Marine reptiles are especially difficult for paleontologists to classify, because, in the course of evolution, creatures living in marine environments tend to take on a limited variety of body forms--which is why, for example, Ichthyosaurus looks so much like a large bluefin tuna. This trend toward convergent evolution can make it difficult to distinguish between various orders and suborders, much less individual species within the same genus. Here's a look at how marine reptiles are currently classified.
Superorder: Ichthyopterygia The "fish flippers," as this superorder is called, includes the ichthyosaurs--the streamlined, tuna- and dolphin-shaped predators of the Triassic and Jurassic periods. This abundant family of marine reptiles--which includes such famous genera as Ichthyosaurus and Ophthalmosaurus--largely went extinct at the end of the Jurassic period, supplanted by the pliosaurs, plesiosaurs and mosasaurs (described below).
Superorder: Sauropterygia The name of this superorder means "lizard flippers," and it's a good description of the ocean-going reptiles that swam the seas of the Mesozoic Era, starting from about 250 million years ago to about 65 million years ago--when the sauropterygians went extinct along with their dinosaur cousins.
Order: Placodontia The most ancient of all marine reptiles, placodonts flourished in the oceans of the Triassic period, between 250 and 210 million years ago. These creatures tended to have squat, bulky bodies with short legs, reminiscent of turtles or overgrown newts, and probably fed along shallow coastlines rather than in the deep oceans. Typical placodonts included Placodus and Psephoderma.
Order: Nothosauroidea Paleontologists believe these Triassic reptiles were much like modern seals, scouring shallow waters for food but coming ashore periodically on beaches and rocky outcroppings. Nothosaurs were about six feet long, with streamlined bodies, long necks and webbed feet, and they probably fed exclusively on fish. You won't be surprised to learn that the typical nothosaur was Nothosaurus.
Order: Pachypleurosauria One of the more obscure orders of extinct reptiles, pachypleurosaurs were slender, smallish (about one and one-half to three feet long), small-headed creatures that likely led an exclusively aquatic existence and fed on fish. The exact classification of these marine reptiles--the most commonly preserved of which is Keichousaurus--is still a matter of ongoing debate.
Order: Plesiosauria This order accounts for the most familiar marine reptiles of the Jurassic and Cretaceous periods, and its genera often attained dinosaur-like sizes. Plesiosaurs are divided by paleontologists into two main suborders, as follows:
Suborder: Plesiosauroidea The prototypical plesiosaur was a large, streamlined, long-necked fish eater possessing big flippers and sharp teeth. Plesiosaurs probably weren't as accomplished swimmers as their close cousins, the pliosaurs (described below); it's likely they cruised slowly along the surface of rivers, lakes and oceans, extending their long necks to snap up unwary prey. Among the most famous plesiosaurs were Elasmosaurus and Cryptoclidus.
Suborder: Pliosauroidea Compared to plesiosaurs, pliosaurs had much more fearsome body plans, with long, toothy heads, short necks, and barrel-shaped bodies; many genera resembled modern sharks or crocodiles. Pliosaurs were more agile swimmers than plesiosaurs, and may have been more common in deeper waters, where they fed on other marine reptiles as well as fish. Among the scariest pliosaurs were the gigantic Kronosaurus and Liopleurodon.
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