Paleoenvironment, paleoecology, and evolution of Maniraptoran "dinosaurs"

Abstract

Maniraptora is a taxonomic group that includes the well-known primitive bird, Archaeopteryx , that is thought to have had limited power of flight, the small, four-winged, feathered glider, Microraptor and the terrestrial runner Bambiraptor . All are herein considered important links in the origin of flight and a subsequent transition to terrestriality in some forms. In cladistic classifications, dromaeosaurid "dinosaurs" were only considered terrestrial cursors. The discovery of a gliding stage within the dromaeosaurs, a group purportedly closest to birds, confounds the currently suggested biologic framework. Any evolutionary framework lacking predictability for origin of flight scenarios must be fundamentally flawed. Paleoclimate was a significant factor for evolution of birds and birdlike dinosaurs during the Mesozoic. It is characterized by faunal and floral changes reflecting climatic change. For instance, the first known birds such as Archaeopteryx were arboreal and evolved during a warm period in the Late Jurassic. The Solnhofen quarries that produced Archaeopteryx have a windblown faunal and floral component from a forested area indicating a typical Jurassic forest with large trees. During the Early Cretaceous, the Jehol Biota climate was warm and forested providing a suitable arboreal habitat for Microraptor . The cooling trend at end of the Cretaceous opened up the environment making it difficult for poor fliers or gliders as forested areas became less dense. Terrestrial forms and birds with full flight capabilities could survive best in these new environments. Furthermore, birds with specialized manus claws for tree climbing were common in the Early Cretaceous and are so far unknown in the Late Cretaceous. This indicates a change in the avian community with fliers developing an increased ability to take off from flat surfaces. Dromaeosaurs survived well after their initial radiation during the Jehol Biota. Only terrestrial forms, such as Bambiraptor , have been found during the Late Cretaceous. Birds of modern aspect probably replaced the primitive dromaeosaurs, Microraptor and its kin, since they were more efficient fliers. A majority of cladistic analyses show Microraptor as the plesiomorphic sister group to the more terrestrial dromaeosaurs. The geologically younger Bambiraptor provides examples of the morphological changes necessary for the transition to ground dwelling and how this transition was accomplished