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The timeless swimmer
7/10/13

A rigorously selective evolution…

Malawania is therefore a strange evolutionary exception. “Animals, and in particular, marine reptiles, tend to evolve steadily, notably, under the influence of a very strong selective pressure in this ecological environment”, explains the palaeontologist. Initially, ichthyosaurs, like all marine reptiles, descended from a land animal. Their arrival into this new environment “obliged” them to evolve rapidly with regard to hands, the spinal column and metabolism… The ichthyosaurs then diversified, occupying several levels of the food chain.

The ichthyosaurs were originally coastal animals, feeding on small animals or algae in the case of some species. The ichthyosaurs specialized in deep and rapid swimming, becoming first-rate pelagic hunters.  Other characteristics also changed. Living further and further away from the coasts and having lost the faculty to move on dry land, the ichthyosaur had to adopt a metabolism that resembled that of warm-blooded animals. Unable to find a sunbaked rock in the middle of the ocean to warm itself in the manner of reptiles on land, it depended on its muscular activity to warm its body. Because of this, the animal had a more voracious appetite than that of other reptiles in order to satisfy its energy needs. Gradually, coastal types of Ichthyosaur disappeared and the animal adopted the wider oceans as its habitat. By now, it had become very different to its land-dwelling ancestor and other marine reptiles that were its contemporaries.

But then, even the Baracromia, which were already quite “sophisticated”, continued to evolve during the Jurassic and Cretaceous periods. Some of the Opthalmosaurinae, for example, acquired the biggest eyes known in the animal kingdom with pupils that were nearly 10 centimeters in diameter, enabling them to see into the depths and detect prey up to 30 meters further away than the other ichthyosaurs (2). Thanks to the study of these pupils but also to the study of histology, and the detection of the “Caisson’s disease”, it was deduced that certain representative examples of the species could see and dive to depths of 500 meters, while others remained much closer to the surface. The different generations of ichthyosaurs constantly diversified and specialized.

“And then we came across Malawania, which displayed characteristics that were undoubtedly primitive, which made it the last known representative of a line that was confined to the early Jurassic”, said the surprised researcher.

Phylogenetic ichtyosaur

 

(2) Fischer V, Arkhangelsky MS, Uspensky GN, Stenshin IM, Godefroit P (2013) a new Lower Cretaceous ichthyosaur from Russia reveals skull shape conservatism within Opthalmosaurinae. Geological Magazine In press.

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