Ichthyosaur

The Ichthyosaur is a two-part fossil that can be donated to the museum. It was introduced in and returned in. It is superseded by Ophthalmosaurus in.

In
After donating the final part of the fossil in, its plaque in the museum will read:

"Though they bear a striking visual similarity to dolphins, ichthyosaurs were reptiles rather than mammals. Interestingly, adult specimens have been found with juvenile bones inside their abdominal cavities. This suggests that rather than laying eggs, ichthyosaurs gave birth to live young in the water."

Real-world information
Ichthyosaurs are among the earliest marine reptiles. They evolved 250 million years ago, shortly after the Permian-Triassic extinction event and at least 15 million years before dinosaurs. Their evolutionary relationship to modern reptiles is contentious; most studies suggest they are more closely related to snakes and lizards than to crocodiles and birds, but some suggest the opposite. The ichthyosaurs diversified quickly throughout the Triassic and remained diverse through the Early to Middle Jurassic. They eventually went extinct 90 million years ago.

Ichthyosaurus was also the first known prehistoric marine reptile. Its discovery in 1811/1812 by siblings Joseph and Mary Anning gave credence to the then-controversial idea of extinctions. While it was proposed in 1798 by Georges Cuvier, extinction was viewed as conflicting with contemporary religious views, as it seemed to imply that God's creations could be imperfect, and many believed that any oddities in the fossil record could be explained by living, albeit undiscovered, animals. The discovery of Ichthyosaurus, along with Mary Anning's later discoveries (like plesiosaurs) and the discovery of dinosaurs in the 1820s, dealt the first major blow to this view and gave rise to the field of modern paleontology.