Ichthyosaur – Stenopterygius acutirostris

Ichthyosaur. Artist: Frey Micklethwait. Source: Museum Victoria.

Ichthyosaurs were marine reptiles which looked remarkably like modern dolphins. However the two types of animals were unrelated, ichthyosaurs were reptiles which lived in the sea at the same time as the dinosaurs, while dolphins are mammals which adapted to the marine habitat long after the ichthyosaurs and dinosaurs had become extinct. The ancestors of ichthyosaurs and the ancestors of dolphins became sea-dwelling creatures at different times and evolved strikingly similar fish-like body forms. Both ichthyosaurs and dolphins, however, continued to be air-breathing animals.

We know that the ichthyosaurs gave birth to live young rather than laid eggs because there are fossils of females with the young half expelled from the body of the mother. Fossil evidence also reveals that ichthyosaurs ate cephalopods (like squid), fish, and even an occasional pterosaur. They had large eyes, a long snout, and their limbs were modified into broad fins. Their tail bones ended in a 'kink' which ran along one of their two large tail flukes.

Some of the earliest ichthyosaurs, which lived in the Late Triassic of North America about 225 million years ago, were giants that were more than 10 metres in length. In their history, which spanned more than 100 million years, successive families arose, persisted for a time and then became extinct. To all but the specialist, there was little difference between them—other than size. All of the ichthyosaurs had become extinct long before the end of the Cretaceous Period.

On display at Melbourne Museum is a fossil skeleton of Stenopterygius acutirostris in rock matrix from Germany.