Turning to the euryapsid reptiles, we find four highly adapted groups of sea reptiles, some of which reached ponderous proportions. Euryapsids are believed to have evolved from diapsids, having lost the lower temporal opening in the process. They were a very important part of the marine environment during the Mesozoic and, in a sense, were as dominant in that setting as the dinosaurs were on the land. A subject that often is neglected in the course of discussions about the reasons for the demise of the dinosaurs at the end of the Cretaceous concerns the reasons behind the extinction all of the euryapsid sea reptiles at the end of the Cretaceous.

Nothosaurs lived from early to late Triassic times but were most common in the Middle Triassic (242-227 mya). They were relatively small compared with the plesiosaurs that followed them. Nothosaurs had moderately long necks and limbs modified as flippers. They are thought to have been possible ecological equivalents of modern seals and otters. Nothosaurs had sharp, conical teeth modified for catching fish. The structure of the nothosaur shoulder girdle is unique among reptiles; it provides little space for the attachment of trunk-supporting body muscles, such as occurs in land reptiles.

Plesiosaurs are thought to have evolved from nothosaurs in the Middle Triassic. They persisted until almost the end of the Cretaceous but were most abundant in the Middle Jurassic (180-159 mya) and slowly dwindled in numbers until their extinction. Plesiosaurs had legs modified as paddles that flapped up and down like aquatic wings. This action not only pushed them through the water but also gave them a lift that allowed them to "fly" through the sea in the same manner as seaturtles and penguins. The bones of the shoulder and hip girdles of plesiosaurs were expanded greatly below, forming an armor on the bottom of the animal that left no "soft underbelly" for attack by such predators as sharks. Two general kinds of body types were prominent in the plesiosaurs, long-necked forms and short-necked forms.

In the first group, a small head was positioned at the end of a very long neck. The body was heavy and bulbous. The teeth were conical and sharp; for this reason it is assumed that these plesiosaurs fed mostly on fishes. The elasmosaurid clade of long-necked plesiosaurs reached a length of more than 40 ft (12.2 m) and had enormous paddles and very small heads. Some researchers have suggested that the Loch Ness monsters (if they actually existed) were long-necked plesiosaurs. If this is true, remarkable physiological changes that allowed them to adapt to icy waters must have occurred since the Mesozoic.

Short-necked plesiosaurs have practically no neck at all and a massive head. It has been suggested that they were the ecological equivalents of the killer whales of present times, as they were consummate carnivores. Kronosaurus, the largest marine reptile that ever lived, was a massive animal that reached a length of at least 42 ft (12.8 m). This animal was found on the property of a rancher in Australia and ended up in the Museum of Comparative Zoology at Harvard. One of the problems of collecting such a large animal is how and where to exhibit it. As frequently happens, a room at the museum had to be remodeled to put this "reptilian killer whale" on exhibit.

Placodonts were marine reptiles with short, stout bodies that lived from Middle to Upper Triassic times (242-206 mya). The limbs were only moderately paddlelike. It once was suggested that placodonts were related rather closely to nothosaurs, but there is really no good evidence to support this hypothesis. The most distinguishing feature of placodonts is the form of their teeth, which are flattened rather than pointed. In the well-known genus Placodus, the teeth along the margin of the cheek region and on the palate are large and very flat, whereas the long, narrow front teeth protrude from the end of the somewhat narrowed snout. It is thought that Placodus used the front teeth to grasp mollusks and the hind ones to crush them. Some placodonts, such as Henodus, were superficially like turtles, in that they had an upper shell composed of numerous small polygonal bones. A lower shell was not present.

The most highly specialized marine reptiles were the ichthyosaurs, whose body took on the appearance of modern tunas, sharks, and porpoises. Ichthyosaurs lived from early Triassic to Middle Cretaceous times (248-112 mya). The skull of ichthyosaurs is streamlined, with a long snout; the eyes are very large. The body is spindle-shaped, and the limbs are reduced to fins. The tail fin is fishlike. The individual vertebrae in the spinal cord are in the form of very short and compact biconcave discs, very similar in appearance to those of modern sharks. It is estimated that some ichthyosaurs were very active and could swim 30-40 mph (48.3-64.4 km/h). Why the ichthyosaurs became extinct in the Middle Cretaceous (ca. 121-99 mya), long before the dinosaurs and other marine reptiles, remains a mystery.

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