A new feathered dinosaur from China, belonging to an obscure and strange carnivorous group, bears a seemingly bony wrist structure that may have had a role in flight. See Letter p.70
When the first dinosaurs with feathers or feather-like structures were brought to light by Chinese scientists in the mid-1990s, they cemented the hypothesis of the dinosaurian origin of birds and provided spectacular evidence about the origin of flight and the primordial functions of feathers1,2. In the ensuing two decades, the picture of the evolution of feathers and flight has become richer and more complicated as other feathered dinosaurs have been discovered, seemingly on a monthly basis3. But things have just gone from the strange to the bizarre. On page 70 of this issue, Xu et al.4 present a feathered dinosaur from a completely unexpected branch of the dinosaur tree — and it sports a never-before-seen skeletal element that the authors think may be related to flight.
The dinosaur, named Yi qi, is a member of the unusual group of theropod (carnivorous) dinosaurs called Scansoriopterygidae5 (Fig. 1). Although scansoriopterygids are not well known — there are only three kinds, and only incomplete remains of these — they seem to be small (the skull of Yi qi is about 4 centimetres long), with smaller and fewer teeth than other theropods, and with long hands. Unusually for theropod dinosaurs, the third finger is longer than the second. It is especially long in Yi qi, and its forelimb is further distinguished by a long 'styliform element' coming off the wrist. The authors are refreshingly agnostic about the exact function of this new structure, partly because it is so different from anything previously known. However, their find opens two cans of worms: about interpreting unique structures in fossils and about what it means to fly.
The styliform element, which may be a hypertrophied wrist bone or a neomorphic calcified structure, is longer than any of the animal's fingers and is curved at both ends. It is probably not a true finger, such as the fourth finger that forms the main outboard spar of the wing in pterosaurs — flying reptiles that were related to their dinosaur contemporaries, but that evolved flight independently of birds. Instead, the styliform element of Yi qi has no joints and comes directly off the carpal bones without the intermediary of a metacarpal (palm bone), so it is probably not a finger (the authors confusingly label the three fingers of the dinosaur-bird hand as II–III–IV instead of the standard I–II–III, which corresponds to our thumb, index and middle fingers6). How the structure is attached to the wrist is not clear, because its proximal end seems quite squared off; this means that we also do not know if or how it could move. Superficially, it looks the same as the animal's other bones, a conclusion supported by the authors' energy dispersive spectrometry analysis. It would be interesting to see if it has true bone cells, to confirm that it is indeed bone and not calcified cartilage or other calcified tissue.
What could this element be except a support for some kind of aerofoil? The authors infer this on the basis of its position and the presence of membranous tissue in the wrist area. But although they consider a variety of analogous structures in living and extinct flying and gliding animals, none is exactly like the styliform element in Yi qi. Furthermore, Yi qi's body is not preserved below the ribcage, so reconstructions of the pelvis, hindlimbs and tail must be conjectured from what is known of other scansoriopterygids (Fig. 1). Further anatomical analysis of this structure and how the rest of the body related to it — such as whether the tail created lift or drag — will require other discoveries.
In the meantime, the authors do not commit themselves to whether this animal could flap or glide, or both, or neither. That is a good position to take7, but we can parse it further. To fly actively, an animal must be able to execute a flight stroke that can generate a vortex wake that propels it forward8. No evidence presented so far suggests that Yi qi had this ability. Furthermore, in flapping animals the outboard skeletal elements (wrist, hand and so on) are primarily responsible for thrust, the essential component of powered flight8,9, but these are not particularly long in Yi qi. So, at present we can shelve the possibility that this dinosaur flapped.
As for gliding, if Yi qi's styliform element helped to support a membranous aerofoil, it can be used to reconstruct the planform of the wing, as Xu and colleagues have done. But in a gliding animal, the centre of lift of the aerofoil should be fairly congruent with the centre of gravity of the body — if the bulk of the animal's weight falls too far behind the centre of lift, the back end will sag and the animal will stall9. That is clearly the case in the authors' reconstruction of Yi qi, but an aerofoil that was swept back more, if anatomically possible, might have mitigated this problem. Still, we are left in a quandary: an animal with a strange structure that looks as if it could have been used in flight, borne by an animal that otherwise shows no such tendencies. And so far, there is no other plausible explanation for the function of this structure.
Despite this aeronautic uncertainty, the paper is a milestone for another reason. The Yi qi fossil was found by a farmer, which is the case for many Chinese fossils. But Xu and colleagues provide more complete information about the geographical and geological provenance of their specimen than has accompanied other recent Chinese fossils collected by non-scientists. The Supplementary Information to the paper documents how the authors verified the provenance of the specimen and even excavated fossils in associated local sediments. Moreover, they examined the specimen meticulously to be sure that none of its elements had been faked or restored. This is a key advance and sets the standard for future publications of specimens procured from third parties. The authors are due thanks for this diligence from the entire palaeontological community.Footnote 1
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The Open Ornithology Journal (2016)