Amplistoma

Amplistoma is a genus of fossil cephalopods belonging to the extinct subclass Ammonoidea. These ancient marine mollusks are characterized by their coiled shells, which typically evolved into a planispiral form, meaning the shell coils in a single plane. The shell of Amplistoma is often described as evolute, with earlier whorls being largely visible from the outside. The sutures, where the shell chambers connect internally, are typically complex and ornate, a common feature that helps in the classification of ammonites.

Fossils of Amplistoma are found in marine sedimentary rocks dating primarily from the Jurassic period. They are particularly common in deposits from the Middle Jurassic epoch. The geographical distribution of Amplistoma fossils indicates a widespread presence in ancient seas, with specimens discovered on multiple continents. Paleontologists study Amplistoma for its role in understanding ammonite evolution, paleoecology, and biostratigraphy, as its distinctive shell morphology and geological occurrence make it a useful index fossil for dating rock layers. The precise meaning of the genus name, Amplistoma, is not definitively known, but it likely refers to characteristics of its shell aperture or overall form.