The fossil record shows that ichthyosaurs were around from 245 million years ago, ands survived up to 90 million years ago, which is almost as long as the dinosaurs. Ichthyosaur fossils are found worldwide, suggesting they migrated just like modern whales. Fossils of the most primitive known ichthyosaurs, from the early Triassic, were not found until 1927 and even these only amounted to disarticulated fragments on the whole. Although the first ichthyosaurs were more lizard shaped, they soon assumed a difinitive, torpedo shaped body and the formerly stocky legs of their ancestors changed into flippers. They also developed a stabilising dorsal fin and a boneless tail fluke, perfect for life in the open ocean. With these extreme adaptations, most of the ichthyosaurs lost key terrestrial tetrapod features like wrist and anklebones that would have made it possible to recognise their cousins on the land. Lack of fossil evidence of the earliest ichthyosaurs led to them being assigned to almost every major vertebrate group. However, as techniques in deciphering the phylogeny of animal species improved, palaeontologists started to agree that ichthyosaurs were indeed reptiles of the group, Diapsida. But exactly where they branched of remained a mystery until the discovery in Asia of the earliest ichthyosaur fossils known so far. The first major discovery was made on a north-western beach on the island of Honshu, Japan. Hokkaido University excavated 2 near complete specimens in 1982. Over a period of 15 years the fragile skeletons were painstakingly cleaned. When Ryosuke Motani went to see one of the fossils he realised that it was what palaeontologists had been expecting to find for years, an ichthyosaur that looked like a lizard with flippers. It was named Utatsusaurus and is the nearest thing to a missing link between its terrestrial ancestors and all ichthyosaurs. It is now thought that Ichthyosaurs branched off from the rest of the Diapsids near the separation of lepidosaurs and archosaurs. Ichthyosaurs rapidly diversified from then on, becoming one of the dominant marine reptile groups througout the Jurassic and in to the Cretaceous.
Most ichthyosaurs are preserved in a flattened state, however some are preserved in three dimensions within nodules. Below is a sketch of a Temnodontosaurus skull, that was preserved in three dimensions.
Author: P. T. Hadland
Last updated: Date 21st November 2005
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