Protoavis

Protoavis ("first bird") is the name given to archosaurian fossil bones from the Late Triassic (Norian stage), found near Post, Texas by Dr. Sankar Chatterjee. These fossils have been described as a primitive bird which, if the identification is valid, would push back avian origins some 60-75 million years.

The identification of the skeleton as a bird is controversial. Dr. Lawrence Witmer examined the braincase critically and was not able to confirm bird features. He was, however, able to confirm features known only in coelurosaurian theropods. This makes protoavis the earliest known coelurosaur.^[1]

The original describer of Protoavis texensis, Sankar Chatterjee of Texas Tech University, interpreted the type specimen to have come from a single animal, specifically 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 like that of modern birds.^[2] 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.^[3] Reconstructions usually depict it with feathers, as Chatterjee originally interpreted structures on the arm to be quill knobs, the attachment point for flight feathers found in some modern birds and non-avian dinosaurs. However, re-evaluation of the fossil material by subsequent authors such as Lawrence Witmer have been inconclusive regarding whether or not these structures are actual quill knobs.^[4][1]

However, this description of Protoavis assumes that Protoavis has been correctly interpreted as a bird. Many paleontologists doubt that Protoavis is a bird, or that all remains assigned to it come from a single species, because of the circumstances of its discovery and unconvincing avian synapomorphies in its fragmentary material. When they were found at a Bull Canyon Formation (formerly known as the 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 bones that may reflect an incident of mass mortality following a flash flood.

Dispute

Chatterjee 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 Protoavis 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, an avicephalan diapsid:^[5]

"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."^[6]

If it really is a single animal and not a chimera, Protoavis would raise interesting questions about when birds began to evolve, but until better evidence is produced, the animal's status remains uncertain. Furthermore, paleobiogeography suggests that true birds did not colonize the Americas until the Cretaceous; the most primitive lineages of unequivocal birds found to date are all Eurasian.^{[citation needed]} 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. Paleontologist Zhonghe Zhou stated:

"[Protoavis] has neither been widely accepted nor seriously considered as a Triassic bird ... [Witmer], 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."^[7]

In discussions of evolution

Scientists such as Alan Feduccia have cited Protoavis in an attempt to refute the hypothesis that birds evolved from dinosaurs.^[8] However, the only consequence would be to push back the point of divergence further back in time.^[6] At the time when such claims were originally made, the affiliation of birds among 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. Chatterjee himself has since used Protoavis to support a close relationship between dinosaurs and birds.^[9]

"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."^[6]

References

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