Did Birds Evolve From Dinosaurs? Video

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Did Birds Evolve From Dinosaurs?

Sharon White

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.

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