Marine Reptiles Across the Ages

Around 312 million years ago ("MYA"), Amniotes developed adaptations that would keep them alive on land at all stages of their life cycle, allowing them to colonize the interior of the continents, and diversify into many groups, including reptiles. Chief among these was the amniotic egg, which had internal membranes to keep the embryo moist on dry land and allow for gas exchange from the air rather than the water. One would assume that reptiles had left the sea for good. But this is not the case. Below, I will discuss five different species of marine reptiles, each with different adapttions for life in the sea.

Nothosaurus lived in the Triassic period, about 240 MYA, and was small compared to some of the later marine reptiles, about the size of a seal. It still had a very reptilian body plan, but there were some key differences. Its feet were webbed, its tail was muscular, for swimming, and it had pointy, thin teeth designed for holding on to slippery prey. It is believed to be only semi-aquatic. Although not as impressive as some of the later marine reptiles, it is important, because something like Nothosaurus was the ancestor of the mighty plesiosaurs.

Metriorhynchus lived in the late Jurassic period, about 167 MYA. It was a crocodyliform, and had salt glands for excreting excess salt, flippers, and a finned tail. It is known to have scavenenged Leedsichthys, a massive fish, and was probably also a predator. It had long, slightly curved, conical teeth.

Elasmosaurus lived about 80 MYA, during the late Cretaceous period. It was fully aquatic, and had flippers, a short tail, a small head, and of course its defining feature: a neck longer than its body and tail combined. It probably hunted schools of fish. They were probably viviparous, i.e., had live birth. (There are fossils of other species of plesiosaur with fetuses.)

Tylosauruss was an apex predator. It had multiple rows of teeth, a long tail with a fin at the end, paddle like flippers, and scales (most marine reptile skin impressions that we have are smooth). Tylosaurus was a squamate, related to snakes and lizards, though the scientific community is still debating whether it is more closely related to snakes (because of adaptations like its extensible jaw), or lizards. It lived in the late Cretaceous, about 80 MYA.

Archelon was a magnificent chelonian, reaching up to 12 feet in length. It had many of the adaptations of modern sea turtles, including large flippers for swimming, a leathery carapace instead of a hard shell, and a robust beak for crushing shells. It likely ate mollusks and crustaceans. It lived in the late Cretaceous, about 70 MYA.