Protoavis and Brief Insight in the Herbivory in Theropoda

Everyone who is interested in the evolution of birds should at least be aware of the controversial Protoavis texensis. For those who have not the slightest of idea, this taxon was found in 1987 and described in 1991 by Sankar Chatterjee after finding them in a quarry in Texas, which date back to the Late Triassic period. After studying the remains of the animal, Chatterjee claims that he discovered the world's oldest known bird - a creature older than Archaeopteryx lithographica by 75 million years! It would have been a remarkable and revolutionary discovery IF not for the extremely fragmentary nature of the bones. This post will discuss briefly on Protoavis and its controversial history and a possible connection with the evolution of herbivory in theropods.

What in the World is Protoavis?
Chatterjee promotes it as the oldest known bird. Not only just that, but he also argues that Protoavis is the sister taxon to Pygostylia (the clade that contains our crowned birds and stem-bird avialans with a short, stubby tail) as it shows anatomical features more akin to Pygostylia than to earlier stem-bird maniraptorans (e.g, Chatterjee, 1997). This would mean that not only would be more older than Archaeopteryx but more derived as well. Not just that, but it would also mean that all of our major coelurosaurian lineages (especially those of Maniraptora and Paraves) have already been established at this time some 225 million years ago!

It was not surprising that there is a lot of criticism. The critics who have examine the material have stated that there is nothing uniquely avian about this animal - there seems to elements similar to more primitive theropods such as coelophysoids (after Chiappe & Witmer, 2003). Not to mention, Chatterjee seemed to have destroyed the matrix where it comes from (resulting in some of the bones to break as well), and thus losing context. This resulted in the on-going debate whether Protoavis texensis is a bird, an indeterminate theropod or even a chimera of different species that happen to mix up (after Chiappe & Witmer, 2003).

Where are the Bloody Photos???

As far as I am aware of, this is the only photo there is of Protoavis texensis. Photo by Chatterjee.

Before we get to the next topic, I wanna address something. One of the things I have found most interesting the most is, for a fossil that is claimed to be very extraordinary, there is hardly any photos. The only photo I am aware of is a somewhat decent quality photo that is found on the Wikipedia page for Protoavis. It is too hard to make out any sort of positive judgement if the bones do belong to a single species and, if so, are they truly avian? I am surprised that Chatterjee has failed to produced additional photographs, especially on individual bones.

The next closet thing to see the bones for people (or at least for me) are the drawings Chatterjee provided in his book The Rise of Birds: 225 Million Years of Evolution. There are two editions and I have read both. The drawings are, however, more or less the same. The drawings are pretty detailed and look pretty avian. But the problem is that the drawings are based on his interpretations. Other scientists had given a look and offer contradicting interpretations as the kind of animal Protoavis is. Another thing worth mentioning is it is frown upon to conduct any sort of phylogenetic studying based on photos and drawings of poor-quality fossils as you miss some important detail.

Not exactly from the book, but this is the sort of thing that occurs a lot. Note the comparison and contrasts with the photo above. Reconstruction by Chatterjee.

The best thing is for someone take more photos of the fossil or do a digital 3D scan/modeling and post it online. It would still not be using the actual fossil, but it is better than using a picture and studying in the realm of 2D format. With that said, let us move on.

Herbivory in Theropoda
Until in the last score Theropoda was seen as a clade of mostly flesh-eating dinosaurs. This includes some of the most iconic dinosaur genera of all time such as Tyrannosaurus, Velociraptor, Allosaurus, and so forth. Yet there has been a stacking amount of evidence is showing that theropods had evolved herbivorous or omnivorous diets independently from time and time again. Indeed living theropods the birds have done a fantastic job in exploiting different diets.

