ammonoideos

Ammonoideos, commonly called ammonoids, are an extinct group of marine cephalopods characterized by their spiral, chambered shells. They first appear in the Early Devonian and persisted until the end of the Cretaceous period, about 400 to 66 million years ago, making them one of the most enduring cephalopod lineages. The shells are typically planispirally coiled and subdivided into multiple chambers by septa; the animal lived in the final chamber and used the gas-filled rear chambers to regulate buoyancy.

Morphology and classification: Ammonoideos show a range of shell shapes, from planispiral to helically coiled, but most species feature flat coils. Sutures between septa are diverse and have historically been used to classify groups into early goniatitids, ceratitids, and the more complex ammonites.

Ecology and life history: Ammonoideos were marine predators with broad ecological roles across Paleozoic and Mesozoic seas. They inhabited a range of depths and climates. They are believed to have reproduced sexually; details of soft-body anatomy remain poorly known due to the fossil nature of the remains.

Paleontological significance: Ammonoideos are among the most useful index fossils for dating and correlating Paleozoic and Mesozoic rocks because of their rapid evolutionary rates and wide geographic distribution. They became extinct at the end of the Cretaceous period during the Cretaceous–Paleogene mass extinction event, along with many other marine organisms.

They are closely related to nautiloids and, more broadly, to modern cephalopods.