|Did Birds Evolve from Dinosaurs?|
By Dr Marc Surtees
The modern form of the theory that birds are related to theropod dinosaurs (two-legged carnivores ranging from the chicken-sized Compsognathus to the gigantic Tyrannosaurus) dates from the 1970s when Ostrom described similarities between Archaeopteryx, a Jurassic bird (see below) and Deinonychus, a theropod.
Deinonychus (discovered in 1969) was 2.5 to 4 metres long and is thought to be 110 Myr old. It has long been believed that it and similar "raptors" were feathered.
The Chinese Fossils
Fossils from Cretaceous sediments in China are now claimed to confirm this theory. These sediments are interpreted as remains of ancient lakes which once covered the Liaoning Province in north-eastern China, during the Cretaceous period. These layers contain many fossils of plants, insects, molluscs, crustaceans, fish, frogs, salamanders, turtles, lizards, mammals and birds, such as Confuciusornis sanctus, together with well preserved theropod dinosaurs such as Sinosauropteryx prima.
Their importance lies in the fact that some have remains of filaments that are thought to be precursors to feathers (proto-feathers) and that others had feathers.
One of the first fossils from this region was Confuciusornis (Hou et al, 1995) a bird from Jurassic sediments. However, the real excitement was caused by the discovery of small theropod dinosaurs which appeared to have remains of filament-like structures which some consider to be proto-feathers, (which is sometimes called "dino-fuzz").
I will briefly review five of these creatures: Sinosauropteryx, Caudipteryx, Sinornithosaurus, Microraptor and Protarchaeopteryx.
Sinosauropteryx (whose name means "Chinese dragon feather") was the first of the theropod dinosaurs that was found to have dino-fuzz. It was about one metre in length and had a very substantial tail with 64 bones. It had hollow leg bones, long legs and short arms.
The discovery of dino-fuzz on this and other theropod dinosaurs has led to considerable speculation about which dinosaurs had feathers. National Geographic was so enthusiastic about this that, in one issue, it published pictures of a feathered Tyrannosaurus rex (Sloan, 1999).
Caudipteryx was a turkey-sized animal with well-preserved feathers. This creature had feathers on the end of its short tail and beautifully preserved long symmetrical feathers attached to its very short forelimbs.
Sinornithosaurus is similar in size to Sinosauropteryx and is also said to have remains of proto-feathers or feather-like covering.
Microraptor gui has been described as a four-winged dinosaur. It had asymmetrical flight feathers on all four limbs and is considered by some to be evidence that bird ancestors where gliding creatures that lived in trees, rather than the alterative ground runners. But questions remain about how it managed to fly and how it would get off the ground if it ever landed.
Protarchaeopteryx had a tail with a spray of symmetrical feathers. Wing feathers have also been described for this creature. It had long hands (longer than the arms). The legs were long and powerful with three forward-facing toes.
When evaluating fossils which have feathers it is important to determine whether or not the feathers are asymmetric or symmetric. All flying birds have asymmetric wing or flight feathers, therefore it is reasonable to assume that any fossil with asymmetric feathers was a flying creature. However, symmetric feathers are found on the wings of flightless birds.
The suggestion that some of the Chinese fossils have proto-feathers has been contested by Feduccia, who described one of the first bird fossils from Liaoning, and is an expert on fossil birds. He and his team studied the evidence and the way in which reptile skin decomposes and looked at fossils of other animals which have "dino-fuzz" (Feduccia et al, 2005). They concluded that there is no good evidence that fossilized structures found in China are rudimentary feathers. Instead, the fossilised patterns appear to be "bits of decomposed skin and supporting tissues that just happen to resemble feathers to a modest degree" (Williamson, 2005).
Archaeopteryx was first discovered in 1861 in the Jurassic sediments of the Solnhofen region of Germany. All experts agree that while it has some unusual features it was a true bird and was able to fly, even if not very strongly. It had air-filled bones like birds we find today and asymmetrical flight feathers. The brain structure is similar to that of modern birds. Alonso et al (2004) carried out a study of endocasts of an Archaeopteryx skull and concluded, among other things, that it had an enlarged forebrain with neurological and structural adaptations and enhanced somatosensory integration required for a lifestyle involving flying ability.
Fossil bird tracks?
Late Triassic "bird-like" footprints from the Santo Domingo Formation in Argentina were reported by Melchor et al (2002). These footprints are very similar to those made by modern birds, with claw marks and a backward-facing toe (the hallux) and many other features that are found in modern bird footprints. However, the authors conclude that "these bird-like footprints can only be attributed to an unknown group of theropods showing some avian characters". That is, while they acknowledge that these prints have "clearly avian characters" and the most likely conclusion is that these are bird footprints, this is not possible because there are no known Triassic bird fossils. Further, although they acknowledge that there are no known theropod dinosaurs with feet showing these avian characteristics, they maintain that they must have existed.
