Which came first, the thecodont or the egg?

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Which came first, the thecodont or the egg?

A discussion of avian origins

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        Although most paleontologists today would agree that the debate over the origin of birds has been settled, there are still those who do not accept the widely acknowledged theory that modern birds evolved from theropod dinosaurs.  Despite the evidence to the contrary, these researchers are determined to expose the supposed failings of this theory.  While they provide little or no solid scientific evidence to prove how birds did evolve, these skeptics do have some compelling arguments that refute the dinosaur/bird theory.  To better understand this dispute, we must examine the cases made by both sides.

        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.

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        Further support comes from comparisons of bird and theropod skeletal morphology. Paleornithologist Luis Chiappe of the American Museum of Natural History's Department

of 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, ...

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