Tuesday, February 5, 2013

The feathered dinosaur that didn't force anyone to rethink anything about bird evolution

Eosinopteryx brevipenna is a newly-discovered troodontid from the northeast of China. This little animal has been causing quite a stir among news outlets and paleontological communities, as it has been touted as the force behind a new way of thinking of the evolution of birds and flight. While the discovery of this creature is of course an exciting one, I believe that its impacts on our understanding of avian evolution have been largely overstated by various media.

E. brevipenna lived 150Ma in what were once swamp-like subtropical forests, and coexisted with many diverse and interesting species, from quill-backed heterodontosaurs to bat-like pterosaurs. Two of its close relatives also shared this environment; together, these three species have shed much information on the evolution of birdlike traits (including flight), as well as on adaptive radiation.

Anchiornis huxleyi, a troodontid closely related to Eosinopteryx brevipenna. The colors in this reconstruction are consistent with the fossilized feathers of this species.
Let’s travel all the way back to 2009, when the fossil of a curious Chinese troodontid was described. This was Anchiornis huxleyi, an extremely birdlike species which was the first Mesozoic dinosaur to have its color officially determined. Apart from woodpecker-like coloration, A. huxleyi displayed a number of other avian features, including long forearms which bore flight feathers (though these were not as aerodynamic as gliding microraptorines or true birds) and a highly mobile wrist. Troodontids such as A. huxleyi did not give rise to true birds, yet they displayed traits remarkably similar to them. This fact spurred a reevaluation of the relationship between non-avian and avian dinosaurs, and led to many questions about just where to draw the line between dinosaur and bird, or if this was even possible anymore.

Yeah, yeah, the same reconstruction of Xiaotingia zhengi used in every article about the species. Reconstruction by Xing Lida and Liu Yi.
From the same time and place as E. brevipenna (which I promise I’ll get back to writing about in a second) and A. huxleyi came Xiaotingia zhengi, a species remarkably similar to the famous Archaeopteryx lithographica, often considered the first bird. In fact, the two were very closely related, and therein lay the problem. The discovery of X. zhengi led to game-changing questions:  was this the new first bird? Could A. lithographica even be considered a “bird” anymore, if X. zhengi was to be classified as a non-avian dinosaur? If not, where do both species fit in the evolutionary history of deinonychosaurs and birds? (Xu et. al, 2011)

Since then, many new feathered species have been discovered, and each one is reported to have earth-shaking effects on the taxonomy of non-avian and avian, and how the two are related. The exact position of these species in the sprawling cladograms of deinonychosaurs becomes more and more refined with each related fossil found. The most recent analyses of these fossils place the aforementioned animals within Deinonychosauria, the clade which includes dromaeosaurids and troodontids, and those species helped further refine our understanding of this complex evolutionary history and reach the current conclusion.

So, back to my main topic. Finally, right? Anyway…

E. brevipenna has been portrayed by several media outlets as “forcing” paleontologists to reevaluate the evolution of birds, flight, and feathers. In honesty, E. brevipenna, while it is an interesting and unique species, does less to shed light on bird evolution than A. huxleyi and X. zhengi did.

The only specimen of E. brevipenna. The avian features of this troodontid, such as feathers and a mobile wrist, can clearly be seen in this beautiful fossil.
The almost completely-preserved fossil of E. brevipenna shows an animal very similar to A. huxleyi. The two were, in fact, sister species; they occupied the same habitat at the same time and were incredibly closely related, yet they were extremely different. While A. huxleyi was adapted for a life in the trees, where it could climb and glide from tree to tree in pursuit of prey, E. brevipenna had short feathers on its arms, legs, and tail. Not only were the feathers shorter, but compared to those of its arboreal relative, they were much simpler and would not have provided support for flight even if they were longer. Even the skeletal anatomy of the shoulders and arms of E. brevipenna prohibit the ability of the wings to flap. (Godefroit et. al, 2013)

These traits point to one way of life for our new little troodontid: it was a ground-dweller. Ground-dwelling feathered dinosaurs are by no means rare; the majority of feathered non-avian dinosaurs discovered so far are ground-dwelling. However, E. brevipenna evolved from ancestors with the power of flight, or at least the ability to glide. Its shortened feathers and stiff arms allowed it to be a cursorial animal, pursuing prey on the ground, while the closely-related A. huxleyi remained in the trees.

E. brevipenna: the proud ground-dweller surveying its domain. Reconstruction by Emily Willoughby.
This phenomenon, when closely-related species evolve to fill different niches, is an example of the classic concept of adaptive radiation. The most notable example of adaptive radiation can be seen in the different beak sizes of mockingbirds in the Galapagos, and our fossilized example portrays a similar scenario: the two species at hand evolved to exploit completely different food sources.

While E. brevipenna is a remarkable species which reveals a great deal of new information on the evolution and ecology of small-bodied theropods, it does not have the profound effect on the evolutionary history of birds or feathers as so many sources claim. It did not shake the very roots of the study of feather evolution. However, it does provide yet another example of the diversity of feathers in the Mesozoic Era, a time when many more varieties of feathers were found on a wider range of species, helping each one fill a different niche in a different environment. The discovery of this novel little animal cannot be overlooked, though it needs to be looked at for its true value as an example of adaptive radiation in an extinct family.

Claeys, P.; Demuynk, H.; Dyke, G.; Escuillie, F.; Godefroit, P.; and Hu, D. 2013. Reduced plumage and flight ability of a new Jurassic paravian theropod from China. Nature Communications. doi:10.1038/ncomms2389

Du, K.; Han, F.; Xu, X.; and You, H. 2011. An Archaeopteryx-like theropod from China and the origin of Avialae. Nature. doi:10.1038/nature10288

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