Assuntos
Extinção Biológica , Planetas Menores , Animais , Mudança Climática , Fenômenos Geológicos , México , Água do Mar , Vertebrados , Erupções VulcânicasRESUMO
Description and analysis of the complex structure of enamel can be facilitated through the application of a system of hierarchical levels of structural complexity. Five interdependent levels are distinguished. These are the levels of: 1) crystallites, 2) prisms, 3) enamel types, 4) schmelzmuster, and 5) dentition. This system provides a basis for analysis of both variation of particular structures and variation of structural types throughout a mammal's dentition. Optimally, in wide ranging systematic and biomechanical studies, all levels of structural complexity should be considered, but lack of information about one level does not prevent significant analyses at other levels.
Assuntos
Esmalte Dentário/ultraestrutura , Animais , Humanos , Mamíferos/classificaçãoRESUMO
Abundant skeletal remains demonstrate that lambeosaurine hadrosaurid, tyrannosaurid, and troodontid dinosaurs lived on the Alaskan North Slope during late Campanian-early Maestrichtian time (about 66 to 76 million years ago) in a deltaic environment dominated by herbaceous vegetation. The high ground terrestrial plant community was a mild- to cold-temperate forest composed of coniferous and broad leaf trees. The high paleolatitude (about 70 degrees to 85 degrees North) implies extreme seasonal variation in solar insolation, temperature, and herbivore food supply. Great distances of migration to contemporaneous evergreen floras and the presence of both juvenile and adult hadrosaurs suggest that they remained at high latitudes year-round. This challenges the hypothesis that short-term periods of darkness and temperature decrease resulting from a bolide impact caused dinosaurian extinction.
RESUMO
Fragmentary mandibles of Purgatorius unio Van Valen and Sloan from the Puercan (approximately early Paleocene), Garbani Locality, Montana, preserve associated postcanines. Their morphology indicates that this mammal was an early paromomyid primate and suggests that primate ancestry does not include currently known members of the palaeoryctid and leptictid Insectivora or of the Condylarthra.