A fuss about feathers and fuzz

For the past 24 hours the palaeontological community has been abuzz, following the unveiling of a new Siberian dinosaur in the high-impact journal Science (Godefroit et al. 2014). The find, a small 1-m long bipedal herbivore named Kulindadromeus, is a member of the major dinosaur group Ornithischia and was closely related to the group that includes the ornithopods (such as Iguanodon) and ceratopsians (such as Triceratops). It comes from rocks of Middle–Late Jurassic age, and is dated at somewhere between 169–144 million years old. In many respects, the anatomy of Kulindadromeus is unremarkable – representing a fairly standard small ornithischian – but the important thing about this animal is the evidence for preserved skin structures, which have led the authors to conclude that it represents a rare example of a feathered ornithischian.

Until relatively recently, all dinosaurs were viewed as scaly – a proposal supported by numerous dinosaur cadavers with skin impressions that show standard reptilian scales around the body. This was overturned by a series of spectacular and beautiful discoveries from Germany, Canada and especially China of exceptionally preserved dinosaur skeletons with clear evidence of ‘protofeathers’ or more complex branched down-like or true feathers (e.g. Norell and Xu 2005). Since the first of these discoveries in the mid-1990s more evidence has accrued to show that feathers were not an exclusively avian feature, but had their deep evolutionary origins somewhere in the theropod family tree. An additional twist was provided to the story in the 2000s, with the description of unusual quill-like and branched structures in the ornithischians Psittacosaurus and Tianyulong (e.g. Zheng et al. 2009), which have been suggested by some to be evidence that feathers and their precursors were actually widespread across dinosaurs as a whole and not restricted to birds and their theropod ancestors.

Kulindadromeus possesses several interesting skin structures, which include scales, simple filaments and unusual (and so far unique) structures with several ribbon-like filaments arising from a single basal plate. The authors of the paper have suggested that these structures support the view that ornithischians were also feathered (at least as juveniles) and that this in turn indicates that feathers had a deep origin within Dinosauria. Nevertheless, some questions remain regarding the identity of the structures found in Kulidadromeus.

Firstly, the authors have so far been unable to determine the original composition of the filament-like structures – were they definitely composed of beta-keratin, as would be expected if they were feathers, or do they represent something else? Secondly, the odd plate-like structures with several filaments arising have no precedent among birds or theropods, nor do they conform to any of the hypothetical feather morphologies conceived by developmental biologists who study feather origins. As a note of caution, reptiles in general are known to do many odd things with their skin – many produce armour within the skin and the variety of scale form and function among living reptiles is enormous, ranging from the flat, horny scutes of turtle shells, to the thin scales of snakes, and the long fringing scales of iguanas, as well as the feathers and protofeathers seen in birds and theropods. Even among other extinct reptiles, we have the bizarre elongate scales of Longisquama, the elaborate midline crests of hadrosaurs and the fuzzy pycnofibres of pterosaurs to show us that lots of experimentation was going on with epidermal and dermal structures and their derivatives. So, are the structures in Kulindadromeus feathers, a side branch of the feather story, or were they an independent invention of a novel skin covering, as has occurred many times in the evolutionary history of reptiles? Indeed, a rival team working on other material from the same locality concluded that these features were ‘bristle-like scales’ (Saveliev and Alifanov 2014).

Personally, I think the jury is still out on this and that some of the enthusiastic responses to the announcement of Kulidadromeus may prove to be premature. Questions remain over the detailed morphology of these structures and their composition. Also, the authors conducted no analyses to support their claim that the discovery of these features in Kulindadromeus affects our overall picture of dinosaur skin evolution. Discovery of beta-keratin and of features identical to those seen in theropods would help to cement the case for feather-like structures in Kulindadromeus, but what we really need to solve the mystery of ornithischian feathers once and for all are older fossil deposits that have the potential for the exceptional preservation of skin structures. This would enable us to see how deep these features really go, but unfortunately these deposits remain elusive, at least at present.

References

Godefroit, P. et al. 2014. A Jurassic ornithischian dinosaur from Siberia with both feathers and scales. Science 345, 451–455. (doi:10.1126/science.1253351)

Norell, M. A. &  Xu, X. 2005. Feathered dinosaurs. Annual Reviews of Earth and Planetary Science 33, 277–299. (doi:10.1146/annurev.earth.33.092203.122511) 

Saveliev, S. V. & Alifanov, V. R. 2014. A new type of skin derivatives in ornithischian dinosaurs from the Late Jurassic of Transbaikalia (Russia). Doklady Biological Sciences 456, 182–184.

Zheng, X-T. et al. 2009. An Early Cretaceous heterodontosaurid dinosaur with filamentous integumentary structures. Nature 458, 333–336. (doi:10.1038/nature07856)å