Ichthyosaur

Ichthyosauria Temporal range: Lower Triassic – Upper Cretaceous
Dolichorhynchops
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Sauropsida
Superorder:	†Ichthyopterygia
Order:	†Ichthyosauria Blainville, 1835
Families
†Grippidae †Utatsusauridae †Parvinatatoridae †Ichthyosauridae †Leptonectidae †Mixosauridae †Ophthalmosauridae †Shastasauridae †Stenopterygiidae †Teretocnemidae

Drawing from an 1814 paper by Everard Home for the Royal Society showing skull of Ichthyosaur found by the Annings. Note sclerotic ring in eye orbit.

Ichthyosaurs are an extinct order of marine reptiles from the Mesozoic era.

They are similar in shape to dolphins and to fast predatory fish like tuna (convergent evolution). They are found in marine strata from the earliest Triassic to the Cretaceous, with essentially the same body shape.

Although isolated ichthyosaur vertebrae are quite common, the first fossil which showed the ichthyosaur form was found by Mary Anning (1799–1847) and her brother Joseph. Mary Anning was an early British fossil collector, dealer and paleontologist. Many of her finds may be seen today at the Natural History Museum, London.

The first find

Mary Anning's first great discovery was made shortly after her father's death when she was just twelve. In 1810 her bother Joseph found what he thought was a crocodile skull, but the rest of the animal was not in evidence. Mary kept searching and a year later a storm weathered away part of the cliff and exposed part of the rest of the skeleton of the 17 ft (5.2m) long creature, which she was able to dig out of the cliff and collect with a little help from local quarrymen.^p33–41 Anning later discovered three more species.

Interpretation

In 1821 William Conybeare and Mary’s old friend Henry De la Beche, published a paper which analyzed the specimens found by Mary and others. They concluded that ichthyosaurs were a previously unknown kind of marine reptile, and they decided there had been at least three species.

Anatomy

They have no neck, streamlined form, paddles, and fish-like tail. Ichthyosaurs averaged about 2–4m (6–13 ft) in length; a few were smaller, and some species grew much larger. They had 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 metres (7.9 ft) Stenopterygius weighed around 163–168 kilograms (360–370 lb) whilst a 4 m (13 ft) Ophthalmosaurus icenicus weighed 930–950 kg (1.03–1.04 short tons).

Biologist Stephen Jay Gould said the ichthyosaur was his favourite example of convergent evolution. Its similarities to fish are not homologous, caused by common descent, but analogous, caused by selection in the same environment:

This sea-going reptile with terrestrial ancestors converged so strongly on fishes that it actually evolved a dorsal fin and tail in just the right place and with just the right hydrological design. These structures are all the more remarkable because they evolved from nothing – the ancestral terrestrial reptile had no hump on its back or blade on its tail to serve as a precursor.

Eyes

The eyes of ichthyosaurs are large in comparison to the size of the body, suggesting they hunted by sight even in dimly-lit waters or at night. In fact, ichthyosaurs hold the record for eye size: the largest was 264mm in diameter (~10.4 inches), from the species Temnodontosaurus platyodon. This is the largest eye ever recorded for any vertebrate; the colossal squid eye is larger still. The eyes of ichthyosaurs are protected by sclerotic rings. These are circlar bony plates inside the outer edges of the cornea of the eye. Their function is mechanical support of the eye. They are well preserved in fossil ichthyosaur skulls.

Fins

Unidentified ichthyosaur: body outlined (in paint) shows the tail and dorsal fins

At first, the downward-pointing tail bones were hard to interpret. Then, later in the 19th century, a remarkable find was made. Some specimens of Mixosaurus and Stenopterygius were well preserved in the black mudstones of Germany. These fossils were preserved in such fine sediment that the fossils were almost perfect, with only a few bones disturbed or missing. The outline of the body shape could be seen as a thin carbon film.^p10 Even though the soft parts had rotted away long ago, they left their imprint behind. For the first time, scientists could see clearly that the tail of ichthyosaurus was shaped like the tail of a fish. And on its back, like a shark, there was a dorsal fin.

Live birth

Live birth was always assumed for ichthyosaurs, because with their paddles they could never have manoeuvred on land to lay eggs like turtles do. As reptiles, they produced cleidoic eggs, but they were entirely aquatic and their eggs developed inside the mother. Once more, evidence came from the early Jurassic of Germany, where several fine examples clearly show embryos inside the rib cage of the adult. Other examples exist of the young actually being born.^p125 They came out tail first. No doubt, some complication of birth had killed both mother and offspring together.

Extinction

Ichthyosaurs became extinct during the Upper Cretaceous, about 30 million years before the K/T extinction event. There was an ocean anoxic event at the Cenomanian–Turonian stage boundary. The deeper layers of the seas became anoxic and poisoned by hydrogen sulphide (H₂S). As life died off in the lower (benthos) levels of the sea, so did the predators at the top of the food chain. The last pliosaurs and ichthyosaurs became extinct.^p251

Ichthyosaurs had been dwindling in numbers for some time; they were no longer the force they once were in the Upper Triassic and Lower Jurassic. By the middle Jurassic, all belonged to the single clade Ophthalmosauria. By the Cretaceous only three genera survived. Only one genus, Platypterygius, is known at the time of the anoxic event in the Upper Cretaceous.

No firm reasons can be given for the decline of the ichthyosaurs. By the Cretaceous they certainly had more competitors than in the Triassic, and more elusive prey. The adaptive radiation of teleost fish meant their new prey were fast swimming and highly evasive. The teleosts also included fast-swimming predatory fish, which must have been competitors. This, together with the extinction event, was apparently enough to end their long history.

The ichthyosaurs and pliosaurs were replaced in the marine ecology by the giant mosasaurs. The mosasaurs were probably ambush hunters, whose sit-and-wait strategy apparently proved most successful.

References

• Ellis, Richard 2003. Sea Dragons: predators of the prehistoric oceans. University Press of Kansas.
• McGowan C. & Motani R. 2003. Ichthyopterygia. Handbook of Paleoherpetology, Part 8, Verlag Dr. Friedrich Pfeil.
• Motani R. 2000. Rulers of the Jurassic seas. Scientific American vol.283, #6.