There were many marine animals that thrived during the Jurassic period, ranging from small seabed dwellers to large swimming predators. Most of these animals were similar to those found in the Triassic period, although many had diversified and increased in number during the Jurassic period.

Modern shark families developed at this time; bony fishes with symmetrical tails, the teleosts, diversified (they account for the great majority of modern fishes—over 20,000 species). The first oysters evolved; modern squids and cuttlefishes appeared; and squid-like belemnites diversified. The ammonoids almost disappeared during the late Triassic extinctions; one family survived (out of eight) and quickly diversified during the Jurassic period.

The ichthyosaurs, such as the Ichthyosaurus and Stenopterygius, flourished in the Jurassic period oceans. Plesiosaurs were also abundant, such as the Muraenosaurus, a Late Jurassic period reptile with a 36-foot (11-meter) neck (it included 40 vertebrae); and the Liopleurodon, a short-necked pliosaur with a huge, elongated head was 39 feet (12 meters) long, with a 10-foot (3-meter) head.