Ichthyosaurs (ICK-thee-oh-sores) were giant marine reptiles that resembled dolphins. Ichthyosaurs belonged to the order known as Ichthyosauria or Ichthyopterygia. Ichthyosaurs lived and thrived during much of the Mesozoic. Ichthyosaurs are believed to have first appeared around 250 million years ago. These is also evidence that at least one species of Ichthyosaurs may have survived until about 90 million years ago, right into the Late Cretaceous period.

Who discovered Ichthyosaurs?

Mary Anning who was only 12 years old at the time, who was an amateur British fossil hunter has been credited with the first discovery of an Ichthyosaur fossil.

She discovered the Ichthyosaurs remains on the south coast of the UK nicknamed the “Jurassic Coast” sometime between 1809 and 1811.

Types of Ichthyosaurios

Nobu Tamura (http://spinops.blogspot.com), compiled by Levi bernardo, CC BY 3.0, via Wikimedia Commons

How big was Ichthyosaurs?

Ichthyosaurs measured around 8 feet (2.4 metres) in length and weighed around 163 to 168 kilograms (360 to 370 pounds). Although Ichthyosaurs looked like dolphins, however they were very different.

Ichthyosaurs had fin-like limbs, which were possibly used for stabilisation and direction control underwater, rather than propulsion. Ichthyosaurs speed through the water came from its 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. Ichthyosaurs had a porpoise-like head and a long, toothed snout, which gave Ichthyosaurs its Dolphin like appearance.

Built for speed

As already mentioned, Ichthyosaurs were very strong swimmers fully adapted to life in the seas. They were better adapted than any other reptiles, although they still needed to go to the surface periodically to breathe air.

Built for speed, some Ichthyosaurs appear also to have been deep divers. It has been estimated that Ichthyosaurs could swim at speeds up to 25 miles per hour (40 kilometres per hour).

What did Ichthyosaurs eat?

Ichthyosaurs were carnivores and they would have eaten early fish, and squid like creatures using their strong jaws and sharp teeth to snap at them at speed.

It is believed that Ichthyosaurs main source of food would have been Belemnites, which were early squid like animals.


James St. John, CC BY 2.0, via Wikimedia Commons

Due to the various sizes of Ichthyosaurs and as they were one of the most successful marine reptiles ever, it is believed that Ichthyosaurs likely had a wide range of prey. One distinctive feature of Ichthyosaurs were their large eyes, protected within a bony ring, suggested that they may have hunted at night.

Evolution of Ichthyosaurs

During the middle Triassic Period, Ichthyosaurs evolved from unidentified land reptiles that moved back into the water, in a development parallel to that of modern-day dolphins and whales. They were particularly abundant in the Jurassic Period, until they were replaced as the top aquatic predators by plesiosaurs in the Cretaceous Period

A reptile that didn’t lay eggs

Like modern day cetaceans such as whales and dolphins, they were viviparous which means that the embryo develops inside the body of the mother Ichthyosaurs, from which it gains nourishment. Although they were reptiles and descended from egg-laying ancestors, viviparity is not as unexpected as it first appears.

All 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 do today.


Heinrich Harder (1858-1935), Public domain, via Wikimedia Commons

When did Ichthyosaurs live and Die?

Ichthyosaurs were still common in the middle of the Jurassic period, but at this point Ichthyosaurs had now decreased in diversity. All remaining species of Ichthyosaurs all belonged to the single clade called Ophthalmosauria.

Ichthyosaurs seemed to decrease further in diversity in the Cretaceous Period. By this time only a single genus is known which was called Platypterygius, and although it had a worldwide distribution of this species existed, there was little diversity species-wise.

This last Ichthyosaur genus fell victim to the mid-Cretaceous or Cenomanian-Turonian extinction event. Ichthyosaur weren’t the only marine reptile to go extinct in this event, even the giant Pliosaurs could not escape. However, it’s important to note that less hydrodynamically efficient animals like Mosasaurs and long-necked Plesiosaurs flourished.


Last Updated on 15/07/2022 by