![]() ![]() are recognized within the prosimians: the Lemuroidea, Daubentonioidea and Lorisoidea. The prosimians have unfused hemi-mandibular bones, a humid rhinarium lacking hair, and laterally oriented nostrils (nasal holes). In this trait they differ from the insectivorous mammals that do not have a postorbital bar and from the haplorhines that have a fully closed orbit ( Fig. ![]() Strepsirrhini includes primates in which the orbital and temporal fossae are clearly communicated and with an orbital rim defining the border of the orbits. Many of these anatomical traits greatly vary within the primates, especially between the prosimians and anthropoids. Primates always have a well-developed Calcarine fissure that defines the posterior lobe of the brain. Primates) we shall include placental mammals that always have a clavicle bone in the shoulder girdle, nails in at least one finger instead of claws (if not in all fingers), a complete osseous ring surrounding the orbit, which may not be fully separated from temporal fossa, a trend to show the orbits in a ventral (frontal) position, instead of lateralised, and a variable degree of opposable thumbs. Another significant fossil is Eosimias, a 40-45 Ma middle Eocene specimen between simians and prosimians, close to the phylogenetic position of the tarsiers. More recent specimens are Teilhardina, a well-known, early Eocene (56-47 Ma) marmoset-like primate from Europe, North America and Asia, and Darwinius masillae, a species within the adapoidea, a 47 Ma basal strepsirhini from the middle Eocene, representative of the early haplorrhini diversification. Other remains of ancient primates date to 55 Ma, such as Cantius, Donrussellia, and Altanius. However, one of the oldest known primates from the fossil record is Altiatlasius, dated to the Late Paleocene in Africa and to the Paleocene– Eocene transition in the northern continents. Molecular analyses have estimated the origin of the primate branch in the mid- Cretaceous period, around 85 Ma, well before the extinction of the dinosaurs. Primate diversification within the mammals, goes back at least 65 Ma. giganticus, a large mammal from the Mesozoic era, had a dental formula (3124/2125) with mixed primitive (5 molars) and derived (2 premolars) traits. robustus fed on small vertebrates and young dinosaurs, either by scavenging or opportunistic hunting, while R. giganticus were terrestrial mammaliforms from deposits in China that date to the early Cretaceous period, 125-123.2 Ma. Castrocauda shows that these animals were not all tiny insectivorous shrew-like organisms, but also highly adapted, despite the reptilian domination. The docodonts include late Mesozoic mammaliform organisms as Castrocauda, whose most distinguishing dental feature was their relatively sophisticated molar teeth. Dated to 164 Ma, in the Jurassic period, it shows evidence of a hair fur covering the body (hair predates many other derived traits of modern mammals) and a tail adapted to aquatic environments, similar to modern Ornithorhynchus. ![]() ![]() Geologic periods based on major extinction events.Ĭastrocauda is one of the most ancient mammalian fossils. Therapsids heterodontia was likely due to a shift in dietary habits that required a diversification of the functional use of teeth.įigure 2. Therapsids show some anatomically differentiated teeth, such as a projecting canine and a non-single cusped tooth that can be called an incipient molar tooth. This primitive condition serves to capture preys but not for food processing or chewing. Modern amphibians and reptiles have homodondt teeth all their teeth show a conical shape wit a single, projecting point or cusp. Triassic mammal-like reptiles ( Therapsids) share with mammals a unique trait: heterodont dentitions. Mammal differentiation dates back to the Triassic/Jurassic transition, 206 Ma ( Fig. This time-line classification is useful for defining the evolutionary landmarks of our ancestors. Primates greatly diversified during the Tertiary period, and the hominines diverged during the Pliocene. The evidence of massive extinctions of organisms underlies the geologic time-scale classification. ![]()
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