The ichthyosaurs (colloquially also fish dinosaurs) are a group of extinct reptiles. They lived exclusively in the sea and became extinct 93 million years ago at the beginning of the Upper Cretaceous. Ichthyosaurs moved by striking with the tail fin, using the forelimbs for steering. They possessed very large eyes, the largest ichthyosaur eye found was also the largest eye of any vertebrate, at 26.4 cm in diameter. The eyes were also very bright, suggesting that ichthyosaurs hunted at great ocean depths. Ichthyosaurs ate mainly belemnites, ammonites, fish and sometimes other vertebrate. Ichthyosaurus was a very widespread genus, which means that it can be found all over the world.
Ichthyosaurs evolved greatly during their 150 million year era. Early representatives of the ichthyosaurs, like Catorhynchus still had a lizard-like elongated body and lived very close to the coast in shallow water. At the end of the Middle Triassic all elongated ichthyosaurs disappeared and were replaced in the Upper Triassic by the shastasaurids, forms with a more spindle-like physique. However, ichthyosaurs reached their greatest species diversity in the Lower Jurassic. Therefore, fossils of the genus Ichthyosaurus are relatively common in Holzmaden and can be found in a great variety of forms.

Three of the best known ichthyosaur species in the Posidonia Shale are Stenopterygius, Eurhinosaurus, and Temnodontosaurus. The latter was the apex predator of the time. Stenopterygius, by far the most common species in Holzmaden and the surrounding area, grew to a length of only three to four meters. Ichthyosaurus teeth have a relatively smooth tip and if preserved a long root.