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, 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.
The main explanation that Alan Feduccia uses to disprove the theory of a theropod origin of birds is based on the origin of avian flight. The dinosaur/bird theory implies that birds achieved flight from the ground up. He claims that the “ground up” theory is impossible due to the physical limitations of theropods. Theropods were adapted to run on the ground. Part of this adaptation was their body shape; it was flattened from the sides, making it vertically long and narrow. Birds are just the opposite; they are flattened on the top and bottom, making their bodies more horizontally spread out, which is good for flight.
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.
Works Cited
DiSilvestro, Roger L. “In quest of the origin of birds.” Bioscience 47 (1997): 481-486.
Feduccia, Alan, and Larry D. Martin. “Theropod-bird link reconsidered.” Nature 391 (1998): 754.
Novas, Fernando E., and Pablo F. Puerta. “New evidence concerning avian origins from the Late Cretaceous of Patagonia.” Nature 387 (1997): 390-393.
Novas, Fernando E., and Pablo F. Puerta. “New evidence concerning avian origins from the Late Cretaceous of Patagonia.” Nature 387 (1997): 390-393.
DiSilvestro, Roger L. “In quest of the origin of birds.” Bioscience 47 (1997): 481-486.
Feduccia, Alan, and Larry D. Martin. “Theropod-bird link reconsidered.” Nature 391 (1998): 754.
DiSilvestro, Roger L. “In quest of the origin of birds.” Bioscience 47 (1997): 481-486.