Old Earth Ministries Online Dinosaur Curriculum

Free online curriculum for homeschools and private schools

From Old Earth Ministries (We Believe in an Old Earth...and God!)

NOTE:  If you found this page through a search engine, please visit the intro page first. 


Lesson 73 - Ichthyosaur

Ichthyosaurs (Greek for "fish lizard") were giant marine reptiles that resembled fish and dolphins. Ichthyosaurs thrived during much of the Mesozoic era; based on fossil evidence, they first appeared approximately 245 million years ago (mya) and disappeared about 90 million years ago, about 25 million years before the dinosaurs became extinct. During the middle Triassic Period, ichthyosaurs evolved from as-yet unidentified land reptiles that moved back into the water, in a development parallel to that of the ancestors of modern-day dolphins and whales. They were particularly abundant in the Jurassic Period, until they were replaced as the top aquatic predators by another reptilian order named plesiosaurs in the Cretaceous Period.


Quick Facts


Length:  6 - 13+ feet

Date Range:   245 - 90 Ma, Triassic -  Cretaceous Period



Above: CGI reconstruction of Ichthyosaurus communis   (Picture Source)
Below: Mounted fossil of Temnodontosaurus trigonodon   (Picture Source)


Ichthyosaurs averaged 24 meters (6.613 ft) in length (although a few were smaller, and some species grew much larger), with a porpoise-like head and a long, toothed snout. Built for speed, like modern tuna, some ichthyosaurs appear also to have been deep divers, like some modern whales. It has been estimated that ichthyosaurs could swim at speeds up to 40 kilometres per hour (25 mph). Similar to modern cetaceans such as whales and dolphins, they were air-breathing.

According to weight estimates by Ryosuke Motani a 2.4 meters (7.9 ft) Stenopterygius weighed around 163168 kilograms (360370 lb) whilst a 4 meters (13 ft) Ophthalmosaurus icenicus weighed 930950 kilograms (1.031.05 short tons).

It has been determined by teeth records that several sea-dwelling reptiles, including Ichthyosaurus, had a warm-blooded metabolism similar to that of mammals. They had the ability to generate endothermic heat to survive in colder habitats.

Although ichthyosaurs looked like fish, they were not. Ichthyosaurs had fin-like limbs, which were possibly used for stabilization and directional control, rather than propulsion, which would have come from the large shark-like tail. The tail was bi-lobed, with the lower lobe being supported by the caudal vertebral column, which was "kinked" ventrally to follow the contours of the ventral lobe.

Apart from the obvious similarities to fish, the ichthyosaurs also shared parallel developmental features with dolphins, lamnid sharks, and tunas. This gave them a broadly similar appearance, possibly implied similar activity levels (including thermoregulation), and presumably placed them broadly in a similar ecological niche.


They were viviparous (bore live young). Some adult fossils have even been found
Adult and juvenile Stenopterygius quadriscissus   (Picture Source
 containing fetuses. Although they were reptiles and descended from egg-laying ancestors, viviparity is not as unexpected as it first appears. Air-breathing marine creatures must either come ashore to lay eggs, like turtles and some sea snakes, or else give birth to live young in surface waters, like whales and dolphins. Given their streamlined bodies, heavily adapted for fast swimming, it would have been difficult for ichthyosaurs to move far enough on land to lay eggs.


For their food, many of the fish-shaped ichthyosaurs relied heavily on ancient cephalopod kin of squids called belemnites. Some early ichthyosaurs had teeth adapted for crushing shellfish. They also most likely fed on fish, and a few of the larger species had heavy jaws and teeth that indicated they fed on smaller reptiles. Ichthyosaurs ranged so widely in size, and survived for so long, that they are likely to have had a wide range of prey. Typical ichthyosaurs have very large eyes, protected within a bony ring, suggesting that they may have hunted at night.

Return to the Old Earth Ministries Online Dinosaur Curriculum homepage.

horizontal rule