Feathered Dinosaurs: How do we know?

For a long time feathered dinosaurs just looked weird to me. Seeing fuzzy Deinonychus or some other dromeosaur with a splash of plumage never looked quite right and I didn’t understand why in the course of a few years predatory dinosaurs went from being scaly to being covered in down. Most of the books I had seen didn’t explain it beyond “These dinosaurs were closely related to birds,” something I didn’t dispute but was not enough to make me feel comfortable with feathered raptors. Even after I started taking a greater interest in paleontology I still had problems with reconstructions of feathered dromeosaurs when no specimens with feathers had yet been found. As I learned more, however, I came around to the notion that any Deinonychus or Velociraptor that appears without feathers is a naked dinosaur, a growing body of evidence showing that there were probably many more feathered dinosaurs than had previously been suspected.

The notion that birds and dinosaurs are closely related has been around for a very long time, kicked off by the discovery of Archaeopteryx and Compsognathus in the mid-19th century and popularized by T.H. Huxley. (It should be noted, though, that similarity was not always considered to reflect an ancestor-descendant relationship.) Even though the term “dinosaur” had only just been coined by Richard Owen in 1842 by the 1870’s the Solnhofen fossils, trackways, and the bipedal Hadrosaurus and Dryptosaurus spurred the first Dinosaur Renaissance, revealing bipedal (and even bird-like) animals rather than the pachyderm-like creatures Owen brought to life at the Crystal Palace. The smaller dinosaurs like Compsognathus and Hypsilophodon were particularly important as they were considered to be more representative of the form of bird ancestors, flightless birds like rheas and emus being the next step in the hypothetical evolutionary system. Still, it wasn’t until the second Dinosaur Renaissance of the late 20th century that the notion of birds as living dinosaurs began to hatch.

A particular problem plagued the hypothesis that birds evolved from dinosaurs, however; no feathered dinosaurs had been found. Archaeopteryx was considered to be a bird, too derived to be comfortably called a feathered dinosaur, and while it became increasingly important as a transitional fossil a transition from what was debated. Eventually the morphological problems that faced the dinosaur hypothesis, like the supposed absence of clavicles, were overcome with new evidence and increased study but more than Archaeopteryx was needed to confirm the predictions being made. Enter Sinosauropteryx prima, described in 1996 (Chen et al 1998). Although there was some skepticism about whether or not the preserved structures were really feathers (Unwin 1998, Thomas & Garner 1998) a flood of feathered dinosaurs coming out of China soon followed and overwhelmingly supported the notion that birds evolved from dinosaurs.

Specimens like “Dave” (a probable Sinornithosaurus, Ji et al 2001, the first specimen of Sinornithosaurus millenii also bearing filamentous feathers [Xu et al 1999]) threw greater weight to the notion that birds had evolved from predatory dinosaurs. (For a fairly recent reviews see Norell & Xu 2005 and Zhou 2004.) Even more surprising, however, was the discovery of Beipiaosaurus inexpectus, a therizinosauroid dinosaur with integumentary feathers. While they are theropods relatively closely related to the raptors therizinosauroid dinosaurs are also very different, having long necks, huge claws on their hands, and perhaps a herbivorous diet. If these dinosaurs had feathers it raises the question of whether other coelurosaurs, which include the ostrich-like ornithomimosaurs and the tyrannosaurs, also had feathers. It is entirely possible that even the terrifying Tyrannosaurus had plumage during at least some stage of its life, perhaps to regulate body temperature when small but shedding the feathers as it quickly became larger, and even the recently described Jurassic tyrannosauroid Guanlong (Xu et al 2006) may have had feathers. This hypothesis has yet to be confirmed but it may be that feathers are not just indicative of dromeosaurs but of coelurosaurs as a group.

Feathers pop up again and again in differing forms and arrangements in coelurosaurs, and if we think about this evolutionarily the prediction that at least all dromeosaurs should be feathered is clear. If feathers were not inherited from a common ancestor then they would have to evolve several times within the coelurosaurs, such large-scale convergence being unlikely. If different groups of coelurosaurs were feathered it is reasonable to assume that most, if not all, were, and any that lacked feathers would have secondarily lost them for one reason or another. This prediction is still being worked out but future studies may help confirm or refute it, especially since there is more than one way to detect the presence of feathers in the fossil record.

The discovery that truly blew me away was revealed in a short note printed in Science last year. Until recently I had assumed that the detection of feathered dinosaurs relied entirely upon exceptional preservation, feathers being so delicate that in most cases they will not be preserved (indeed, Archaeopteryx specimens in which the feathers were not well-preserved were mistaken for Compsognathus and pterodactyls). If the feathers are not preserved we can still make the case that the dinosaur would have been feathered based upon its relationships but confirming the idea would require an exceptionally-preserved specimen, the chances of finding one becoming increasingly less likely with increasing body size. What Turner et al (2007) found, however, were quill knobs on the forearm of Velociraptor, the very same structures seen on living birds with secondary feathers. The terrors of Jurassic Park had more than the sparse mohawks they sported in the third installment; they had what would appear to be flight feathers, perhaps being used for display since they certainly could not fly.

Regardless of what they were used for there is now an osteological character that can be observed to see if some dinosaurs had secondary feathers, something that can be useful for large animals unlikely to have their feathers preserved in the fossil record. What’s more, the detection of feathers associated with Shuvuuia suggests that there may be evidence of feathers associated with some theropods in deposits that would not be considered to exhibit “exceptional preservation.” The potential that feathers may surround some dinosaurs in non-lagerstatten deposits requires greater care and attention to detail while excavating. In such situations where the presence of feathers might be ambiguous chemical tests can help resolve the issue, the presence of feathers on Shuvuuia (Schweitzer et al 1999) confirmed when tests turned up beta-keratin. This simultaneously placed Shuvuuia close to birds and illustrated that proteins may survive much longer than expected in the fossil record.

As Mark Norell once explained to me when I asked him about this question we can be as confident that dromeosaurs had feathers as we are that Australopithecus was covered in hair (a statement similar to one attributed to him in Science in 1999). Reconstructions of feathered dinosaurs may sometimes look silly, yes, but our aesthetic preferences should not dictate whether we accept or reject a scientific reality. Dromeosaurs, and possibly most coelurosaurs, had feathers, and that is one of the most exciting notions to come out of paleontology in recent years. There are still plenty of questions about how birds evolved even if their familial relationships have become better understood, though, and I have tried to focus on the presence of feathers here rather than their origin. Interdisciplinary studies, particularly of development (Wagner 2005, Vargas & Fallon 2005), have become more important to understanding the origin of birds and old dichotomies have come into question as research continues (Glen & Bennett 2007). As Richard Prum wrote in a 2002 review, “ornithology is extant dinosaur biology,” and the origin & evolution of birds will likely be a hot area of research for years to come.

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Xu, X.; Clark, J.M.; Forster, C.A.; Norell, M.A.; Erickson, G.M.; Eberth, D.A.; Jia, C.; Zhao, Q. (2006) “A basal tyrannosauroid dinosaur from the Late Jurassic of China.Nature, Vol. 439, pp. 715-718
Zhou, Z. (2004) “The origin and early evolution of birds: discoveries, disputes, and perspectives from fossil evidence.Naturwissenschaften, Vol. 91, pp. 455-471

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