Encyclopedia of Evolution.pdf - Online Reading Center

ody and require a large breastbone (sternum) with a keel
for muscle attachment. Modern birds do not have bony
tails; instead, the tail vertebrae are fused into a short pygostyle.
• Miniaturized organs. Bird organs, in many cases, work very
efficiently for their size. Their small lungs allow very efficient
oxygen uptake relative to their size, which is essential
for flight. Reproductive organs remain atrophied except
during the reproductive season. Birds excrete nitrogenous
wastes in the form of dry uric acid rather than urea dissolved
in water; therefore they need to carry very little
water, which is heavy. They excrete white uric acid and
dark intestinal contents through a single opening, the cloaca.
Bird brains are relatively smaller than the brains of
most mammals, since brains (and thickened skulls to protect
them) are heavy.
• High metabolic rate. Birds have high body temperatures
(several degrees higher than those of mammals), which
encourage the rapid metabolism necessary for supplying
energy to flight. The small size of their cells and the small
amount of DNA in each also allows rapid cell growth.
This means that birds have to eat a lot, in some cases half
of their body weight each day, or more! To “eat like a
bird,” a human would have to eat at least 50 pounds of
food daily.
The evolutionary origin of birds from among reptiles is
evident from modern characteristics and from fossil evidence.
Modern characteristics. Birds and reptiles share many
skeletal and muscular features. Some reptile skull features are
present in embryonic but not adult birds. Birds, like reptiles
and amphibians, have nucleated red blood cells. Also, feathers
are just scales that develop greater complexity. Birds have
scales on their feet. Birds, like reptiles, excrete uric acid and
have nasal salt glands; both of these adaptations allow them
to excrete wastes and maintain salt balance without excessive
secretion of water. Bird claws resemble reptile claws;
one bird, the hoatzin, retains claws on the wings in the juveniles.
Birds, like reptiles, have a cloaca, a common opening
for the expulsion of intestinal and kidney wastes. In many
birds, as in many reptiles, the cloaca also releases sperm, and
mating occurs when males and females press their cloacae
together. In some reptiles, and some birds (ratites such as
ostriches, cassowaries, and kiwis; and the geese, ducks, and
swans), males have penes; bird penes resemble reptile, not
mammal, penes.
Fossils. Protoavis, from the Triassic period, is the earliest
fossil that may have been a bird, but its identity as a bird
is uncertain. The oldest undisputed bird fossil is arcHaeopteryx,
which lived 150 million years ago during the Jurassic
period, at the same time as the dinosaurs. The skeleton
of Archaeopteryx was mostly reptilian, but this species had
feathers. Archaeopteryx was intermediate between earlier
dinosaurs and modern birds.
Fossils of birds and other feathered dinosaurs reveal an
entire series of characteristics intermediate between Mesozoic
dinosaurs and modern birds (see figure on page 62). Sinosau-
ropteryx had down feathers that would not have been suitable
for flight. Caudipteryx had flight feathers on its forelimbs
and tail, but these feathers were symmetrical. Confuciusornis
had down feathers on its trunk and asymmetrical flight feathers
on its wings. The skeleton of Confuciusornis was mostly
reptilian but had reduced tail vertebrae and a toothless beak
instead of a toothed jaw. These birds were apparently part of
one of the lineages of coelurosaur dinosaurs.
The feathered dinosaur Archaeoraptor lianingensis, the
discovery of which was announced in 1999, turned out to
be a hoax. It was not an evolutionary hoax, since there was
no need for one. Several other feathered dinosaurs, including
Sinornithosaurus and Beipiaosaurus, had already been
legitimately described from the same fossil bed in China (see
figures on page 63). The hoax was probably intended for
monetary gain, since fossils can bring a lot of money to a
Chinese peasant.
During the Mesozoic era, feathered coelurosaurian
dinosaurs with a whole range of intermediate characteristics
ran around all over the place. And it was not just the coelurosaurs.
In 2004 scientists announced the discovery of a
130-million-year-old tyrannosaur fossil, named Dilong paradoxus,
that had primitive featherlike structures. Scientists are
uncertain which of these feathered dinosaurs may have been
the ancestor of modern birds, precisely because fossils of so
many animals intermediate between reptiles and birds have
been found.
Numerous birds during the Cretaceous period also
had characteristics intermediate between those of reptiles
and of modern birds. At least 12 genera are known. Most
had toothed jaws rather than toothless beaks, and a sternum
not enlarged to the size of that of modern birds. Sinornis
had teeth and an enlarged sternum, but the sternum was not
keeled as in modern birds. Hesperornis in North America
had very small wings, large feet, and was probably a flightless
aquatic diving bird. Other Cretaceous birds resembled
modern birds in their small size and ability to perch. Birds
diversified into numerous lineages, most of which became
extinct.
Before the end of the Cretaceous period, three modern
lineages of birds may have evolved and survived the Cretaceous
extinction. These are the lineages that today include
waterfowl (represented by the Cretaceous Presbyornis),
loons (represented by Neogaornis), and gulls (represented by
Graculavus). The albatross lineage may also have been differentiated
by the Cretaceous. Although fossil evidence is
lacking, cladistic analysis suggests that the earliest divergence
in the history of modern birds was between the paleognaths
(large flightless birds such as ostriches) and all other birds,
the neognaths. Certainly after the Cretaceous Extinction, the
evolutionary radiation of birds was spectacular. Modern bird
groups include:
• waterfowl (ducks, geese, swans)
• swifts and hummingbirds
• wading birds
• herons, storks, and New World vultures
birds, evolution of