Main Menu
-
5 users online.
Did Birds evolve from Dinosaurs?
The more popular theory of birds as dinosaurs got some major support
in 1861. A fossil was found in a limestone quarry in Solnhofen,
Germany that displays features of both dinosaurs and birds. It had a
theropod head and claws, but it was also covered in flight feathers.
The specimen, named Archaeopteryx lithographica, is regarded as the
oldest bird in the fossil record, dating to around 145 million years
ago. Most scientists take Archaeopteryx as clear evidence that birds
evolved from dinosaurs.
A recent find in Argentina provides another example of a theropod with
some very birdlike traits. The Unenlagia comahuensis, discovered by
Fernando Novas and Pablo Puerta, has a number of features that give it
a predisposition for flight. Most notable is its laterally oriented
shoulder joint. The shoulder socket of all non-avian dinosaurs faces
down and back. Unenlagia's distinctive shoulder would have allowed it
to fold its forelimbs in much the same way a bird folds its wings. It
also permitted Unenlagia to perform a 'flight stroke,' composed of an
upward motion and a full, down and forward stroke. All of this
fossil evidence seems to point to a theropod dinosaur that could
theoretically learn to fly.
Further support comes from comparisons of bird and theropod skeletal
morphology. Paleornithologist Luis Chiappe of the American Museum of
Natural History's Departmentof Ornithology points out numerous similarities
that seem to imply a common ancestry. For example, the theropod foot
has three main toes.
Their alignment and proportions are identical to the three main toes
of a modern bird. Other shared skeletal features include the fused,
semilunate carpal bones and a hip socket unique to birds and
dinosaurs. Chiappe also references behavioral similarities between
theropods and birds. There is fossil evidence suggesting that
Oviraptor sat on its eggs to insulate them, in the same way modern
birds do.
Dr. Alan Feduccia leads the opposing theory about avian evolution. He
is the head of the Biology Department at the University of North
Carolina. To get an idea of just how contradictory the two sides of
this debate are, we must first look at how Feduccia responds to the
so-called "Rosetta Stones" of avian evolution: the fossil remains of
Archaeopteryx. Feduccia claims that Archaeopteryx is not a close
relative to the theropod dinosaurs as is traditionally assumed. He
believes that it is instead something new and different, resulting
from a separate evolutionary line than the dinosaurs. While he does
not give an explanation of exactly what Archaeopteryx is descendant
from, he makes a fairly strong case that it is not simply a feathered
theropod.
Feduccia claims, "If you viewed [the Archaeopteryx fossil and a
theropod skeleton] through binoculars at 50 yards, yes, they'd look
alike," but he believes "the similarity is very superficial."He
points out that the teeth of theropods have a sharp, serrated edge,
while those of Archaeopteryx are peg-like and reptilian. Also,
Archaeopteryx has long forelimbs and proportionally shorter hind
limbs. These proportions are the opposite of theropods, which have
longer hind limbs and shorter forelimbs. This evidence is used to
imply that Archaeopteryx evolved from something that was not a
theropod.
The next argument that Feduccia makes is about the development of
feathers. The dinosaur/bird theorists believe that feathers evolved as
insulation for the newly endothermic dinosaurs and were pre-adapted
for flight. While this makes sense, Feduccia believes that flight
feathers are much too intricate to be evolved for simple insulation.
He says, "Feathers are the most complex appendages ever produced by
the vertebrate integument." To support his argument, Feduccia
notes that the feathers of flightless birds, such as the ostrich, are
simple and almost hair-like. These minimal feathers lack the flat
surface found in the feathers of flighted birds.
Feduccia believes that the ancestors of birds were arboreal, and that
they first learned to fly by gliding from tree to tree on large, scaly
flaps along their forelimbs. He concludes that these ancestors were
the thecodonts, the earliest archosaurs. This theory does make
chronological sense. By placing the ancestors of birds at around 200
million years ago, Feduccia's theory avoids the problem in the
dinosaur/bird theory of where Archaeopteryx came from. The bird-like
theropods that are referenced in the dinosaur/bird theory are about 40
million years younger than Archaeopteryx, making it difficult to draw
a connection between theropods and the first birds.
Unfortunately for Feduccia, his theory does not allow for the
possibility of very much fossil evidence. If early birds were indeed
living in forests, it would be much more difficult to preserve their
remains than if they were terrestrial theropods. The forest ecosystem
provides that if an arboreal thecodont were to die and fall to the
forest floor, its remains would be scavenged and decomposed,
preventing any sort of fossil preservation. Since they would have
lived exclusively in the trees, it is not likely that we will find one
somewhere conducive to fossilization, such as the bottom of a muddy
lake or swamp. Additionally, these small organisms would have adapted
lighter bones in order to decrease body weight and make flight easier.
Their hollow, brittle bones make would break much more easily than
those of a terrestrial organism, making preservation even more
unlikely.
Therefore, as logical as this thecodont ancestor theory is, the
dinosaur/bird supporters still have the upper hand. It is important to
keep in mind that while Dr. Feduccia's argument makes logical and
rational sense, he has little or no fossil evidence to back it up. His
theory lacks the measurability of tangible proof, which is the
backbone of modern science.
What is most interesting about this debate is the fact that new
fossils are being discovered every day. In addition to the thousands
of archeological digs going on all over the world, there is always the
possibility that some quarry worker or vacationing scientist will
stumble upon a fossil that could be the next missing link in avian
evolution. Since the fossil record is so incomplete, a single find
could change everything that we know about where birds come from.
Perhaps someone will someday find a feathered thecodont from 150
million years ago, and Dr. Alan Feduccia will be proven right. Until
that day, however, it seems that the most likely answer to the
question of what birds evolved from will remain theropod dinosaurs.
Article Source: MxGet Article Directory
Author's Bio
The article was produced by the writer of masterpapers.com.
Sharon White is a senior writer and writers' consultant in example term paper writing.
Get some useful tips for
dissertation coach and English research paper writing.
Total views: 15 Word Count: 951
Rating: Not yet rated
Login to vote