Download Origin of Birds

The Origin of Birds
By: Shannon B. Carpenter
The earliest known fossils, which were identified to be a type of bird, link their ancestry to
reptiles, or small dinosaurs of the Triassic period. The fossilized birds from the Triassic/Jurassic
Period have many characteristics in their skeletal structure that could suggest their derivation from
reptiles; even living birds still show several similarities in skeletal makeup that can only otherwise be
seen in long extinct reptiles. The Archaeopteryx, which means “ancient winged creature”, is the
earliest fossil bird known. The first fossil of Archaeopteryx was discovered in 160 million year old
limestone rocks of Southern Bavaria. There are many problems associated with the classification of
Archaeopteryx as being a bird as this creature has traits typical of both dinosaurs and birds. Also
discovered recently were a number of Chinese fossils of the same type of feathered dinosaurs; animals
with down-covered bodies, strong legs, and stubby arms. These winged creatures are examples of
some of the strongest evidence yet that birds evolved from dinosaurs. The Confuciornis,
Protoarchaeopteryx, and the Caudipteryx were three species of these feathered dinosaurs that lived in
the middle to late Jurassic Period of China.
There is a difference in opinion as to whether the Archaeopteryx should be classified as a bird
or a dinosaur. Sir Richard Owen of the British Museum of Natural History first declared that
Archaeopteryx was a bird based on the presence of feathers and a specialized
collarbone, found only in modern birds, called a furcula. Without its feathers,
Archaeopteryx looks almost exactly like a small coelurosaur. In fact, one
specimen of Archaeopteryx was misidentified as a coelurosaur for over 100
years until it’s feathers were noticed by pterosaur expert, Dr. Peter
Wellnhofer. The Archaeopteryx was a land “bird” of limited flying powers
(seen in the picture to the right). Researchers have calculated that Archaeopteryx could almost
quadruple its running speed with thrust from flapping its wings. This “bird” was about the size of a
raven, with a very compact skull structure. The skull has a large preorbital, narial and temporal fossae
and a post temporal bar separating the orbit and temporal fossa. The Archaeopteryx also had short
cylindrical teeth in both its upper and lower jaws. The compact skull structure and short cylindrical
teeth are both characteristics of many fossil reptiles discovered from this time period, not of modern
birds. Some other features that this creature had that were reptilian like were a long bony tail, claws on
both its feet and on its feathered fore-limbs, and it’s bones were solid not hollow, making it seem even
more like todays modern birds. The orbit of the Archaeopteryx’s skull has a ring of sclerotic plates.
This skull is an intermediate form between that of Pseudosuchian reptiles and that of flying birds like
the Pigeon. The brain, which has been reconstructed by many geologists, has been found to be similar
in shape to that of reptiles, but different from that of all other known birds.
The vertebrae of the Archaeopteryx have distinct reptilian traits. “ The
vertebrae are certainty reptilian in structure for they are flat or slightly bioconcave
on their articular ends and nowhere exhibit the saddle-shaped articulations found
in other birds whether fossil or present.” (Swinton, Pg.21) The pectoral girdle is only
one-third the length of the scapula, proving that the pectoral muscles were short
and gave the creature poor flight capabilities. This creature is similar to modern
birds because it has a “cupped-shaped joint between the coracoid and the scapula
also houses the end of the clavicle, whose other end is joined distally to its fellow,
the whole structure being known as the furcula (Latin for a
‘forked prop’) but more familiarly and generally called ‘the merrythought’ or wish-bone…” (Swinton, Pg. 22)
The trait that stands out the most as being “bird-like” is
the fact that the Archaeopteryx had distinct feathers, typical also
to modern birds. These feathers can clearly be seen in the picture to the right. It is
still unknown as to whether they were used for regulating its body temperature or
for flight (because of the short pectoral muscles of this creature, the feathers were
possibly used to glide not for flight). Feathers may have originally been intended
for insulation, but as the creature evolved, become a more important device for it’s
flight capabilities. Two models of the evolution of flight have been proposed. In the
"trees-down" model, birds evolved from ancestors that lived in trees and could glide
down, analogous to today's flying squirrels. In the "ground-up" model, the ancestors
of birds lived on the ground and made long leaps.
