#index fossil

• 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.
• Acteonella — Romanian term (derived from the fossil genus Acteonella) for a type of prehistoric marine gastropod snails, characteristic of the Cretaceous period. Its structural shell is thick, oval or fusiform in shape, smooth, and provided with distinct columellar plications. Acteonella fossils are frequently used in geology as index fossils for dating sedimentary strata and reconstructing ancient equatorial marine environments.
• Aegoceras — An extinct genus of ammonoid cephalopods within the order Ammonitida and family Liparoceratidae, which thrived during the Early Jurassic epoch (predominantly in the epicontinental seas covering modern Western Europe). The shells of these ammonites are evolute, characterized by widely open whorls, rounded whorl sections, and prominent, well-spaced transverse ribs, holding significant stratigraphical value as index fossils for mapping geological strata.
• Alveolina — A genus of large fossil marine foraminifera, characteristic of the Late Cretaceous and Eocene periods, having a spherical, ellipsoidal, or fusiform calcareous shell structurally divided into numerous small chambers or alveoli; a microfossil belonging to this genus. These organisms represent essential index fossils for dating sedimentary geological layers and reconstructing ancient tropical marine environments.
• 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.
• 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.
• Asprokastron — The Greek-origin name (meaning 'White Castle') under which the historic fortress and city at the mouth of the Dniester River, later named Cetatea Albă, was known in medieval Byzantine and Genoese documents. It represents a commercial and strategic node of crucial importance on the Black Sea coast, historically contested between the Byzantine Empire, the Republic of Genoa, the Principality of Moldavia (under the reign of Stephen the Great), and the Ottoman Empire.