#fossil

• 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.
• Anapsids — A group of amniote vertebrate reptiles structurally characterized by the complete absence of temporal fenestrae or openings in the posterior part of the skull (solid skull). From a historical and evolutionary standpoint, the subclass Anapsida includes the most primitive reptiles from the Paleozoic era, as well as turtles in classical morphological classifications, although modern genetic and phylogenetic studies suggest that turtles are actually secondarily modified diapsids.
• Ancyloceras — A genus of fossil ammonites from the Early Cretaceous period, characterized by a partially uncoiled or unusually bent shell, often with an initial spiraled portion followed by a straight shaft and a final hook-like bend. It represents a reference organism in biostratigraphy for the precise dating of marine geological strata.
• Anchitherium — A genus of fossil perissodactyl mammals from the Miocene epoch, considered a three-toed ancestor of the modern horse (genus Anchitherium). It was about the size of a small pony, adapted to forest life, featuring low-crowned teeth suitable for chewing soft leaves, and feet ending in three functional toes that prevented sinking into damp soil.
• Anchitherium — Scientific Latin spelling for anchiteriu; an extinct genus of primitive three-toed equids that lived during the Miocene epoch across North America, Europe, and Asia. This small-sized mammal represents a major evolutionary link, bridging the gap between primitive forest browsers and open-steppe equids.
• Anisian — A geological stage or substage located at the base of the Middle Triassic epoch, chronologically spanning from approximately 247 to 242 million years ago. As an adjective, the term refers to rocks, sediments, or fossils formed during this time interval, internationally defined by the appearance of specific ammonite and conodont assemblages within reference Alpine geological sections.
• Anisodont — Having teeth that are unequal in size, shape, or structure; the term specifically describes a type of asymmetric dentition found in certain extinct reptiles or mammals, where the elements of the dental arch visibly differ from one another. As a masculine noun, it defines an organism or a fossil specimen characterized by this anatomical conformation adapted for specific feeding or predatory habits.
• Annularia — A fossil plant genus of the order Equisetales, characteristic of the Carboniferous and Permian periods, representing the star-shaped whorled foliage of the ancient trees known as Calamites. The linear or lanceolate leaves were structurally joined at the base into a distinct ring, forming a circular fossil imprint that provides crucial clues about the structure of Paleozoic swamp forests.
• Anoplotherium — A fossil ungulate mammal from the late Eocene and Oligocene epochs, characterized by the complete absence of prominent canine teeth and horns, possessing a continuous dentition and a long, robust tail. This extinct genus (Anoplotherium) serves as an essential biostratigraphic marker for reconstructing the fauna and forest environment of the European Paleogene period.
• Anthracotherium — An extinct genus of hippopotamus-like artiodactyl ungulates within the family Anthracotheriidae, that lived during the Oligocene and Miocene epochs across Eurasia and Africa, adapted to a semi-aquatic lifestyle.
• Anthracothere — A fossil ungulate mammal belonging to the family Anthracotheriidae, structurally related to modern hippopotamuses and pigs, which lived during the Eocene and Oligocene epochs, whose skeletal remains have been frequently discovered within geological layers rich in lignite and fossil coal. This large-bodied animal possessed a specific bunodont dentition and short limbs adapted to aquatic or swampy environments, being an essential stratigraphic benchmark for dating carboniferous deposits.
• Anthracosaur — A generic term for large fossil amphibians with transitional characteristics between amphibians and reptiles, which lived during the Paleozoic era, particularly during the Carboniferous and Permian periods. These primitive tetrapods were top predators in swamp ecosystems, featuring massive skulls, sharp labyrinthodont teeth, and a lifestyle adapted to both deep aquatic environments and early terrestrial movement.
• Anthropolit — A fossilized human remain or a human body petrified through incrustation with mineral substances (lime, silica) inside cave sediments or ancient geological layers. This rare technical term is used in fossil studies to denote prehistoric skeletal specimens that have undergone a complete process of diagenesis and structural mineralization.
• Anthropomorphids — A rare taxonomic or systematic name, historically used in primate classification to denote the family or extended group of higher anthropomorphic apes that present the closest structural, skeletal, and genetic similarities to humans. The study of these fossils and living specimens helps clarify the intermediate links in the hominid evolutionary tree.
• Anthropopithecus — An archaic or historical scientific name given to a hypothetical genus of transitional fossil primates, considered linking bridges between higher apes and humans; ape-man. In the 19th century, the term was used by paleontologists to classify fossils presenting a structural mixture of simian and hominid traits, prior to the establishment of modern Homo and Australopithecus taxonomies.
• Apatosaurus — A genus of gigantic sauropod dinosaur, a herbivore that lived during the Late Jurassic period in what is now North America. This remarkable prehistoric animal was characterized by an extremely long neck, a long whip-like tail used for defense, and massive pillar-like limbs that supported a structural weight of dozens of tons, historically known also under the popular name of brontosaurus.
• Aptian — An upper geological stage of the Lower Cretaceous, chronologically situated between the Barremian and the Albian, structurally characterized by massive sedimentary deposits rich in ammonites and the expansion of specific marine faunas; as an adjective, it describes the rocks, formations, or fossils that formed during this period of Earth's history.
• Araucaria araucana — An endangered species of evergreen conifer in the genus Araucaria, native to the Andean slopes of Chile and western Argentina, popularly known as the monkey puzzle tree or Chilean pine. Notable for its sharp, triangular, scale-like leaves arranged spirally on thick branches, this relict tree can live for over a thousand years.
• Araucariaceous — Common name used to denote a member of the conifer family Araucariaceae or to describe elements characteristic of these relict Southern Hemisphere trees. As an adjective, the term qualifies the anatomical structures, leaves, or massive woody cones specific to this ancient plant lineage.
• 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.