Ichthyosaurs - (pronounced IHK thee uh sawrs)

Ichthyosaurs (meaning 'fish' and 'lizard') were giant marine reptiles that resembled fish and dolphins.

 

Ichthyosaurs lived during a large part of the Mesozoic era, having appeared about 230 million years ago, slightly when the dinosaurs appeared (230 million years ago); and disappeared about 90 million years ago, about 25 million years before the dinosaurs became extinct.

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.

Ichthyosaurs were 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.

They belong to the order known as Ichthyosauria or Ichthyopterygia. Ichthyosaurs averaged 2 to 4 metres in length, 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. It has been estimated that ichthyosaurs could swim at speeds up to 40 km/h (25 mph).

Similar to modern cetaceans such as whales and dolphins, they were air-breathing and also were viviparous (embryo develops inside the body of the mother, from which it gains nourishment, as opposed to in an egg). 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.

Given their streamlined bodies, heavily adapted for fast swimming, it would have been difficult for ichthyosaurs to scramble successfully onto land to lay eggs.

They measured around 2.4 metres (8 feet) and weighed around 163 to 168 kg (360 to 370 lb).

Although ichthyosaurs looked like fish, they were not. Ichthyosaurs had fin-like limbs, which were possibly used for stabilisation 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.

Ichthyosaurs were carnivores and they ate fish, octopus and other swimming animals with their strong jaws and sharp teeth. They also relied heavily on ancient cephalopod called belemnites. However, 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.

Ichthyosaurs were still common in the Middle Jurassic, but had now decreased in diversity. All belonged to the single clade Ophthalmosauria.

Ichthyosaurs seemed to decrease in diversity even further with the Cretaceous. Only a single genus is known, Platypterygius, and although it had a worldwide distribution, there was little diversity species-wise. This last ichthyosaur genus fell victim to the mid-Cretaceous (Cenomanian-Turonian) extinction event (as did some of the giant pliosaurs), although ironically less hydrodynamically efficient animals like mosasaurs and long-necked plesiosaurs flourished.

Ichthyosaur fossils have been found in North and South America and Europe. The Ichthyosaur is the state fossil of Nevada, USA.

Scientific Classification:

Kingdom: Animalia

Phylum: Chordata

Class: Sauropsida

Subclass: Diapsida?

Superorder: Ichthyopterygia

Order: Ichthyosauria