Reptiles return to the sea

By about 248 million years ago, only a few million years after the Great Dying, several reptile lines have moved back into the sea. The earliest known ichthyosaurs, such as Chaohusaurus and Cartorhynchus from the Early Triassic of China, are small, slender swimmers. Cartorhynchus, described by Ryosuke Motani and colleagues in 2014, is about 40 cm long, with a short snout for suction feeding and flippers that could still bend enough to move over land. Alongside them appear the sauropterygians, the group of nothosaurs, placodonts and eventually plesiosaurs. Chaohusaurus, about a meter long, swam in shallow seas in what is now Anhui Province, China, and probably chased fish and squid.¹⁴

The oceans offered open niches. The Great Dying had wiped out most marine animals, but squid-like cephalopods and fish recovered quickly and made a rich food supply with few big predators. Air-breathing amniotes with strong limbs and, in the ichthyosaur case, live birth at sea were well placed to exploit it. A pregnant Chaohusaurus fossil from China shows a baby being born, proof of live birth in the water.²⁴

Ichthyosaurs evolved dolphin-shaped bodies, vertical tail flukes and, in some, such as Ophthalmosaurus, eyes more than 20 cm wide. Triassic giants include Shonisaurus popularis, about 15 m long, whose bones lie in Nevada's Berlin-Ichthyosaur State Park. Ichthyosaurs lasted until about 90 million years ago, while plesiosaurs and later mosasaurs kept the sea reptile tradition going to the end of the Cretaceous.³⁴