Protoavis

Systematik

Protoavis texensis ("First bird from Texas") is the name given to archosaurian fossil bones from the Late Triassic found near Post, Texas. These were described as a primitive bird which if valid would push back avian origins some 60-75 million years.

Protoavis is claimed to have been a 35 cm tall bird that lived in what is now Texas, USA, between 225 and 210 million years ago. Though it existed far earlier than Archaeopteryx, its skeletal structure is allegedly more bird-like. Protoavis has been reconstructed as a carnivorous bird that had teeth on the tip of its jaws and eyes located at the front of the skull, suggesting a nocturnal or crepuscular lifestyle. The fossil bones are too badly preserved to allow an estimate of flying ability; although reconstructions usually show feathers (see link below), judging from thorough study of the fossil material there is no indication that these were present (Paul, 2002; Witmer, 2002).

However, this description of Protoavis assumes that Protoavis actually existed and, if so, that it has been reconstructed correctly. Almost all paleontologists doubt that Protoavis is a bird, or even a good species, because of the circumstances of its discovery. When they were found at a Dockum Formation quarry in the Texas panhandle in 1984, in a sedimentary strata of a Triassic river delta, the fossils were a jumbled cache of disarticulated dinosaur and other bones that may reflect an incident of mass mortality following a flash flood.

The discoverer, Sankar Chatterjee of Texas Tech University, was convinced that some of these crushed bones belonged to two individuals - one old, one young - of the same species. However, only a few parts were found, primarily a skull and some limb bones which moreover do not well agree in their proportions respective to each other, and this has led many to believe that the Protavis fossil is chimeric, made up of more than one organism: the pieces of skull appear like those of a coelurosaur, while most parts of the limb skeleton suggest affinities to ceratosaurs and at least some vertebrae are most similar to those of Megalancosaurus (Renesto, 2000), which despite whiat its name may suggest is not a dinosaur but rather an avicephalan diapsid:

Everywhere one turns; the very fossils ascribed thereto challenge the validity of Protoavis. The most parsimonious conclusion to be inferred from these data is that Chatterjee's contentious find is nothing more than a chimera, a morass of long-dead archosaurs. (EvoWiki, 2004)

If it really existed, Protoavis would raise interesting questions about when birds began to diverge from the dinosaurs, but until better evidence is produced, the animal's status currently remains uncertain. Furthermore, biography suggests that birds did not colonize the Americas until the Cretaceous; the most primitive lineages of unequivocal birds found to date are all Eurasian. Certainly, the fossils are most parsimoniously attributed to primitive dinosaurian and other reptiles as outlined above. However, coelurosaurs and ceratosaurs are in any case not too distantly related to the ancestors of birds and in some aspects of the skeleton not unlike them, explaining how their fossils could be mistaken as avian; Archaeopteryx itself was initially believed to be a small theropod dinosaur. Zhou (2004) sums up the matter:

[Protoavis] has neither been widely accepted nor seriously considered as a Triassic bird [... Witmer (2001, 2002)], who has examined the material and is one of the few workers to have seriously considered Chatterjee’s proposal, argued that the avian status of P. texensis is probably not as clear as generally portrayed by Chatterjee, and further recommended minimization of the role that Protoavis plays in the discussion of avian ancestry.

Sometimes it is claimed that Protoavis is a refutation of the hypothesis that birds evolved from dinosaurs (e.g. Feduccia, 1999). But this is not true; the only consequence would be to push back the point of divergence further back in time and possibly cause the dromaeosaurs to be included in the bird clade. Note that at the time when these claims were originally made, the affiliation of birds and maniraptoran theropods which today is well-supported and generally accepted by most ornithologists was much more contentious; most Mesozoic birds have only been discovered since then. Note also that Chatterjee himself (1997) has used Protoavis to support a close relationship between dinosaurs and birds.

As there remains no compelling data to support the avian status of "Protoavis" or taxonomic validity thereof, it seems mystifying that the matter should be so contentious. The author very much agrees with Chiappe in arguing that at present, "Protoavis" is irrelevant to the phylogenetic reconstruction of Aves. While further material from the Dockum beds may vindicate this peculiar archosaur, for the time being, the case for "Protoavis" is non-existent. (EvoWiki, 2004)

References

• Chatterjee, Sankar (1987): Skull of Protoavis and Early Evolution of Birds. Journal of Vertebrate Paleontology 7(3)(Suppl.): 14A.

• Chatterjee, Sankar (1991): Cranial anatomy and relationships of a new Triassic bird from Texas. Philosophical Transactions of the Royal Society B: Biological Sciences 332: 277-342. HTML abstract

• Chatterjee, Sankar (1995): The Triassic bird Protoavis. Archaeopteryx 13: 15-31.

• Chatterjee, Sankar (1997): The Rise of Birds: 225 Million Years of Evolution. Johns Hopkins University Press, Baltimore. ISBN 0801856159

• EvoWiki (2004): Chatterjee's Chimera: A Cold Look at the Protoavis Controversy. Retrieved 2006-OCT-5.

• Feduccia, Alan (1999): The Origin and Evolution of Birds (2nd ed.). Yale University Press, New Haven. ISBN 0300078617

• Melchor, Ricardo N.; de Valais, Silvina & Genise, Jorge F. (2002): Bird-like fossil footprints from the Late Triassic. Nature 417: 936-938. doi:10.1038/nature00818 (HTML abstract)

• Ostrom, J. H. (1991): The bird in the bush. Nature 353(6341): 212.

• Ostrom, J. H. (1996): The questionable validity of Protoavis. Archaeopteryx 14: 39-42.

• Paul, Gregory S. (2002): Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds. Johns Hopkins University Press, Baltimore. ISBN 0801867630

• Renesto, S. (2000): Bird-like head on a chameleon body: new specimens of the enigmatic diapsid reptile Megalancosaurus from the Late Triassic of northern Italy. Rivista Italiana di Paleontologia e Stratigrafia 106: 157–180. HTML abstract

• Witmer, Lawrence M. (2001): The role of Protoavis in the debate on avian origins. In: Gauthier, J & Gall, L. F. (eds): New perspectives on the origin and early evolution of birds: 537-548. Special Publication of the Peabody Museum of Natural History, Yale University, New Haven, Conn., USA. ISBN 0912532572

• Witmer, Lawrence M. (2002): The debate on avian ancestry: phylogeny, function, and fossils. In: Chiappe, Luis M. & Witmer, Lawrence M. (eds): Mesozoic birds: Above the heads of dinosaurs: 3-30. University of California Press, Berkeley, Calif., USA. ISBN 0520200942

• Zhou, Zhonghe (2004): The origin and early evolution of birds: discoveries, disputes, and perspectives from fossil evidence. Naturwissenschaften 91(10): 455-471. doi:10.1007/s00114-004-0570-4 (HTMl abstract)

External links

• Protoavis reconstruction (note that most details are entirely conjectural)
• Dinosaur Encyclopaedia: Protoavis

Vorlage:Bird-stub