When is a bird not a bird?

Birds were once thought to have a large number of features exclusive to the group. One by one those features have also been identified in fossils of certain theropod dinosaurs. Now feathers join the list.

Among all living creatures, only birds have feathers. So when the fossil of a single isolated feather was found in the Late Jurassic rocks of Bavaria shortly after Darwin published On the Origin of Species in 1859, it was enough to demonstrate that, astonishingly, birds must have existed in those remote times (now known to be some 150 million years ago). When the skeleton of Archaeopteryx was discovered in the same area in 1861, it fulfilled that plumose prediction. But the skeleton was recognized as a bird's mainly because it had feathers on its wings and tail. Nothing else except an odd, boomerang-shaped wishbone seemed to ally it to living birds. In fact, some other specimens were assigned to Archaeopteryx only belatedly, because no feathers were preserved in the fossils concerned. Those specimens had instead been taken for small carnivorous (theropod) dinosaurs.

On page 753 of this issue, Ji et al.1 show unequivocally that clothes don't make the bird. They describe two small theropod dinosaurs from geological beds in the Liaoning province of China1,2,3,4. The age of the beds is disputed; although initial reports suggested they are Late Jurassic, radiometric dates and other evidence now point to the Early Cretaceous4 — that is, around 145 million years or maybe later. These theropods have both plumulaceous (down-like) and vaned, barbed feathers on the body, arms, legs and tail. But these animals were clearly not birds, and they were clearly not capable of flight.

One form, Protarchaeopteryx, has been briefly described in a Chinese journal2. It has down-like feathers on its body and tail, and vaned, barbed, symmetrical feathers along at least the end of the tail, in a fan-like pattern. The second, Caudipteryx, has remiges (primary feathers) attached to the second (longest) finger of the hand, though the arms are much shorter than in birds. These feathers are also vaned and barbed, and down-like feathers are also preserved. Were preservation more complete, we would have an even fuller idea of the plumage of these animals. This is the most that can be said at the moment, but new specimens continue to emerge from this locality.

If these are true feathers — and it can scarcely be doubted — what does this do to our conception of birds? The bottom line is that it simply forces us to revise our idea of the association of feathers with the animals we call birds. This is why.

Systematists define the names of organisms (taxa) by their ancestry; in this case, birds (Aves) consist of Archaeopteryx plus living birds (Neornithes) and all the descendants of their most recent common ancestor (Fig. 1). But we diagnose (recognize) birds by unique features that only they possess; these are inherited from that most recent common ancestor. (Ji et al.1 give three such features, none of which relates to feathers.) So the diagnostic characteristics follow from our definition of ancestry. Why worry if feathers turn out to be shared by a wider group than birds alone? We still define birds as Archaeopteryx and its later relatives. Protarchaeopteryx and Caudipteryx may have feathers but they're not birds, because they're not members of that ancestral club of Archaeopteryx and living birds. Ji et al.1 show that these animals belong to a group of dinosaurs known as the maniraptoran coelurosaurs, which include the small theropods most closely related to birds. However, their analysis of evolutionary relationships does not encompass a broader group of coelurosaurs, which might eventually help to pin down the position of Protarchaeopteryx.

Figure 1: Distribution of known feather types and other features in selected groups of theropod dinosaurs, including birds (Aves).

The nodes (junctions) on the diagram provide the names and shared features of the groups connected above the nodes. Hence, the Avetheropoda (everything but Coelophysisin this diagram) have wishbones and breastbones and have lost the fourth and fifth fingers. The fossils of Protarchaeopteryx and Caudipteryx have both the vaned and down-like feathers seen in living birds, so we infer that their common ancestor (group Eumaniraptora) had them. (This may be true for the Velociraptor-Deinonychus group, by inference, unless Protarchaeopteryx later turns out to be more closely related to birds than they are.) Down-like feathers are not known in specimens of Archaeopteryx. But the base of its vaned feathers are down-like9, like those of Protarchaeopteryx and living birds, and the reasonable inference is that Archaeopteryx did in fact possess them. Sinosauropteryx has only the beginnings of down-like feathers, which could be seen as an aberration or a foreshadowing of their later elaboration in the Eumaniraptora, but their similarity to down-like feathers is very strong4. Similar structures have been reported in the ornithomimid Pelecanimimus10, but these now appear to be other soft-part structures11.

By admitting that plumage did not first spring full-blown on the wings of Archaeopteryx, we are free to examine how feathers evolved in the first place. At the 1996 meeting of the Society of Vertebrate Paleontology, Dr Chen Pei-ji disclosed photographs of an amazing specimen — a small theropod dinosaur, also from the Liaoning beds, with a fringe of hair-like or down-like structures along its neck and backbone. This animal, dubbed Sinosauropteryx by Ji and Ji3,4, turns out to be closely related to the basal (relatively ‘primitive’) coelurosaur Compsognathus, a chicken-sized, short-armed form found in the same deposits as Archaeopteryx5. Without even seeing the specimens, critics of the theropod-bird connection tried to pass off these remains as artefacts of preservation. They claimed that the structures were frayed internal collagen fibres, not external or epidermal in origin at all, and that similar structures in sea snakes suggested that these theropods lived in an aquatic environment6. Those doubts can now be put to rest3,4.

The evolution of carnivorous dinosaurs through basal coelurosaurs into birds shows some unmistakable trends7. As their evolutionary history has become better known, we have seen wishbones, breastbones, hollow bones, long arms and hands, sideways-flexing wrists, nesting behaviour and rapid growth rates all disappear from the avian catalogue of exclusive features. Like feathers, as Ji and colleagues' work1 shows, these features first evolved in coelurosaurs for purposes completely unrelated to flight7. The proto-feathery fringe of Sinosauropteryx, now known to extend to its flanks as well as along its midline4, was obviously not made for flight and it is questionable whether it could have served any kind of function in insulation. Camouflage, display and species recognition come to mind as other possibilities8.

Caudipteryx and Protarchaeopteryx go it one better, evolving long feathers with a central rachis (shaft). Were these feathers airworthy? Their vanes are symmetrical and very even, suggesting interlocking barbs, although most flying birds have asymmetrical feathers. However, the arms of Caudipteryx are only 60% as long as the legs, and they are only two-thirds as long in Protarchaeopteryx (in Archaeopteryx they are of more or less equal length). Evidently, the arms and feathers were not large enough for flight (the plumage is not yet known well enough to say if it was effective as insulation, or what other functions it may have served). The feathers of Archaeopteryx seem to be a natural extension of this trend, so to speak, but they are not much different qualitatively. So the available evidence suggests that structurally airworthy feathers may have evolved before they were long enough, or their possessors able to use them, for flight.

The work of Ji et al.1 should lay to rest any remaining doubts that birds evolved from small coelurosaurian dinosaurs. These new discoveries will excite the public and scientists alike by showing that down-like and later vaned body feathers evolved before flight feathers, and that a full complement of feathers was present in coelurosaurs before birds were invented.


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Padian, K. When is a bird not a bird?. Nature 393, 729–730 (1998). https://doi.org/10.1038/31576

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