In the seas, there were more and more invertebrate species. Belemnites and ammonites increased and were the most important of the invertebrates during the Jurassic Period. Gastropods, or sea snails, were a mollusk that grew plentiful in the Jurassic period. Corals began to produce reefs in the warm seas. There were many types of bony fishes, sharks and rays swimming in the tropical seas.

The seas of the Jurassic Period were filled with predators. The ammonites, belemnites, and fish were all excellent hunters. This may be the reason for some species? numbers growing fewer. There were not as many brachiopods or crinoids as in the previous periods. These animals, that stayed fixed on the sea floor, could not complete with the moving predators for food.

The marine reptiles that had gotten their start during the Triassic Period increased in numbers, too. Ichthyosaurs and plesiosaurs were most plentiful during the Jurassic Period. They shared the sea with early crocodiles.

As the land pulled apart, the seas rose. Warm shallow seas again covered parts of Laurasia (North America and Eurasia). With the warm seas touching the land, the climate became more humid and tropical. Plants grew thick and tall. There were so many cycads that the Jurassic is also known as the Age of Cycads. Cycads are a seed plant with leaves that look like palms. They have heavy trunks. Cycads still exist in modern times, but they are not as large or as plentiful as the ones that lived during the Jurassic Period. All of this plant life fed the many different types of herbivorous dinosaurs that developed. Of course lots of herbivores means good eating for the carnivores. So many different types of dinosaurs lived during this period that it has been called the Age of the Dinosaurs.

Dinosaurs were the big story during the Jurassic period. The first dinosaurs developed in the Triassic Period but in the Jurassic and Cretaceous Periods they dominated. There were Sauropods-dinosaur giants, theropods-two legged hunters and ornithopods-bird hipped plant eaters. Large and small, herbivore and carnivore, dinosaurs ruled the earth. But that is another story.