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The origin of birds
The discovery that birds evolved from small carnivorous
dinosaurs of the Late Jurassic was made possible by recently
discovered fossils from China, South America, and other countries,
as well as by looking at old museum specimens from new perspectives
and with new methods. The hunt for the ancestors of living birds
began with a specimen of Archaeopteryx, the first known
bird, discovered in the early 1860s. Like birds, it had feathers
along its arms and tail, but unlike living birds, it also had teeth
and a long bony tail. Furthermore, many of the bones in
Archaeopteryx's hands, shoulder girdles, pelvis, and feet
were distinct, not fused and reduced as they are in living birds.
Based on these characteristics, Archaeopteryx was
recognized as an intermediate between birds and reptiles; but which
reptiles?
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The Berlin specimen of Archaeopteryx
lithographica.
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In the 1970s, paleontologists noticed that
Archaeopteryx shared unique features with small
carnivorous dinosaurs called theropods. All the dinosaur groups on
this evogram, except the ornithischian dinosaurs, are theropods.
Based on their shared features, scientists reasoned that perhaps
the theropods were the ancestors of birds. When paleontologists
built evolutionary trees to study the question, they were even more
convinced. The birds are simply a twig on the dinosaurs' branch of
the tree of life.
As birds evolved from these theropod dinosaurs, many of their
features were modified. However, it's important to remember that
the animals were not "trying" to be birds in any sense. In fact,
the more closely we look, the more obvious it is that the suite of
features that characterize birds evolved through a complex series
of steps and served different functions along the way.
Take feathers, for example. Small theropods related to
Compsognathus (e.g., Sinosauropteryx) probably
evolved the first feathers. These short, hair-like feathers grew on
their heads, necks, and bodies and provided insulation. The
feathers seem to have had different color patterns as well,
although whether these were for display, camouflage, species
recognition, or another function is difficult to tell.
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This fossil of Sinosauropteryx preserves evidence of
hair-like feathers.
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In theropods even more closely related to birds, like the
oviraptorosaurs, we find several new types of feathers. One is
branched and downy, as pictured below. Others have evolved a
central stalk, with unstructured branches coming off it and its
base. Still others (like the dromaeosaurids and
Archaeopteryx) have a vane-like structure in which the
barbs are well-organized and locked together by barbules. This is
identical to the feather structure of living birds.
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At right, asymmetrical flight feathers are present in a fossil of a
dromaeosaurid that may have had the ability to glide.
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Another line of evidence comes from changes in the digits of the
dinosaurs leading to birds. The first theropod dinosaurs had hands
with small fifth and fourth digits and a long second digit. As the
evogram shows, in the theropod lineage that would eventually lead
to birds, the fifth digit (e.g., as seen in Coelophysoids) and then
the fourth (e.g., as seen in Allosaurids) were completely lost. The
wrist bones underlying the first and second digits consolidated and
took on a semicircular form that allowed the hand to rotate
sideways against the forearm. This eventually allowed birds' wing
joints to move in a way that creates thrust for flight.
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This oviraptorid dinosaur, Citipati osmolskae, may have
been protecting a nest of eggs.
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The functions of feathers as they evolved have long been
debated. As we have seen, the first, simplest, hair-like feathers
obviously served an insulatory function. But in later theropods,
such as some oviraptorosaurs, the feathers on the arms and hands
are long, even though the forelimbs themselves are short. What did
these animals do with long feathers on short arms? One suggestion
comes from some remarkable fossils of oviraptorosaurs preserved in
the Cretaceous sediments of the Gobi Desert. The skeleton of the
animal is hunched up on a nest of eggs, like a brooding chicken.
The hands are spread out over the eggs as if to shelter them. So
perhaps these feathers served the function of warming the eggs and
shielding them from harm.
Birds after Archaeopteryx continued evolving in some of
the same directions as their theropod ancestors. Many of their
bones were reduced and fused, which may have helped increase the
efficiency of flight. Similarly, the bone walls became even
thinner, and the feathers became longer and their vanes
asymmetrical, probably also improving flight. The bony tail was
reduced to a stump, and a spray of feathers at the tail eventually
took on the function of improving stability and maneuverability.
The wishbone, which was present in non-bird dinosaurs, became
stronger and more elaborate, and the bones of the shoulder girdle
evolved to connect to the breastbone, anchoring the flight
apparatus of the forelimb. The breastbone itself became larger, and
evolved a central keel along the midline of the breast which served
to anchor the flight muscles. The arms evolved to be longer than
the legs, as the main form of locomotion switched from running to
flight, and teeth were lost repeatedly in various lineages of early
birds. The ancestor of all living birds lived sometime in the Late
Cretaceous, and in the 65 million years since the extinction of the
rest of the dinosaurs, this ancestral lineage diversified into the
major groups of birds alive today.
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gladiator answered 1 year ago