Mammalian adaptation to herbivory in the aftermath of the KT extinction.

摘要

The extinction event at the K/T boundary was responsible for the demise most animals larger than a kilogram in general. The subsequent radiation of mammals into herbivorous niches recently vacated by the dinosaurs is a well-known phenomenon, reflected in a general body size increase and the adaptation of dentitions to the shearing forces that the breakup of tough plant matter requires. The timing and nature of this transition has not yet been evaluated, however: multituberculates radiated in size and, presumably, function just prior to the K/T extinction, bit the degree to which the placental mammals that replaced them were ecologically similar is unknown. Changes in mammalian diet in the Paleocene was investigated using molar shearing crest length (Shearing Quotient) in series with low-magnification microwear techniques, which allows for the evaluation of an animal's dietary niche (insectivore, carnivore, grazer, browser, hard-object feeder) more precisely than is usually possible by analyzing either tooth shape or microwear alone. Results show an initial transition from predominantly insectivorous communities in the Judithian to a more dietarily diverse mammalian community in the Maastrichtian, in which some multituberculates adapted to a high-fiber (leaves/browse) diet. These larger and more herbivorous forms went extinct at the boundary; the Puercan communities that immediately followed were dominated by insectivory and hard-object feeding/omnivory. Mammalian faunas remained hard-object dominant through the Paleocene even as both taxonomic and size diversity increased. Evidence of predominant high-fiber diets does not re-appear in any taxa until the late Paleocene, when a general drying trend in the Western interior triggered a change from closed-canopy Paleocene rainforests to more open vegetation. These results suggest that Paleocene mammalian communities were strongly influenced by their environment, not resorting to browse consumption until the Paleocene rainforest fruit-fall became unavailable.