Age of the Dinosaurs III: Life in the Seas

Around 250 million years ago there were already animals living in the oceans. They had evolved back from terrestrial animals and were therefore part of the reptile family. (1) Each period in the Mesozoic (the age of reptiles) had its own specific types of sea life, but all of them fed on fish, shrimps and shellfish. (2)

During the Triassic Period there was a huge ocean called the Tethys Sea which stretched across Southern Europe. The oldest group of reptiles that had adapted to life in this ocean were placodonts, who fed on shell fish and still needed to surface in order to breathe. They shared the water with nothosaurs, whose body was more streamlined for aquatic life. It had fins on its legs, but could still walk on land. (1)


1. Nothosaur from the Triassic Period

From the nothosaurs another group of aquatic reptiles emerged: plesiosaurs. This is possibly the most varied group of swimming reptiles and can be split into two groups. One group, characterized by their short necks, is called pliosaurs, while the long-necked group is called elasmosaurs. Both of these groups had species in all kinds of sizes and they occupied the oceans in the Jurassic and Cretaceous Period (1)

But the most well adapted sea reptiles in the Mesozoic were the ichthyosaurs. These reptiles resembled dolphins with their streamlined bodies, back-fin, paired fins on either side, and large fin on the tail. Like dolphins, they had to surface in order to breathe. Ichthyosaurs developed in the Triassic Period and persisted until the early Cretaceous Period, when they were replaced by mosasaurs until the end of the Mesozoic. (1)


2. Platypterygius kiprijanovi, an ichthyosaur species

Aside from the reptiles in the seas, there was one other major development below the surface. From the Jurassic Period onwards the marine reptiles shared the oceans with numerous fish species. The development of bonefish, where most modern fish species are a part of, had begun, but they were still overshadowed by the older group of fish: ray-finned fish. (3)


References:

1. Dixon, D., In the Sea, 2001, Ticktock Publishing
2. Amersham, B., National Geographic Dinosaurs, 1999, Firecrest Books (page 16)
3. Palmer, D., Atlas of the Prehistoric World, 1999, Marshall, London (page 101)

Picture credits:

All pictures from Wikipedia Commons