About 200 million years ago, nearly 20% of marine species and a significant portion of large land vertebrates disappeared in what has been called the Triassic-Jurassic extinction. It is not clear what caused it, perhaps a series of events, for example in relation to volcanism and perhaps also the fact that this mass extinction occurred around the time Pangea fractured.
Still, the paleontologistspaleontologists think that it allowed theradiative explosionradiative explosion of the dinosaursdinosaurswhich become dominant, and mammalsmammals by freeing up ecological niches.
Among the orders of life affected, but not extinct at that time, there is the order of Ichthyosauria (Greek ιχθύς meaning “ fishfish ” And σαύρος meaning “lizard”) introduced by Richard Owen in 1840, the inventor of the term “dinosaur. We owe in particular the identification of this order of vertebrates tetrapodstetrapods to the self-taught British paleontologist Mary Anning (1799-1847) when she was twelve years old, when she discovered the first skeleton ofichthyosaurichthyosaur found complete, on the English coast of Lyme Regis.
Ophthalmosaurus is a marine reptile from the Jurassic and Cretaceous. This ichthyosaur breathed with lungs. The young, which were born directly in the water, therefore had to emerge from their mother’s womb tail first and quickly rise to the surface to breathe in the open air for the first time. To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Automatically translate”. Choose “French”. © BBC Earth, YouTube
An evolutionary convergence with dolphins
Ichthyosaurs are marine diapsid vertebrates which resemble current dolphins and which, like them, must have come to breathe theairair on the surface of the waters. Like them and also whales, they are known to be descended from terrestrial vertebrates. But they are not at the origin of dolphins, this is also an example of what we call evolutionary convergence. The possible adaptations are constrained by the laws of biomechanics, so there are no 36,000 ways to have a particularly hydrodynamic body.
As the morphologymorphology ichthyosaurs are very similar to that of dolphins, we can deduce that their way of life must have been quite similar. However, we know that in addition to fish they ate a lot ofammonitesammonites and belemnites. In fact, we found the remains (rostrarostra And shellsshells) of these species which disappeared like the dinosaurs with the Cretaceous-Tertiary extinction, in their stomachsstomachs.
Remember that dinosaurs are purely terrestrial animals, ichthyosaurs – of which we know about ten families and many more species – are not dinosaurs, any more than the others reptilesreptiles sailors from the age of dinosaurs, even if they were giants.
Many ichthyosaurs were large, but it seems that the most gigantic ones all disappeared during the Triassic-Jurassic crisis. The more modest species will, however, continue to prosper until the beginning of the CretaceousCretaceous higher, all disappearing around 90 million years ago, perhaps due to competition with other predatory marine reptiles such as pliosaurs and mosasaurs.
A still mysterious species
One of the last giant ichthyosaurs, probably the largest species of marine reptile ever described, has just been officially introduced into the world of paleontologypaleontology with a study published this April 17 in the open access journal Plos One by Dean Lomax of the University of Bristol and the University of Manchester (UK), and colleagues. The new species was called Ichthyotitan severnensis. It could have measured 25 meters long.
We do not yet know much about it because only two jaws are known, and still in the form of fragments discovered in recent years in the formation Westbury Mudstone in Somerset, United Kingdom.
In the press release of Plos One, accompanying the publication of the scientific article, Dean Lomax explains: “ In 2018, my team (including Paul de la Salle) studied and described the giant jawbone and we had hoped that one day another would be discovered. This new specimen is more complete, better preserved and shows that we now have two of these giant bones (called surangulars) which have a unique shape and structure. It is quite remarkable to think that gigantic ichthyosaurs the size of a blue whaleblue whale swam in the oceans around what was the United Kingdom during the Triassic period. These jaws provide tantalizing evidence that perhaps one day, a skullskull or a complete skeleton of one of these giants could be discovered. We never know. »
