RESUMEN
We report a time-calibrated stratigraphic section in Colorado that contains unusually complete fossils of mammals, reptiles, and plants and elucidates the drivers and tempo of biotic recovery during the poorly known first million years after the Cretaceous-Paleogene mass extinction (KPgE). Within ~100 thousand years (ka) post-KPgE, mammalian taxonomic richness doubled, and maximum mammalian body mass increased to near pre-KPgE levels. A threefold increase in maximum mammalian body mass and dietary niche specialization occurred at ~300 ka post-KPgE, concomitant with increased megafloral standing species richness. The appearance of additional large mammals occurred by ~700 ka post-KPgE, coincident with the first appearance of Leguminosae (the bean family). These concurrent plant and mammal originations and body-mass shifts coincide with warming intervals, suggesting that climate influenced post-KPgE biotic recovery.
Asunto(s)
Ecosistema , Extinción Biológica , Fósiles , Mamíferos , Plantas , Animales , Tamaño Corporal , Clima , Colorado , Mamíferos/anatomía & histología , Mamíferos/clasificación , Plantas/anatomía & histología , Plantas/clasificación , Reptiles/anatomía & histología , Reptiles/clasificación , TemperaturaRESUMEN
The incidence of thymic germinal centres in normal individuals is disputed by various authors. The present study gives the results of a detailed histological assessment of 75 thymic biopsies obtained during operations to correct congenital heart lesions. Serial sectioning showed structures previously of disputed significance to be obliquely sectioned germinal centres. Comparison with the data of Hammar (1926) showed that there was some stress involution of the glands studied. Despite this 40% of the subjects had thymic germinal centres. Germinal centres were not detected in subjects under 4 years of age and there were insufficient subjects over 16 years of age to allow assessment of the frequency in adult thymus glands.
