Fossil group

Ammonites

Molluscs · Jurassic – Cretaceous

Extinct cephalopods — relatives of squid and the living nautilus — housed in a coiled, chambered shell divided by intricately folded walls. They swam in Mesozoic seas, evolved rapidly, and are among the finest index fossils of the Jurassic and Cretaceous; they died out with the dinosaurs.

A broad fossil group rather than a single taxon — the units below record it among their fauna or flora.