Based on fossil and biological evidence, most scientists accept
that birds are a specialized subgroup of theropod dinosaurs. More
specifically, they are members of Maniraptora, a group of theropods
which includes dromaeosaurs and oviraptorids, among others. As
scientists have discovered more nonavian theropods closely related
to birds, the previously clear distinction between nonbirds and
birds has become blurred. Recent discoveries in the Liaoning
Province of northeast China, which demonstrate many small theropod
dinosaurs had feathers, contribute to this ambiguity. The consensus
view in contemporary paleontology is that the birds, or avialans,
are the closest relatives of the deinonychosaurs, which include
dromaeosaurids and troodontids. Together, these form a group called
Paraves. Some basal members of this group, such as Microraptor,
have features which may have enabled them to glide or fly. The most
basal deinonychosaurs were very small. This evidence raises the
possibility that the ancestor of all paravians may have been
arboreal, have been able to glide, or both. Unlike Archaeopteryx
and the non-avian feathered dinosaurs, who primarily ate meat,
recent studies suggest that the first birds were herbivores. The
Late Jurassic Archaeopteryx is well known as one of the first
transitional fossils to be found, and it provided support for the
theory of evolution in the late 19th century. Archaeopteryx was the
first fossil to display both clearly traditional reptilian
characteristics: teeth, clawed fingers, and a long, lizard-like
tail, as well as wings with flight feathers identical to those of
modern birds. It is not considered a direct ancestor of modern
birds, though it is possibly closely related to the real ancestor.
Early disagreements on the origins of birds included whether birds
evolved from dinosaurs or more primitive archosaurs. Within the
dinosaur camp, there were disagreements as to whether ornithischian
or theropod dinosaurs were the more likely ancestors. Although
ornithischian (bird-hipped) dinosaurs share the hip structure of
modern birds, birds are thought to have originated from the
saurischian (lizard-hipped) dinosaurs, and therefore evolved their
hip structure independently. In fact, a bird-like hip structure
evolved a third time among a peculiar group of theropods known as
the Therizinosauridae. A small minority of researchers, oppose the
majority view, contending that birds are not dinosaurs, but evolved
from early reptiles like Longisquama. The earliest known bird
(avialan) fossils currently known hail from the Tiaojishan
Formation of China, which has been dated to the late Jurassic
period (Oxfordian stage), about 160 million years ago. The avialan
species from this time period include Anchiornis huxleyi,
Xiaotingia zhengi, and Aurornis xui. The well-known early avialan,
Archaeopteryx, dates from slightly later Jurassic rocks (about 155
million years old) from Germany. Many of these early avialans
shared unusual anatomical features that may be ancestral to modern
birds, but were later lost during bird evolution. These features
include enlarged claws on the second toe which may have been held
clear of the ground in life, and long feathers or "hind wings"
covering the hind limbs and feet, which may have been used in
aerial maneuvering. Avialans diversified into a wide variety of
forms during the Cretaceous Period. Many groups retained primitive
characteristics, such as clawed wings and teeth, though the latter
were lost independently in a number of bird groups, including
modern birds (Neornithes). While the earliest forms, such as
Archaeopteryx and Jeholornis, retained the long bony tails of their
ancestors, the tails of more advanced birds were shortened with the
advent of the pygostyle bone in the clade Pygostylia. In the late
Cretaceous, around 95 million years ago, the ancestor of all modern
birds also evolved better olfactory senses.
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