Phylloceratoids

Phylloceratoids are an extinct suborder of ammonites, a group of shelled cephalopods related to modern squid and octopuses. They are characterized by their generally smooth, involute shells, meaning the inner whorls are almost entirely covered by the outer whorls. The name "phylloceratoid" comes from Greek words meaning "leaf-horn," referring to the often leaf-like shape of their septa (internal walls separating chambers).

These ammonites first appeared in the Triassic period and became particularly diverse and widespread during the Jurassic and Cretaceous periods. They are considered phylogenetically primitive among ammonites and are often used as index fossils for dating rock strata due to their rapid evolution and wide geographic distribution. Their shells typically exhibit a rounded or compressed cross-section and lack prominent external ornamentation like ribs or spines, which distinguishes them from many other ammonite groups.

Phylloceratoid ammonites inhabited various marine environments across the globe. Their evolutionary success and long stratigraphic range make them important for understanding ammonite phylogeny and the history of marine life. While their exact ecological roles are debated, their smooth shells suggest they may have inhabited more open marine settings compared to some of the more heavily ornamented forms. They went extinct along with all other ammonites at the end of the Cretaceous period.