pistosaurids – the other lineages that, together with plesiosaurs, form Sauropterygia, a major clade of Mesozoic marine reptiles. Here’s a much-simplified cladogram showing hypothesised sauropterygian affinities. Cartoons depict the main lineages.
Non-plesiosaurian sauropterygians are mostly Tethyan in distribution: that is, they inhabited the warm, equatorial Triassic Tethys Sea. Back then it extended from western Europe all the way to eastern Asia. Only with the evolution of paraxially swimming plesiosaurs did sauropterygians become truly pelagic and hence able to disperse about the seas of the Mesozoic world.
Sauropterygians have highly distinctive skeletal elements. In the skull, they possess the unusual euryapsid skull condition (where the laterotemporal opening is secondarily closed, only the supratemporal opening remaining*), have retracted external nostrils, a closed, extensive bony palate that extends all the way to the braincase, a large retroarticular process (a prong that projects from the posterior end of the lower jaw), and lack several of the bones otherwise primitive for reptiles (like lacrimals,
tabulars**, postparietals and supratemporals). Postcranially, the ilium is small and the scapula lies superficially to the clavicle – a very weird feature that became developed to an extreme in plesiosaurs. Talking of plesiosaurs, here’s a cake, made specially for the Arthur Cruickshank celebration of 2009...
* This assumes that sauropterygians are part of the major reptile clade Diapsida, as they seem to be based on other lines of morphological evidence.
** My mistake. Some authors do indeed say that sauropterygians are unusual in lacking tabulars, but Nick Gardner reminds me that tabulars have been lost across numerous diapsid lineages (as well as elsewhere in Reptilia, as in captorhinids). In fact, absence of the tabular may be a neodiapsid autapomorphy (though the ‘younginiform’ Claudiosaurus may perhaps have tabulars). Tabulars are paired bones at the back of the skull, located above the occipital surface and on either side of the also paired interparietals.
Sauropterygians of various sorts have been covered on Tet Zoo a few times, but I’ve never gotten round to covering any of them in detail. ARGH THE FRUSTRATION.