orthoceratids

Orthoceratids were a diverse and widespread group of extinct marine cephalopods that flourished from the Early Ordovician to the end of the Triassic period. Their fossils are commonly found and are characterized by their distinctive straight or gently tapering, conical shells, known as orthocones. These shells were typically divided internally by horizontal plates called septa, which were pierced by a tube called a siphuncle. The siphuncle served to regulate buoyancy by controlling the flow of gas and liquid within the shell chambers.

Orthoceratids are believed to have been active predators, likely swimming in the ancient seas. Their exact lifestyle is inferred from their shell morphology, the presence of fossilized ink sacs in some specimens, and the discovery of their remains alongside prey. Their shells varied greatly in size, from a few centimeters to several meters in length, making some orthoceratids among the largest invertebrates of their time. They occupied a wide range of ecological niches, inhabiting both shallow and deep marine environments.

The group encompasses a broad taxonomic range, and their classification has been subject to considerable revision over time. Modern paleontology generally considers Orthocerida to be an order within the class Cephalopoda. The widespread distribution and abundance of their fossils make orthoceratids important index fossils for dating geological strata from the Paleozoic and Mesozoic eras. Their extinction at the end of the Triassic coincided with a major marine extinction event.