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Ichtyosaure

Ichtyosaurs

There is an important concept in biology known as "convergent evolution": animals that occupy similar evolutionary niches tend to adopt roughly similar morphologies. The ichthyosaurs are the perfect example: Beginning about 200 million years ago, these aquatic reptiles evolved physical characteristics and behaviors that are very similar to those of modern dolphins and bluefin tunas that populate extant oceans.

The ichthyosaurs (Greek for lizardfish) were also similar to dolphins in another, even more telling way. These aquatic predators are thought to have evolved from a population of archosaurs (a family of land reptiles ancestors of dinosaurs) that returned to the marine environment during the Lower Triassic period. Similarly, the origin of dolphins and whales can be traced to ancient quadruped prehistoric mammals that have gradually evolved into marine life.

The earliest ichtyosaurs

Anatomically speaking, the first ichthyosaurs of the Mesozoic era can easily be distinguished from more advanced genera. The ichthyosaurs of the Middle and Upper Triassic Periods, such as Grippia, Utatsusaurus and Cymbospondylus, had mostly neither the dorsal fin nor the hydrodynamic form of the later species. Some palaeontologists even doubt that these aquatic reptiles were true ichthyosaurs and prefer to call them proto-ichthyosaur or "ichthyopterygians". The vast majority of ichthyosaurs were relatively small but some gigantic species, such as Shonisaurus, could reach lengths of 60 and even 70 feet!

Although evolutionary relationships remain uncertain, there is ample evidence that the correctly named Mixosaurus was a transitional form between early ichthyosaurs and later ones. As one might guess by its name (Greek for mixed lizard), this aquatic reptile combined the primitive features of the early ichthyosaurs - a rigid tail pointing downwards and short fins - with the slimmer, hydrodynamic shape and fast swimming style of the offspring. In addition, unlike a large proportion of ichthyosaurs, fossils of Mixosaurus have been found throughout the world, indicating that this aquatic reptile was particularly well adapted to its environment.

Evolutionary trends in ichthyosaurs

The Lower and Middle Jurassic periods were the golden age of ichthyosaurs with important species like Ichtyosaurus, which is represented by hundreds of fossils, as well as its close relative Stenopterygius. Apart from their hydrodynamic forms, these aquatic reptiles were distinguished by their strong ear bones (which allowed the detection of subtle vibrations created in the water by the movement of prey) and their large eyes (the eyeballs of a species in particular, Ophthalmosaurus, were 4 inches wide).

By the end of the Jurassic period, almost all ichthyosaurs had disappeared from the oceans - but one genus, Platypterygius, survived to the Lower Cretaceous period possibly because it had developed an omnivorous diet (a fossil specimen of this ichthyosaur contained remains of birds and baby turtles). Why have ichthyosaurs disappeared from the world's oceans? The answer may lie in the evolution of faster prehistoric fish (which avoided being eaten) and better adapted aquatic reptiles such as plesiosaurs and mosasaurs.

A recent discovery, however, casts a shadow of doubt over the accepted theories about the evolution of ichthyosaurs. Malawania sailed the waters of Central Asia during the Lower Cretaceous although it kept the ancestral traits of species that had lived tens of millions of years earlier. Clearly, if Malawania could flourish with such basic anatomy, all ichthyosaurs did not compete with other aquatic reptiles and so other factors are at stake in their extinction.

The way of life and behavior of ichthyosaurs

Despite the similarity of some species with dolphins and bluefin tuna, ichthyosaurs were reptiles and not mammals or fish. All these animals, however, share a set of adaptations similar to their marine environment. Like dolphins, it is believed that ichthyosaurs gave birth to live young rather than lay eggs as their terrestrial counterparts did. There are remains of ichthyosaurs, such as Temnodontosaurus, fossilized in the act of giving birth.

Finally, although ichthyosaurs had many characteristics of fish, they had lungs and not gills and therefore had to regularly come up to the surface to breathe. It is easy to imagine an Excalibosaurus bench frolicking above the waves of the Jurassic friendly fighting with each other with their swordfish snouts (an adaptation that allowed some ichthyosaurs to harpoon fishes on their paths).