Marine reptiles of the Jurassic 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
Some 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.Marine reptiles of the TriassicPlacodus, 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.
The best-adapted ocean reptiles were the ichthyosaurs, such as Ichthyosaurus. They were at their most abundant during the Jurassic Period, a time when shallow, warm seas covered much of the Earth. Like modern dolphins, ichthyosaurs were perfectly streamlined, with long flippers for steering and strong tails to propel them quickly through the water. They were the first marine reptiles to spend all of their lives in the water. They gave birth to live young. Different species of ichthyosaur lived through the Jurassic Period and on into the Cretaceous. Some grew up to 15 metres (49 feet) long.
The Jurassic Period saw the emergence of another important group of marine reptiles: the plesiosaurs. Like dinosaurs, they had long necks, small heads and sharp teeth. Instead of legs, their limbs evolved into large, paddle-like flippers which they used like underwater “wings” to pull themselves through the water, beating in a slow, steady rhythm. Plesiosaurs fed on fish and squid, darting their long, flexible necks backwards and forwards to pick off their prey. They spent most of their time in the water, coming ashore only to lay their eggs.
Pliosaurs, short-necked plesiosaurs, such as Kronosaurus and Liopleurodon, were the top predators of the Jurassic seas. Armed with massive, teeth-lined jaws, they could prey on even the largest sea creatures. Their streamlined bodies were built for speed through the water.
Lizards and turtles
Lizards, turtles and crocodiles all lived at the same time as the dinosaurs. No one knows why they survived while the dinosaurs died out at the end of the Cretaceous. A number of other reptile species also adapted to life in the sea. Platecarpus and Tylosaurus were both mosasaurs: sea lizards. Platecarpus fed on fish and ammonites, while Tylosaurus, a 9-metre (30-foot) giant, had a larger appetite, competing with the massive, predatory fish Xiphactinus for prey such as other fish, marine reptiles and sea birds. Archelon was the largest-known sea turtle, measuring some 4 metres (13 feet) in length. Like a modern leatherback, its “shell” was a bony frame with a leathery covering.
The earliest crocodiles evolved in the Triassic Period. Some grew to massive lengths, such as Sarcosaurus (12 metres / 40 feet long), a Cretaceous crocodile. Most crocodiles today live in fresh water. A number of prehistoric crocodiles spent all their lives at sea, while some lived entirely on land. A sea-dweller, Geosaurus had flippers for limbs and a fishy tail, helping it swim more easily. Teleosaurus looked more like a modern-day gharial, a crocodile with a narrow, elongated snout, and lived both on land and in the sea.
Consultant: Chris Jarvis