Orthoceratid

Part of speech: noun

Definitions

1. Referring to an extinct group of marine cephalopods characterized by long, straight shells and a central siphuncle used for buoyancy control
2. Describing ancient cephalopods that possessed elongated, conical shells with internal chambers separated by septa to aid in flotation
3. Pertaining to prehistoric mollusks that featured rigid, straight tubular shells and lived in marine environments during the Paleozoic era

Etymology: Orthoceratids belong to a large group of extinct marine cephalopods that flourished from the Early Ordovician into the Permian, roughly 480 to 250 million years ago. Their name comes from the Greek "orthos" meaning "straight" and "keras" meaning "horn," a reference to the long, cone-shaped shell that stretched out straight rather than coiling like a modern nautilus's. The "-id" ending is the standard zoological suffix marking a family-level group, here formed from the genus name "Orthoceras," which was coined in the early 1800s by the naturalist Alcide d'Orbigny's contemporaries working on fossil cephalopods. The most striking feature of these animals is the siphuncle, a tube running through the chambers of the shell. By pumping gas and liquid in and out of the chambers, the animal could adjust its buoyancy, rising and sinking in the water much as a submarine ballasts its tanks. Many orthoceratid shells were chambered this way, and their empty shells often became filled with sediment, which is why they are so common in limestone.