Protoavis is probably the most controversial of the fossil birds. Chatterjee (1991) believes it to be a Triassic bird, older than Archaeopteryx. Only fragments have been found, but its discoverer considers it to have many features associated with flight, for example a keel-like sternum indicating that it would have been a better flyer than Archaeopteryx. Its skull was lightly built and pneumatised, with a temporal region similar to modern birds. It also had a relatively large brain with an avian brain architecture similar to modern birds with neurosensory specialisations associated with balance, coordination, flight, agility and high metabolic activity. Claw morphology suggests that Protoavis could climb trees, yet the development of a supracoracoideus (the principal muscles that lift the wing) pulley indicates that it was able to fly.
Caudipteryx: Dinosaur or Bird?
Careful analysis of the anatomy of Caudipteryx suggests that it is more like a flightless bird than a dinosaur (Jones et al, 2000). These authors concluded that Caudipteryx ran using a mechanism more similar to that of modern cursorial birds than to typical dinosaurs. The relative hindlimb proportions of Caudipteryx are indistinguishable from those of cursorial birds and it was also likely to have had its centre of mass situated towards the front, rather than towards the rear as in dinosaurs.
An anterior centre of mass is consistent with the brevity of the tail of Caudipteryx, which is shorter than that of almost all bipedal dinosaurs. Additionally, as in cursorial birds but not dinosaurs, the diminutive tail and the complete absence of a femoral fourth trochanter (a bony lump on the thigh bone to which muscles attach) indicate that the caudofemoralis was not an important locomotory muscle.
The order of the fossils in the geological record actually flies in the face of the theory that birds evolved from theropods. The theropods are found in Cretaceous rocks, while birds are found in Jurassic sediments, which is the opposite of what would be predicted by the theory that birds evolved from theropod dinosaurs. These fossils only prove the evolution of birds from dinosaurs if one assumes that they are descended from an unknown common ancestor that predates both Archaeopteryx and the Chinese "feathered theropods". If this assumption is made then I agree that the fossils might show how it happened, but they do not prove that it did happen.
I suggest that Caudipteryx was actually a running bird with a similar life-style and morphology to modern ostriches. Other so-called theropod dinosaurs with feathers may also prove to be running birds, albeit with dinosaurian features.
I would also suggest that creatures with clear evidence of feathers, either flightless (like Caudipteryx and Protarchaeopteryx) or able to fly, (like Archaeopteryx) are all birds.
Furthermore, as the oldest fossils are birds, a case could be made that species like Caudipteryx and the other Chinese fossils descended from birds rather than vice versa, because this is consistent with the sequence of the fossils in the rocks. The Chinese fossils are assumed to be relics of earlier species of bird ancestors because that is consistent with the assumption that birds evolved from theropod dinosaurs, not because the evidence demands it.
There are two other difficulties for the accepted theory that birds evolved from theropod dinosaurs. First the evidence of Triassic "bird-like" footprints conflicts with the accepted dogma. Yet the authors preferred to put their faith in the existence of an unknown theropod dinosaur with bird-like feet rather than suggest that these footprints are evidence that birds existed in the Triassic. They reasoned that these could not be bird footprints because they are older than the oldest known bird fossils.
This brings us to the second problem for the current theory of bird evolution; Protoavis. The fossils of Protoavis, while controversial, are consistent with the existence of Triassic birds. However, the majority refuse to accept that they could be the remains of a bird. Indeed if Protoavis was a bird that would challenge the evolutionists’ theory that birds evolved from theropods whose fossils are 100 million years younger by their reckoning.
Taken together the fossils do not appear to provide indisputable evidence for the theory that birds evolved from theropod dinosaurs. Indeed, birds appear in the fossil record lower than their supposed ancestors, not higher as we might expect. Also, some of the evidence (Protoavis and Triassic bird footprints) appears to refute the current evolutionary story of bird ancestry. Furthermore, the evidence for "protofeathers" has been questioned. However, evolutionists try to explain away the discordant evidence to protect the theory. Therefore, I conclude that the existence of superbly engineered birds remains a significant challenge to neo-darwinian evolution.
Alonso, P. D., Milner, A. C., Ketcham, R. A., Cookson, M. J. and Rowe, T.B. Nature 430, 666-669 (2004).
Chatterjee, S. Phil. Trans. R. Soc. Lond. B 332, 277-346 (1991).
Feduccia, A., Lingham-Soliar, T and Hinchliffe, J.R. Journal of Morphology 266,125–166 (2005)
Hou, L.-H., Zhou, Z., Martin, L. D. and Feduccia, A. Nature 377, 616 - 618 (1995).
Jones, T. D., Farlow, J. O., Ruben, J. A., Henderson, D. M. and Hilleniuet, W. J. Nature 406, 716 -718 (2000).
Melchor, R.N., de Valais, S. and Genise J. F. Nature 417, 936 - 938 (2002).
Sloan, C.P. National Geographic 196, 98 – 107 (1999).
Williamson, D. http://www.unc.edu/news/archives/oct05/feducci100705.htm (2005).
© Dr. Marc Surtees, 2006.
Dr. Surtees has a B.Sc. in Applied Biology and a Ph.D. in Zoology, having previously worked for a pharmaceutical company for 14 years.
Speculations on the chemical origins of life are almost universally covered in school curricula under ‘Evolution’, despite the questionable relevance of the topic for evolution, and its rather uncertain scientific basis.
Moore, A. (2008) Nature 453:31-32