It is now believed that the Archaeopteryx more than likely fed on insects and
other small animals. Most insects of the time could be found on plants, especially
shrubs, and very few were found on the bare ground. While foraging there, the
Archaeopteryx had to climb about on the branches and had to leap or fly from one
bush to another. “In this activity it might have been used the claws of its forelimb
for clinging to branches and its apposable hallux for perching.” (Hecht and Ostrom,
Pg.42) Also, the Archaeopteryx probably fed on small animals that lived close to the
shore (such as, worms, crayfish, and fishes) and climbed on the cliffs where it might
have had its nesting-places. “The stimulus for flight was the new supply of insect
food; in order to catch such food there would need to be a lengthening in the jaws,
and with such feeding habits there could be associated that loss of teeth (no longer
needed) and the slightly different means of jaw movement leading to snapping
ability. These are precisely the changes that took place in the step from reptilian
skull to the Archaeopteryx head and jaws.” (Swinton, 28)
In the Liaoning province in northeast China, a
number of feathered dinosaurs have recently been
discovered (June of 1998) that date back more than 140
million years ago. Two species found here were the
Caudipteryx and the Protoarchaeopteryx. Both animals
closely resemble meat-eating dinosaurs called theropods. “The analysis showed that
neither Caudipteryx nor Protarchaeopteryx was a true bird, but that both were
dinosaurs that were very closely related to birds - indeed, Caudipteryx was
determined to be one of the dinosaurs most closely related to true birds.
Protarchaeopteryx (see below for diagram) was found to be
more primitive, and may be a close relative to the
Velociraptorinae, a group of dinosaurs that takes its name
from its most famous member, Velociraptor…” (Chapman,
web) Both of the creatures were fast runners and were probably unable to fly,
judging from their short arms and long legs. Their feathers may have been for
insulation or display, similar to the Archeopteryx. The Protoarchaeopteryx, which
was about the size of a modern-day turkey, is the more primitive and earlier of the
two fossils. There were feathers that covered its body but there is no fossilized
evidence of wing feathers. The Caudipteryx, on the other hand, had more plumage,
including and large tail fan. It stood about three feet tall. The Caudipteryx (see
below for picture) had long, sharp teeth, which had deep, rounded roots. “These
teeth were only in the front of the upper jaw, and pointed outward, giving it a bucktoothed look. (Enchanted Learning, web) When it was found, it had gizzard stones,
gastroliths, in its gut that were used to grind up its food.
Another feathered dinosaur that was found in the Liaoning province in
northeast China was the Sinosauropteryx (see to the right for diagram). This
creature dates back 125 million years ago. “It is early
Cretaceous, its preservation is remarkable (despite the
very crushed skull and scattered bones of the hands
and feet). Soft tissue traces (including skin patches)
and even the gut's contents are discernible.” (Neat
School House, web) The most remarkable feature is that there seems to be a
mane of hair-like or feather-like structures (downy feathers) that run all along
the spine and tail. It was about 3 feet (1 m) long with 2mm long feathers and
was bipedal, walking on two long legs. The longest featherfilaments were 1.5 inches long (3.8 cm). These filaments were
probably used for heat insulation and were not used for flying.
