#fossil

• Artolith — A rare petrographic formation or sedimentary rock composed partially of fossil accumulations of structural organic material or petrified plant remains; a type of mineral concretion. In niche geology, the term can describe rock fragments impregnated with silicates that preserved the cellular imprint of archaic plant structures.
• 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.
• Asprete fish — An endemic species of small freshwater fish (Romanichthys valsanicola) in the Percidae family, considered a true monument of nature and an extremely rare living fossil; it lives exclusively in Romania, currently isolated only within a restricted sector of the Vâlsan River. It is structurally adapted to fast mountain streams with rocky bottoms, featuring a body covered in very rough ctenoid scales, two separate dorsal fins, and nocturnal behavior, living hidden under boulders.
• 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.
• Fossilized crayfish — The fossilized remnant, petrified imprint, or petrographic cast of a prehistoric crayfish or a part of the exoskeleton of a decapod crustacean from the Astacidae family; these structural fossil formations are used in stratigraphical geology as biofacial indicators for reconstructing ancient freshwater aquatic environments.
• Asteroxylon — A genus of primitive fossil plants (Asteroxylon) from the Lycopodiopsida class, characteristic of the Early Devonian, discovered in the famous Rhynie chert deposits (Scotland); they were ancient terrestrial plants with dichotomously branched stems covered with scale-like leaf-like outgrowths (enations) and featuring a star-shaped central cylinder (actinostele) visible in cross-section.
• Astrobleme — An ancient landform representing a highly eroded, metamorphosed, and partially buried impact structure or crater, created by the crash of a large meteorite, asteroid, or comet onto the Earth's surface. Unlike younger craters, an astrobleme is often identified through gravitational anomalies, the presence of shatter cones, and shock-metamorphosed minerals like coesite or stishovite.
• 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.
• Australopithecine — An extinct homnid primate belonging to the genus Australopithecus, inhabiting Africa approximately 4 to 2 million years ago. Characterized by primitive bipedalism, a small cranial capacity, and human-like dentition, it represents a crucial link in the evolutionary lineage of hominins.
• Australopithecines — A generic term utilized in the plural to designate a broad group of early hominins belonging to the subtribe Australopithecina, including the genera Australopithecus and Paranthropus. As a plural adjective, it describes anatomical traits, cranial structures, or primitive bipedal locomotion specific to these primates.
• Australopithecus — A localized graphical and phonetic adaptation of the scientific Latin name 'Australopithecus', utilized as a synonym in simplified datasets or science popularization texts to designate the extinct bipedal hominin from prehistoric Africa.
• Axel Heiberg — A large island located in the Qikiqtaaluk region of Nunavut, Canada, forming part of the Canadian Arctic Archipelago and being the 31st largest island in the world. Named after the Norwegian diplomat and financier Axel Heiberg, the island is known in the global scientific community for its massive glaciers, spectacular mountainous terrain, and a unique paleontological site of a fossil forest from the Eocene epoch.
• Baculite — A member of the extinct genus Baculites, representing an Upper Cretaceous ammonite characterized by an almost completely straight, elongated, rod-shaped calcareous shell, unlike classic spiraled ammonites. Geologically, baculite fossils are utilized as highly important index fossils for dating marine sedimentary layers and reconstructing prehistoric ecosystems.