Cross-section of the rostrum of Themnodontosaurus sp. ichthyosaur fossil found in Germany dating 180 million years ago from the Toarcian age. The cross-section shows the different stages of tooth growth (large tooth with root, small sprouting tooth without root). This is the mechanism of continuous tooth growth in ichthyosaurs. Ichthyosaurs were a type of extinct giant marine reptile, that were widespread during much of the Mesozoic (252-66 million years ago) era. Collection Jean-Philippe et Francois-Xavier Blouet, paleontologists.