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Nothosaurs as a whole disappeared from the world oceans at the end of the last stage of the Triassic Period, called the Norian. They did so along with several other, more ancient marine reptile orders, including the strange shellfish eating Placodonts and the bizarre reptile family to which the stupendously proportioned Tanystropheus belonged - as well as dozens of mammal-like reptiles (such as Placerias) and crurotarsan archosaurs (such as Postosuchus) and others, on land also. This is due to the large scale mass extinction event that occured at the Triassic-Jurassic Period boundary, which killed off between a third and half of all life on Earth - the final nail in the coffin which saw the end of the older orders of reptiles having influence on the ecosystems on land and sea, and the extinction event which seconded the older and far more devastating Great Permian Mass Extinction (or the Permo-Triassic Extinction Event) in removing the competitors to the Dinosaurs, the Ichthyosaurs, the Plesiosaurs, Pterosaurs and the aquatic crododilians - all of these being the true masters of the Mesozoic in their niches. As for the older Nothosaurs and others, they are merely confined to the Triassic Period, and in spite of being wonderful marine reptiles in their own right to our eyes, were ultimately of minor influence. It is thought that some Nothosaurs gave rise to the earliest Plesiosaurs, a theory that has some weight behind it although little proof. However, Ichthyosaurs seemed to have bested the older forms such as Nothosaurs, being sleek hunters of a far more deadly variety - indeed, as the Nothosaurs were nearing the end of their time on Earth, an ancient Late Triassic Shastosauridae Ichthyosaur, called Shonisaurus sikanniensis, evolved and thrived for a time. Whereas the Nothosaurs peaked at 4 metres, this monstrous Ichthyosaur actually grew to over 22 metres.