Cryptocleidus is an extinct genus of plesiosaur that lived during the Early Jurassic period, around 200 million years ago. Plesiosaurs were a group of marine reptiles that lived during the Mesozoic era, and Cryptocleidus was a relatively small member of this group, growing to about 3 to 4 meters in length.

Cryptocleidus had a distinctive body shape, with a broad, flat body, a short tail, and four large flippers that were used for swimming. Its neck was relatively short compared to other plesiosaurs, with only around 25 vertebrae, and its skull was relatively small with sharp teeth. Cryptocleidus likely fed on small fish and other marine animals, using its sharp teeth to capture and consume prey.

Fossils of Cryptocleidus have been found in various locations around the world, including the United Kingdom, Germany, and Argentina. Its discovery and study have helped researchers better understand the evolution and diversity of marine reptiles during the Mesozoic era.