THE ORIGIN AND EARLY EVOLUTION OF BIRDS

Birds evolved from and are phylogenetically recognized as members of t he theropod dinosaurs; their first known member is the Late Jurassic A rchaeopteryx, now represented by seven skeletons and a feather, and th eir closest known non-avian relatives are the dromaeosaurid theropods such as Deinonychus. Bird flight is widely thought to have evolved fro m the trees down, but Archaeopteryx and its outgroups show no obvious arboreal or tree-climbing characters, and its wing planform and wing l oading do not resemble those of gliders. The ancestors of birds were b ipedal, terrestrial, agile, cursorial and carnivorous or omnivorous. A part from a perching foot and some skeletal fusions, a great many char acters that are usually considered 'avian' (e.g. the furcula, the elon gated forearm, the laterally flexing wrist and apparently feathers) ev olved in non-avian theropods for reasons unrelated to birds or to flig ht. Soon after Archaeopteryx, avian features such as the pygostyle, fu sion of the carpometacarpus, and elongated curved pedal claws with a r eversed, fully descended and opposable hallux, indicate improved flyin g ability and arboreal habits. In the further evolution of birds, char acters related to the flight apparatus phylogenetically preceded those related to the rest of the skeleton and skull. Mesozoic birds are mor e diverse and numerous than thought previously and the most diverse kn own group of Cretaceous birds, the Enantiornithes, was not even recogn ized until 1981. The vast majority of Mesozoic bird groups have no Ter tiary records: Enantiornithes, Hesperornithiformes, Ichthyornithioform es and several other lineages disappeared by the end of the Cretaceous . By that time, a few Linnean 'Orders' of extant birds had appeared, b ut none of these taxa belongs to extant 'families', and it is not unti l the Paleocene or (in most cases) the Eocene that the majority of ext ant bird 'Orders' are known in the fossil record. There is no evidence for a major or mass extinction of birds at the end of the Cretaceous, nor for a sudden 'bottleneck' in diversity that fostered the early Ter tiary origination of living bird 'Orders'.