The possibility that dinosaurs were the ancestors of birds was first suggested in 1868 by
Thomas Henry Huxley.
After the work of
Gerhard Heilmann in the early 20th century, the theory of birds as dinosaur descendants was abandoned in favor of the idea of their being descendants of generalized
thecodonts, with the key piece of evidence being the supposed lack of
clavicles in dinosaurs.
However, as later discoveries showed, clavicles (or a single fused
wishbone, which derived from separate clavicles) were not actually absent;
they had been found as early as 1924 in
Oviraptor, but misidentified as an
interclavicle.
In the 1970s,
John Ostrom revived the dinosaur–bird theory,
which gained momentum in the coming decades with the advent of cladistic analysis,
and a great increase in the discovery of small theropods and early birds.
Of particular note have been the fossils of the
Yixian Formation, where a variety of theropods and early birds have been found, often with feathers of some type.
Birds share over a hundred distinct anatomical features with theropod dinosaurs, which are now generally accepted to have been their closest ancient relatives.
They are most closely allied with
maniraptoran coelurosaurs.
A minority of scientists, most notably
Alan Feduccia and
Larry Martin, have proposed other evolutionary paths, including revised versions of Heilmann's basal archosaur proposal,
or that maniraptoran theropods are the ancestors of birds but themselves are not dinosaurs, only
convergent with dinosaurs.
Feathers
Archaeopteryx, the first good example of a "feathered dinosaur", was discovered in 1861. The initial specimen was found in the
Solnhofen limestone in southern Germany, which is a
lagerstätte, a rare and remarkable geological formation known for its superbly detailed fossils.
Archaeopteryx is a
transitional fossil, with features clearly intermediate between those of modern reptiles and birds. Brought to light just two years after Darwin's seminal
The Origin of Species, its discovery spurred the nascent debate between proponents of
evolutionary biology and
creationism. This early bird is so dinosaur-like that, without a clear impression of feathers in the surrounding rock, at least one specimen was mistaken for
Compsognathus.
Since the 1990s, a number of additional
feathered dinosaurs have been found, providing even stronger evidence of the close relationship between dinosaurs and modern birds. Most of these specimens were unearthed in the
lagerstätte of the Yixian Formation,
Liaoning, northeastern
China, which was part of an island continent during the Cretaceous. Though feathers have been found in only a few locations, it is possible that non-avian dinosaurs elsewhere in the world were also feathered. The lack of widespread fossil evidence for feathered non-avian dinosaurs may be because delicate features like skin and feathers are not often preserved by fossilization and thus are absent from the fossil record. To this point, protofeathers (thin, filament-like structures) are known from dinosaurs at the base of Coelurosauria, such as
compsognathids like
Sinosauropteryx and
tyrannosauroids (
Dilong),
but barbed feathers are known only among the coelurosaur subgroup Maniraptora, which includes oviraptorosaurs, troodontids, dromaeosaurids, and birds.
The description of feathered dinosaurs has not been without controversy; perhaps the most vocal critics have been Alan Feduccia and Theagarten Lingham-Soliar, who have proposed that protofeathers are the result of the decomposition of collagenous fiber that underlaid the dinosaurs' integument,
and that maniraptoran dinosaurs with barbed feathers were not actually dinosaurs, but
convergent with dinosaurs.
However, their views have for the most part not been accepted by other researchers, to the point that the question of the scientific nature of Feduccia's proposals has been raised.
Skeleton
Because feathers are often associated with birds, feathered dinosaurs are often touted as the
missing link between birds and dinosaurs. However, the multiple skeletal features also shared by the two groups represent another important line of evidence for
paleontologists. Areas of the skeleton with important similarities include the neck,
pubis,
wrist (semi-lunate
carpal), arm and
pectoral girdle, furcula (wishbone), and
breast bone. Comparison of bird and dinosaur skeletons through
cladistic analysis strengthens the case for the link.
Soft anatomy
Large meat-eating dinosaurs had a complex system of air sacs similar to those found in modern birds, according to an investigation which was led by Patrick O'Connor of
Ohio University. The lungs of theropod dinosaurs (carnivores that walked on two legs and had bird-like feet) likely pumped air into hollow sacs in their
skeletons, as is the case in birds. "What was once formally considered unique to birds was present in some form in the ancestors of birds", O'Connor said.
In a 2008 paper published in the online journal
PLoS ONE, scientists described
Aerosteon riocoloradensis, the skeleton of which supplies the strongest evidence to date of a dinosaur with a bird-like breathing system.
CT-scanning revealed the evidence of air sacs within the body cavity of the
Aerosteon skeleton.
Another piece of evidence that birds and dinosaurs are closely related is the use by both of
gizzard stones. These stones are swallowed by animals to aid digestion and break down food and hard fibers once they enter the stomach. When found in association with fossils, gizzard stones are called
gastroliths.
Reproductive biology
A discovery of features in a
Tyrannosaurus rex skeleton recently provided more evidence that dinosaurs and birds evolved from a common ancestor and, for the first time, allowed paleontologists to establish the sex of a dinosaur. When laying eggs, female birds grow a special type of bone between the hard outer bone and the
marrow of their limbs. This
medullary bone, which is rich in
calcium, is used to make eggshells. The presence of endosteally derived bone tissues lining the interior marrow cavities of portions of the
Tyrannosaurus rex specimen's hind limb suggested that
T. rex used similar reproductive strategies, and revealed the specimen to be female.
Further research has found medullary bone in the theropod
Allosaurus and the ornithopod
Tenontosaurus. Because the line of dinosaurs that includes
Allosaurus and
Tyrannosaurus diverged from the line that led to
Tenontosaurus very early in the evolution of dinosaurs, this suggests that dinosaurs in general produced medullary tissue. Medullary bone has been found in specimens of sub-adult size, which suggests that dinosaurs reached sexual maturity rather quickly for such large animals.
Behavioral evidence
A recently discovered
troodont fossil demonstrates that some dinosaurs slept with their heads tucked under their arms.
This behavior, which may have helped to keep the head warm, is also characteristic of modern birds.
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