Ammonoidea

Ammonoidea is an extinct group of marine molluscs within the class Cephalopoda, commonly known as ammonites. They first appeared in the Devonian and persisted until the end of the Cretaceous Period, roughly 400 to 66 million years ago, occupying a wide range of marine environments from nearshore shelves to open waters.

A defining feature is their typically planispirally coiled, chambered shells. The animal lived in the last whorl, with an internal sequence of gas-filled chambers separated by septa. A siphuncle running through the chambers provided buoyancy control as the animal moved. Shell ornamentation ranges from smooth to ribbed, with some forms bearing spines. Over time, the sutures between chambers became increasingly complex: early forms show simple goniatitic patterns, while many later ammonoids exhibit ceratitic or highly folded ammonitic sutures.

Ecology and life history: Ammonoids were nektonic carnivores, feeding on smaller invertebrates and possibly fish. They were ecologically successful and highly diverse, occupying a variety of ecological niches. Details of their reproduction and early life stages are not fully known, but many lineages likely possessed planktonic or lecithotrophic larval stages.

Fossil record and significance: Ammonoids are among the most plentiful and widely distributed fossils in Paleozoic and Mesozoic marine rocks. Their rapid evolutionary turnover and broad geographic range make them important index fossils for dating rocks and correlating marine strata.

Extinction: Ammonoids ultimately disappeared at the end of the Cretaceous mass extinction, along with many other marine groups, marking the end of a major cephalopod lineage.