Birds evolved from and are phylogenetically recognized as members
of the theropod dinosaurs; their first
known member is the Late Jurassic Archaeopteryx, now
represented by seven skeletons and a feather, and their
closest known non-avian relatives are the dromaeosaurid theropods such
as Deinonychus. Bird flight is widely
thought to have evolved from the trees down, but Archaeopteryx
and its outgroups show no obvious arboreal
or tree-climbing characters, and its wing planform and wing loading do
not resemble those of gliders. The
ancestors of birds were bipedal, terrestrial, agile, cursorial and
carnivorous or omnivorous. Apart from a
perching foot and some skeletal fusions, a great many characters that
are usually considered ‘avian’ (e.g. the
furcula, the elongated forearm, the laterally flexing wrist and apparently
feathers) evolved in non-avian
theropods for reasons unrelated to birds or to flight. Soon after
Archaeopteryx, avian features such as the
pygostyle, fusion of the carpometacarpus, and elongated curved pedal claws
with a reversed, fully descended
and opposable hallux, indicate improved flying ability and arboreal
habits. In the further evolution of birds,
characters related to the flight apparatus phylogenetically preceded
those related to the rest of the skeleton
and skull. Mesozoic birds are more diverse and numerous than thought
previously and the most diverse
known group of Cretaceous birds, the Enantiornithes, was not even
recognized until 1981. The vast
majority of Mesozoic bird groups have no Tertiary records: Enantiornithes,
Hesperornithiformes,
Ichthyornithiformes and several other lineages disappeared by the end of
the Cretaceous. By that time, a few
Linnean ‘Orders’ of extant birds had appeared, but none of
these taxa belongs to extant ‘families’, and it
is not until the Paleocene or (in most cases) the Eocene that the majority
of
extant 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 Tertiary origination of living bird ‘Orders’.
