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.
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| 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.
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| 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.
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| 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.
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| 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.
References
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
Coverage by Brian Switek on Laelaps: http://phenomena.nationalgeographic.com/2013/01/29/tiny-feathery-dinosaur-raises-jurassic-questions/
Coverage by Jon Tennant on Green Tea and Velociraptors: http://blogs.egu.eu/palaeoblog/2013/01/31/a-new-feathered-dinosaur-worth-getting-ruffled-for/
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