Ichthyosaurus

Ichthyosaurus is an order of extinct marine reptiles that had a shape similar to fish and dolphins.

Ichthyosaurs are the most adapted reptiles to the aquatic lifestyle. Only one group of mammals, cetaceans, was able to achieve a similar degree of fitness. Unique characteristics associated with an aquatic lifestyle are already observed in the oldest known ichthyosaurs from the Triassic period. Such devices include an elongated muzzle, very large eyes, limbs transformed into fins and strongly concave vertebrae.

A key evolutionary feature of ichthyosaurs is live birth. They had a euryapsid structure of the skull, descended from diapsid ancestors. Teeth were replaced repeatedly during life. The limbs were used to maintain balance and control direction. The tail had two blades, the lower of which was supported by a vertebral column. A typical ichthyosaur had very large eyes protected by a bone ring, indicating that they hunted at night. In this regard, some species had huge eyes (up to 20 cm in diameter). Additionally, apparently, there were some skin receptors similar to the lateral line, as indicated by traces of nerves and vessels on the bones of the skull. The skin, devoid of scales, was probably covered with mucus for better gliding in the water. Well adapted to moving at high speed, like modern tuna, some ichthyosaurs also suggest that they were good deep-sea divers, like modern whales. The most likely color is the counter shade (dark top and light bottom) with a bluish tinge.

Some early ichthyosaurs had teeth with which they fed on shellfish - ammonites, nautiloids and squids. It is very likely that they also fed on fish, and some of the larger species had heavy jaws and teeth, which show that they fed on smaller reptiles.

The analysis of some fossil remains of ichthyosaurs showed that the inner layers of the skin passed into an insulating fat layer like in warm-blooded animals.

Ichthyosaurs existed for almost the entire Mesozoic 250-94 million years ago, they reached their greatest heyday in the Jurassic period, until they were replaced by plesiosaurs in the Cretaceous period. In the Cretaceous period, the number of ichthyosaur species decreased sharply. It is assumed that global warming, which occurred in the middle of the Cretaceous period, led to the depletion of oxygen in oceanic waters, which led to the extinction of ichthyosaurs that did not adapt to climate change.