The nothosaurs of the Upper Triassic period (Sect. 4.4) very probably gave rise to the plesiosaurs ('near lizards'). The close relationship between the two taxa is revealed by Pistosaurus (Fig. 24), the only known member of the nothosaur family Pistosauridae. The head of this marine reptile was similar to that of the plesiosaur, but its palate was typically that of a nothosaur. Fossils of Pisto-saurus have been found in the Middle Triassic deposits of France and Germany along with those of other Sauropterygia. The Pistosauridae ranged between 20 cm and 1 m in length. They evidently swam using their supple bodies and wide sweeps of their deep tails to produce thrust (Ellis 2003; Benton 2004). All these giant reptiles lived in the sea and not in fresh water. This is known because their fossils have been found in association with those of sea urchins, squids, and other marine animals.

The most serious gap in the fossil record of Mesozoic aquatic reptiles occurs between the Upper Permian and Lower Triassic periods. That is where two major groups - the ichthyosaurs and the sauropterygians - evolved, along with other minor and disparate taxa including the thalattosaurs and askeptosaurs (Carroll 1997). The placodonts (Sect. 4.4.1), nothosaurs (Sect. 4.4.2), and ple-siosaurs were all derived from sauropterygian ancestry. The Sauropterygia was a monophyletic clade of diapsid reptiles that invaded the oceans of the world in the Early Triassic period. They first radiated during the Middle Triassic and became extinct during the Late Cretaceous.

In contrast, the ichthyosaurs ('fish lizards') - a distinctive and highly specialised group - lie on the lepidosauromorph line of neodiapsids: they were not sauropterygians. The ichthyosaurs were the most obviously aquatic of all marine reptiles (Bauplan 1: Fig. 17), and no transitional forms between them and their distant terrestrial ancestors have yet been found. Ichthyosaurs arose in the Lower Triassic, radiated in the Middle and Upper Triassic, but were outlasted by the plesiosaurs, dying out somewhat before them in the Upper Cretaceous. They, too, however, were in decline before the Cretaceous extinctions (Sect. 5.5).

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