Of course the understanding of the dietary change in theropods is still not 100%. It has the potential, however, to play a major role in the evolution of birds, in regards to the the avian beak (Zanno & Makovicky, 2010). Bird beaks are covered in a special keratin call rhamphotheca that covers the outing. Herbivory might have had evolved early on in the evolution of crowned birds (Zhou & Zhang, 2002). Below is a list of known herbivorous stem-bird theropods:

1. Elaphrosaurs
2. Chilesaurus diegosuarezi
3. Ornithomimosaurs
4. Therizinosaurs
5. Oviraptorosaurs
6. Jeholornis prima

How does this got to do with Protoavis? Well, there are actually some things that Protoavis and  Lineages 1-6 had in common: they have small box-shaped heads with long necks for probable foraging. The premaxilla of lacked any teeth and there seems to be lack of tooth replacement (Zanno & Makovicky, 2010). Given that the rhamphotheca beak had evolved twice in elaphrosaurs (Lineage 1) and coelurosaurs (Lineages 3-6) (Xu et. al, 2009), whose says it did not occur in earlier lineages in the Triassic? In addition the Triassic was a time where vertebrates were continuing to experiment and evolve herbivory (Reisz, 2000) perhaps theropods had experimented eating plants early in their evolution at this time. Perhaps Protoavis was a herbivorous theropod that might have mosaic features linking coelophysoids and more derived neotheropods. Again, this is assuming the remains do not suggest a chimera.

In some of the therizinosaurs and ornithomimosaurs elongated forelimbs were used for grasping and hooking on to branches (Lautenschlager, 2014). This could be a more reasonable reason for the wing-like forelimbs in Protoavis. Protoavis also has a manus that is similar to a modern bird, especially in the reduction of the digits. However this can be seen in elaphrosaurs and Chilesaurus (Novas et. al, 2015) and their arms were too short for them to able to thrust off and fly. This suggests that this did not coevolve with flight (just as how feathers are seen nowadays). Why the forelimbs and fingers are reduced is not clear, but it is possible they have lost their function as they became more cursorial.

Probably a more ideal and less controversial statement? Please note that I might have accidentally made the neck too long or the forelimbs too short. Might update this in the near future. Art by yours truly.

Looking back at the photo with this in mind, it is quite possible that Protoavis is a pretty revolutionary dinosaur. But for a different reason. Instead of the flying, Triassic "bird" that Chatterjee had envision, Protoavis might have been a terrestrial herbivore that used its forelimbs for - not for flying - but for hooking and grasping branches to reach for leaves. Long legs meant it had to be a fast sprinter to escape the jaws and teeth of predatory stem-crocodilians, other dinosaurs, stem-mammals and crowned mammals. But this requires a fresh look at the material with an open, clear mind before hand in order to understand the validity of Protoavis texensis, the biology and its phylogenetic placement. To quote Sherlock Holmes in A Scandal in Bohemia:

"It is a capital mistake to theorize before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts."

Further Reading
Jaime A. Headden did a fantastic job discussing the evolution of herbivorous theropods in his blog "Gilong", which can be read here.

References

• Chatterjee, S. (1997). The Rise of Birds: 225 Million Years of Evolution. Johns Hopkins University Press.
• Chiappe, Luis M., & Witmer, Lawrence M. (Eds.)(2003). Mesozoic Birds: Above the Heads of Dinosaurs. University of California Press, California, 7-9.
• Lautenschlager, S. (2014). Morphological and functional diversity in therizinosaur claws and the implications for theropod claw evolution. Proceedings of the Royal Society B: Biological Sciences, 281(1785), 20140497.
• Novas, F. E. et, al. (2015). An enigmatic plant-eating theropod from the Late Jurassic period of Chile. Nature.
• Reisz, R. R., & Sues, H. D. (2000). Herbivory in late Paleozoic and Triassic terrestrial vertebrates. Evolution of Herbivory in Terrestrial Vertebrates. Cambridge University Press, New York, 9-41.
• Xu, X., et, al. (2009). A Jurassic ceratosaur from China helps clarify avian digital homologies. Nature, 459(7249), 940-944.
• Zhou, Z., & Zhang, F. (2002). A long-tailed, seed-eating bird from the Early Cretaceous of China. Nature, 418(6896), 405-409.
• Zanno, L. E., & Makovicky, P. J. (2010). Herbivorous ecomorphology and specialization patterns in theropod dinosaur evolution. Proceedings of the National Academy of Sciences, 201011924.