ammonites

Ammonites are an extinct group of marine molluscs in the subclass Ammonoidea, within the class Cephalopoda. They first appeared in the Devonian and diversified through the Paleozoic and Mesozoic eras, becoming one of the most successful cephalopod groups before their extinction at the end of the Cretaceous period, about 66 million years ago. They are related to modern cephalopods such as squids, octopuses, and cuttlefish, and are more distant from the nautiluses.

Most ammonites had planispirally coiled shells, though some lineages developed irregular or elongated forms. The shell consists of a series of gas-filled chambers separated by septa. The animal occupied the outermost chamber in life, while the inner chambers provided buoyancy. A tube-like siphuncle connected the chambers and helped regulate buoyancy by adjusting the gas and fluid within them.

Sutures, the lines where the septa meet the outer shell, formed intricate folded patterns and are a key feature for identifying ammonite species. Ammonites varied greatly in size, with some shells a few centimeters in diameter and others reaching substantial dimensions. While soft body parts are rarely preserved in fossils, they are generally assumed to resemble those of other cephalopods.

Ecology and distribution: Ammonites inhabited a wide range of marine environments, from nearshore shelves to open oceans, and occupied diverse ecological niches as predators of small invertebrates and possibly fish. Their widespread geographic distribution and rapid evolutionary turnover make them valuable index fossils for dating rock layers.

Extinction and significance: Ammonites disappeared at the end of the Cretaceous, during the Cretaceous–Paleogene mass extinction. Their extensive fossil record and short-lived, rapidly evolving lineages make them important tools in biostratigraphy and the study of ancient marine ecosystems.