#cephalopod

• Acanthoceras — A genus of extinct fossil cephalopod mollusks (ammonites) within the Acanthoceratidae family, characteristic of the Late Cretaceous epoch. These marine organisms were distinguished by their massive, planospiral coiled shells heavily ornamented with prominent ribs and rows of sharp tubercles or spine-like outgrowths along the outer whorls, serving as critical index fossils for dating geological strata of that period.
• Octopus — An old, regional, or popular name borrowed from Greek for the octopus (or other eight-armed cephalopod mollusks); a legendary marine animal described in old folk books (such as the Physiologus) as a bizarre creature with many legs. In an archaic literary sense, the term was used to illustrate gigantic marine creatures or abyss monsters.
• Ammonite — A fossil marine cephalopod mollusk from the Ammonoidea subclass, characterized by an external, planospiral, and chambered calcareous shell, which lived from the Devonian period until the end of the Cretaceous. The fossils of these organisms are widely used in stratigraphical geology as index fossils for the precise dating of sedimentary rock layers from the Mesozoic era.
• Ammonites — The plural form of amonit; an extensive group of fossil marine cephalopod mollusks characterized by chambered spiraled shells, similar to the modern nautilus. In historical geology, these representatives serve as essential index fossils for delineating and dating sedimentary biozones of past eras, providing clues about prehistoric ocean climate and dynamics.
• Ammonitids — Scientific name or comprehensive taxonomic classification (Ammonitida) that groups together the orders and families of true ammonites, characterized by shells with highly complex and branched suture lines. These fossil marine cephalopod mollusks dominated the seas during the Jurassic and Cretaceous periods, disappearing completely during the mass extinction at the end of the Mesozoic era.
• Ammonoids — Scientific name or higher taxonomic classification (Ammonoidea) that includes all species of fossil marine cephalopod mollusks related to true ammonites, globally distributed from the Devonian to the Cretaceous. These prehistoric organisms are distinguished by the unique structure of their external, spiraled, and chambered shells, serving as essential index fossils in stratigraphy.
• Argonaut — 1. In Greek mythology, any of the heroes or demigods who embarked on the ship Argo, under Jason's command, to participate in the glorious expedition to Colchis to recover the Golden Fleece. 2. In zoology, a genus of marine cephalopod mollusks within the octopod order, characterized by extreme sexual dimorphism; the female secretes a fine, paper-thin calcareous shell used exclusively as a cradle to protect eggs.
• Arietites — A fossil genus of ammonites (extinct marine cephalopod mollusks) from the Early Jurassic period, characterized by a large, planospiral shell with prominent transverse ribs and a distinct central keel; the name suggests the resemblance of the coiled shell to the horns of the Egyptian god Ammon or a ram, serving as a critical index fossil for dating geological strata of the Sinemurian stage.
• Aspidoceras — An extinct genus of ammonites (coiled marine cephalopod mollusks) in the family Aspidoceratidae, which lived during the Late Jurassic and Early Cretaceous periods. Fossils of this genus are recognized by their robust, evolute shell, characterized by distinctive rows of nodes or spines arranged on the flanks, used by paleontologists as index fossils for dating marine geological strata.
• Aulacoceras — An extinct genus of dibranchiate cephalopods from the order Belemnoidea, characteristic of the Triassic period. This ancient marine organism was notable for an elongated, conical, calcareous internal shell marked on its outer surface by prominent longitudinal grooves.