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Ichthyosaurs were sea reptiles which evolved in the Middle Triassic, reached the peak of their development in the Jurassic, and became extinct in the Upper Cretaceous. The presented skeleton of an ichthyosaur Ichthyosaurus communis was preserved in slates.

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Ichthyosaurs were sea reptiles which evolved in the Middle Triassic, reached the peak of their development in the Jurassic, and became extinct in the Upper Cretaceous. The presented skeleton of an ichthyosaur Ichthyosaurus communis was preserved in slates.

Elaborated by Barbara Kietlińska-Michalik (The Geological Museum of the Institute of Geological Sciences, Polish Academy of Sciences), © all rights reserved

Bibliography:
Franciszek Bieda, Paleozoologia, vol. I, Warsaw 1966;
Urszula Radwańska, Podstawy paleontologii, Warsaw 2007.

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Ichthyosaurs

Ichthyosaurs, known otherwise as fish lizards, reigned supreme in the waters of the oceans for 155 million years. No other tetrapod (with the exception of cetaceans) staged a return to the aquatic environment in such style. However, to this day we do not know where ichthyosaurs came from and why they became extinct so long before the dinosaurs...

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Ichthyosaurs, known otherwise as fish lizards, reigned supreme in the waters of the oceans for 155 million years. No other tetrapod (with the exception of cetaceans) staged a return to the aquatic environment in such style. However, to this day we do not know where ichthyosaurs came from and why they became extinct so long before the dinosaurs.
The first species was correctly recognized as a marine reptile and received the name Ichthyosaurus communis (i.e. “common fish lizard”). Mary Anning, a young fossil collector came across its skeleton in 1811. The incident occurred in Lyme Regis Bay in southern England. Numerous remains of the inhabitants of the early Jurassic sea can be found in the local cliffs to this day. The name “ichthyosaurus” itself was used in 1821, which is two decades earlier than the word “dinosaur”.
These wonderful reptiles occurred in an entire range of sizes. The smallest individuals reached several dozen centimetres in length, and the largest ones – over 15 meters! There were even ichthyosaurs that reached sizes of up to 18 meters, and their skulls were 3 meters in length. A typical genus is the well-known Jurassic Ichthyosaurus from which the name of the group has derived. It had a streamlined body in which the head smoothly merged into the torso and paddle limbs used for control. Its main means of propulsion were lateral flexions of the body and tail, which ended with a vertical fin. The final section of the spine (consisting of disc-shaped vertebrae) was directed downwards and simultaneously penetrated the lower part of the caudal fin, whereas the upper lobe did not have a skeletal bone. The ichthyosaur had a very elongated skull, and the jaws were armed with numerous, sharp teeth. The overall impression was like a threatening, drawn „muzzle”. The eyes of ichthyosaurs were very large and the nostrils moved far back to the rear, which was another adaptation of this reptile to the aquatic environment.
Ichthyosaurs were adapted to life in the water to such a degree that they probably never went on land. When they were accidentally cast ashore, they moved as clumsily as whales do today. Because they were not able to lay their eggs on the shore, they reproduced under water. We have even managed to find several skeletons of adult individuals along with skeletons of immature foetuses inside. Thanks to the remains of females that died during delivery, we know that young ichthyosaurs were born tail forward.
The origin of ichthyosaurs is still not entirely clear. These reptiles  appear suddenly, fully formed, in the fossil record. Earlier, the representatives of the Mesozoic species were already characterized by the majority of specialized features that defined this group. Further evolution consisted in specific adaptations to various modes of life – for example, some forms fed on molluscs and thus developed flattened teeth to crush their shells. Others had a short mandible, and the upper jaw was elongated forming a characteristic „sword” just like in swordfish.
The end of the ichthyosaur line appears remarkably mysterious too. In the Triassic they multiplied in number at a dizzying pace, in the early Jurassic they were characterized by a huge variety, after which came their slow decline. Only one species remained during the Cretaceous – Platypterygius – which died out more or less 90 million years ago, so a long time before the dinosaurs disappeared from the earth. Reports of more recent ichthyosaur remains have occasionally appeared, but these were always erroneous trails, for example errors were made during the dating procedure or the find was misinterpreted.

Remains from the banks of the Vistula

In Poland, only single ichthyosaur vertebrae have been found, with a characteristic cross-section, as well as teeth and jaw fragments. In 2009, just outside the town of Tarnowskie Góry, a high school student found a jaw belonging to a relatively small ichthyosaur from the Triassic. The jaws and teeth of these reptiles are also known from the late Jurassic limestones of the Polish Jura – in the vicinity of Częstochowa and Wieluń. This type of fossil was also found in phosphate rock deposits near Annopol on the Vistula. How did the reptilian remains come to be discovered in Poland? The presence of ichthyosaurs in our area was rare, because these areas were then covered by shallow seas. However, if an individual appeared and then died, scavengers would feed on its body. Hence the finds – for example, the ones from Annopol – are so fragmentary.

Literature:
Encyklopedia Audiowizualna Britannica, Kurpisz S.A., Poznań 2006, part I: Zoologia (on the basis of Britannica- Polish edition, pp. 64–65);
Machalski Marcin, Sekrety rybojaszczurów, „Focus.pl” (2010).

 
 
 
 
Elaborated by: Anna Berestecka (Editorial team of Małopolskas Virtual Museums),
Licencja Creative Commons

 This work is licensed under a Creative Commons Attribution 3.0 Poland License.

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Skeleton of an ichthyosaur “Ichthyosaurus communis”

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