Micraster cortestudinarium

Micraster cortestudinarium is an extinct species of heart urchin, a type of irregular sea urchin, that lived during the Late Cretaceous period. It was first formally described by the German paleontologist August Goldfuss in 1826 and is abundantly preserved in the chalk deposits of northwest Europe.

Micraster fossils are most commonly found in the white chalk formations of southern England, northern France, and Germany, where Late Cretaceous marine sediments are exposed. Classic collecting localities include the chalk cliffs of the south coast of England and active or former chalk quarries.

The Micraster lineage is one of the most famous examples of gradual evolutionary change preserved in the fossil record, showing progressive morphological changes across successive species in the chalk seas. This lineage was historically cited as textbook evidence for evolution, as the changes in test shape can be traced stratigraphically through the chalk.

They date to the Late Cretaceous period, when much of northwest Europe was covered by shallow chalk seas. The Cretaceous period ended around 66 million years ago with the mass extinction event that wiped out the dinosaurs, though Micraster lineages were already well established before that boundary.