#fossil record

• Acheloids — A term given to a category or family of marine or fossil organisms, or insects in older taxonomic classifications (sometimes correlated with Coleoptera species or primitive crustaceans), whose anatomical structure resembles a fluid shape or traits historically associated with aquatic deities (Achelous). In a modern sense, the term appears in niche papers to describe specimens with a specific hydrodynamic morphology.
• Allosaurus — A genus of large bipedal theropod dinosaur, carnivorous, that lived during the Late Jurassic period (approximately 155 to 145 million years ago), operating as the apex predator in what is now North America (Morrison Formation). It was characterized by a massive skull equipped with dozens of sharp, serrated teeth, short but powerful forelimbs ending in three fingers with large claws, and a long, heavy tail used for balance.
• 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.
• 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.
• 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.
• 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.
• 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.