Ichthyosaurs - extinct dolphin-like reptiles

Ichthyosaurs were fish-shaped marine reptiles. They cruised the world's oceans during the age of dinosaurs, from 200 to 90 million years ago. Ichthyosaurs were built for speed, with elongated snout, spindle shaped body, shark-like tail fin and stream-lined flippers. Like dolphins they hunted small, swimming prey, breathed at the surface and gave birth to well developed young at sea.

A 1:10 scale model of a juvenile ichthyosaur. When it reached full adult size it may have reached over 8m in length. Based on sub-adult fossils from South Australia and Queensland.

A diagram showing the skeleton of an ichthyosaur, probably Platypterygius longmani

This ichthyosaur backbone discovered on the Moon Plain near Coober Pedy was not opalised. If it had not been excavated, it would have crumbled to dust in a few years.

An ichthyosaur skull discovered by Ben Kear and Robert Jones of the Australian Museum from Cretaceous rocks near Hughenden in Queensland.

Close up of the ichthyosaur skull.

See if you can spot the similarities and differences.

Similarities and differences

Unlike dolphins, ichthyosaurs were reptiles that swam using side to side movements of the tail. Their shapes resulted from evolutionary adaptations or changes to suit life in an aquatic environment. Most mammals use up and down movements of the body, whether running on land or swimming in the ocean. Dolphins are mammals and therefore more closely related to humans than to the reptilian ichthyosaurs.

Ichthyosaurs are (functionally) more closely related to fish in the use of side to side movements of the tail. But they are not related to each other!

The giant marine reptiles of the past, such as plesiosaurs, pliosaurs, ichthyosaurs and turtles, were only very distant cousins to the dinosaurs. Birds and crocodiles are the only surviving close relatives of dinosaurs.