What Is a Nothosaur? Marine Reptiles Caught Halfway to the Water

Nothosaurs had long slender bodies, long necks, small heads with jaws lined with interlocking needle-like teeth, and limbs that were recognisably limbs rather than flippers, with distinct digits that were probably webbed. They ranged from under a metre to around seven metres in the largest forms. Swimming appears to have been powered mainly by the body and tail, with the limbs used for steering and for movement on land, which is a different arrangement from the fully marine groups that came later and used the limbs for propulsion. The teeth interlocked when the jaws closed, forming a trap suited to seizing slippery prey rather than cutting it, which indicates a diet of fish and squid taken whole. The bones are dense, which reduced buoyancy and suited an animal that needed to stay down while hunting.

The group occupies a position that most marine lineages pass through quickly:

• •Limbs retained rather than converted, indicating an animal still capable of moving on land
• •Dense bone, which is characteristic of shallow water animals and contrasts with the lightened bone of open ocean forms
• •No evidence of live birth, so eggs were presumably still laid on shore, which ties them to land
• •A body plan suited to shallow coastal water rather than to the open sea
• •Close relationship to the plesiosaurs, which took the next step and became fully marine
• •An abundant record from the Triassic of Europe and China, which makes the group unusually well documented for something so transitional

The period these animals occupied was a strange one for marine life, since the seas were rebuilding after the end-Permian extinction and a large number of reptile lineages independently entered the water within a comparatively short interval. Alongside nothosaurs there were placodonts, heavy-bodied animals with flattened crushing teeth for shellfish, some with turtle-like shells built from separate bone. Ichthyosaurs were already present and were becoming fully marine. Thalattosaurs and several smaller groups added further variety. The concentration of independent marine invasions in that period is striking and is generally explained by the ecological opportunity left by the extinction, since the previous occupants of those roles were gone and the seas offered resources with reduced competition. Most of those groups did not survive the end of the Triassic.

Reconstructing the feeding behaviour of these animals has been possible in unusual detail because of what the deposits preserve. The tooth arrangement, with long interlocking teeth at the front of the jaws, is a trap rather than a cutting apparatus and matches animals that seize prey and swallow it whole. Neck length and flexibility suggest a strike delivered by rapid extension of the head while the body stayed relatively still, which is a strategy used by several living aquatic predators and which suits an ambush approach in shallow water. Gut contents in some specimens confirm a diet of fish and cephalopods. Trackways attributed to the group have been reported from shallow marine sediments and interpreted as the animal punting along the bottom with its limbs, disturbing prey from the sediment, which is a behaviour known in living rays and would fit an animal that retained functional feet.

Much of what is known comes from a set of quarries in southern Germany and Switzerland, particularly a mountain site worked over decades, where finely laminated marine sediments preserve complete articulated skeletons in large numbers. The material includes animals at various growth stages, occasional gut contents and a range of species from the same environment, which permits the community to be reconstructed rather than only its members. Excavation there began in the nineteenth century and continued intermittently, with campaigns in the twentieth century recovering thousands of specimens and establishing the site as one of the most productive marine reptile localities anywhere. Comparable deposits in China have added substantially in recent decades, including specimens with soft tissue and with embryos, and the two regions together account for most of the current understanding of Triassic marine reptiles.

Long necks, interlocking needle teeth forming a trap for slippery prey, and limbs with webbed digits rather than flippers, swimming with the body and tail. Dense bone, retained limbs and eggs laid on shore mark an animal not yet committed to the sea. They sit close to the ancestry of the plesiosaurs, which took the next step, and are known chiefly from Alpine quarries and from China.