It had two short forelimbs, and many sharp teeth. It ate meat
like other theropods, and probably preyed upon insects and
small animals. It also had a very long tail, which had 64
vertebrate, the longest of any theropod. This tail was probably used for
balance and for making quick turns. The feeding mode of the Sinosauropteryx
was a meat-eater and it is the only dinosaur that has been found with a
mammal in its stomach. (Only the jawbone of this unidentified mammal was
found in the stomach of a Sinosauropteryx.) The most important thing about
this finding is that the shoulder girdle of this dinosaur, allowed a wide range
of up-and-down motion, like flapping. The shoulders of most dinosaurs
enabled only more limited movement of the upper arms. “This is critical for
the bird-kind of motion,” says Xiao-Chun Wu, a paleontologist at the
University of Calgary in Alberta. “This anatomically shows this animal has the
prerequisite for bird-type action of the forelimb.” Wu suggests the dinosaurs
used the up-and-down motions to keep balance while running. (ABCNEWS,
web) These findings show paleontologists that most likely, bird flight evolved
from the ground up. “If feathers appeared first on ground-dwelling dinosaurs,
then they must have originally served some purpose unrelated to flight. Some
scientists speculate that down, like the Sinosauropteryx structures, evolved
first and insulated the bodies of small theropods. Large plumes later may have
served as a display for attracting mates.” (Science News, web) The existence of
Protarchaeopteryx, Sinosauropteryx, and Caudipteryx lends scientists to
believe that birds probably descended from theropod dinosaurs and not from
four-legged tree-living reptiles. Some scientists do believe that these newfeathered fossils are from a time after that of the Archaeopteryx, which
suggests that perhaps the fossils’ resemblance to birds could be a case of
convergent evolution and that their feathers evolved for insulation, not flight,
signifying a warm-blooded physiology.
Two of the first birds to resemble today’s modern birds almost identically is
the Ichthyornis (meaning "fish bird") and the Hesperornis. This creature was ternlike, that lived alongside dinosaurs like T. rex and the Triceratops. It lived in flocks,
nested on shorelines, and hunted for fish in Cretaceous seas. Ichthyornis lived
during the Cretaceous Period about 135-70 million years ago. Ichthyornis was
about 8 inches (20 cm) long. It had a large head, toothed jaws, and a long beak.
This powerful flyer is the oldest-known bird that had a keeled breastbone (sternum)
similar to that of modern birds. The first Ichthyornis found was in 1872 in Kansas,
USA, by a member of paleontologist Othniel C. Marsh's Yale University expedition.
The Ichthyornis was named in Marsh of 1872. Similar fossils of this creature have
also been found in Kansas and Texas, USA, and in Alberta, and Canada. More than
a century ago, Yale paleontologist Othniel Charles Marsh (1880) clearly argued that
three Mesozoic taxa, Archaeopteryx, Hesperornis, and Ichthyornis, differ so widely
from one another that the evolution of birds must have taken place at a much
earlier time, perhaps at the end of the Triassic. He predicted that Triassic birds with
a freely movable quadrate bone would be found to fill the major morphological and
evolutionary gaps in avian history. The Hespernis lived about 70 million years ago
at the end of the age of dinosaurs. This creature had a long pointed bill, which
contained teeth, like that of the Ichthyornis. From the tip of its
beak to the end of its tail, the bird was about 1.8 meters long. The
Hespernis (see right for diagram) had strong legs, which were
adapted for swimming, but its forelimbs were only composed of small, nonfunctional
wings. The best fossils of this creature come form the shallow seas that in
Cretaceous times covered the region that is now Kansas. Hesperornis, Ichthyornis,
and other Cretaceous diving birds, all had skeletons that were modified for flight,
which includes fused pelvis bones, fused hand bones and a short, fused tail. These
birds are greatly similar to modern day birds in physical appearance however, one
characteristic that seems to remain from their reptilian ancestors is the fact that
they still have true socketed teeth.
There are many conjectures as to the origin of birds and their role in the
development of today’s modern birds. No one knows for sure whether or not the
Archaeopteryx is a direct ancestor of today’s living birds but there is a large amount
of evidence, which suggests this may be true. Specific birds such as the
Hesperornis, and Ichthyornis both are almost exact replicas of today’s birds with the
exception that both have teeth. This is the last step in the evolutionary process that
separates present day birds from latter day reptiles.
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