Marine reptiles of the Jurassic. In this illustration, part of the sea has been "cut away" so we can see the marine reptiles in...Read More >>Marine reptiles of the Jurassic. In this illustration, part of the sea has been "cut away" so we can see the marine reptiles in the water while pterosaurs fly overhead. The dinosaurs lived only on land. But from the Triassic Period and on through the entire Age of Dinosaurs, various groups of large reptiles swam in the oceans. Most were hunters, chasing prey such as fish, squid and shellfish. In some ways, the marine (sea-dwelling) reptiles were similar to dinosaurs. They had scaly skins, big eyes for seeing well, plenty of teeth, four limbs and a tail. But their limbs were not legs for running—they were paddles or flippers for swimming. The marine reptiles could not breathe underwater like fish. So they had to stay near the surface and poke their heads out now and again for fresh air, before diving below the waves.
Placodonts and nothosaurs
Marine reptiles of the TriassicSome kinds of reptile began to feed on marine life during the Triassic Period, and gradually the body design of marine reptiles adapted to an underwater lifestyle. The first big sea reptiles, in the Triassic, were placodonts and nothosaurs. Placodus, a placodont, was 2 metres (6.6 feet) long. Apart from its long, fishy tail, it looked very much like a land reptile, with its short neck, heavy body and sprawling legs. Perhaps it searched for shellfish along the beach and at the water’s edge, crushing them with its large, flat teeth.
A more streamlined swimmer, Nothosaurus was 3 metres (10 feet) long. Its fossils occur in ancient seabed rocks now on three continents—Europe, Asia and Africa. It probably dived to catch food then waddled with its webbed feet on to the shore to rest, in the same way that seals do today. Over millions of years, new kinds evolved from reptiles like these to become permanent sea-dwellers.
Archelon was unable to withdraw its head or flippers inside its bony shell for protection, so it was an easy target for large predators such as mosasaurs or sharks.
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