Out with a bang!

Dinosaur extinction has always excited the imagination – what forces could possibly account for the disappearance of such a dominant and charismatic group? More than 100 different theories have been published to account for the extinction, ranging from the interference of meddling aliens, to collisions between the Earth and roving lumps of galactic antimatter and the possibility of some malign dinosaur pandemic flu. Almost all of these theories have been shown to be lacking in evidence (or just plain crazy). However, several events at the end of the Cretaceous Period have stood the test of scientific scrutiny. These are: the impact of a large meteorite (around 10 km in diameter); the climatic effects of the extensive volcanic eruptions that formed the stacked lavas of the Deccan Traps in India; and a series of more general, long-term global environmental changes, including sea-level rises and climatic shifts. Recently, a multi-disciplinary team reviewed the evidence for the asteroid impact, concluding that the timing of the impact, whose crater is centered on the small town of Chicxulub on the Yucatan Peninsula of Mexico, was the perfect smoking gun to drive the extinction (Schulter et al. 2010). However, dissenting voices (my own included) were quick to reply, noting that the longer-term effects of volcanism and environmental change might have been largely to blame (Archibald et al. 2010).

In order to get a fresh perspective on what happened to the dinosaurs, my colleagues Steve Brusatte and Richard Butler assembled a team of palaeontologists to re-examine the final stages of dinosaur evolution. Together, we took a cold, hard look at the diversity of Late Cretaceous dinosaurs during the last 20 million years of their reign, reanalysing the most recent data on dinosaur distributions and combining this with the latest information on climatic conditions and the timing of Deccan volcanism and the Chixuclub impact. There was much discussion and debate within the team until a consensus emerged: the end result of these deliberations was published online today in the journal Biological Reviews (Brusatte et al. 2014). 

Our new analyses find little support for long-term declines in dinosaur diversity and abundance in the lead-up to the extinction. Some herbivore groups in North America were suffering slightly, but this pattern was not repeated in other areas of the world. Instead, the extinction appears to have been geologically abrupt: an observation that rules out the more gradual declines predicted by extinction models reliant on slower mechanisms of global change. Moreover, the major pulses of Deccan volcanism do not correlate well with this sudden extinction. Consequently, we concluded that the abrupt global extinction of so many dinosaur lineages coincided best, and was most consistent with, the Chicxulub impact. Nevertheless, the stressed herbivore populations in North America may be hinting at the fact that at least some dinosaurs were under environmental strain prior to the impact. It seems reasonable to propose that longer-term mechanisms might have adversely affected at least some dinosaurs, making them more vulnerable to the cataclysmic effects of the impact.

It’s interesting to speculate what might have happened if the asteroid had not hit at this precise moment in time. Dinosaurs were successful and diverse, exploiting a wide range of niches from pole-to-pole. Although re-imagining historical events is always risky, it seems reasonable to speculate that had the asteroid arrived at a time when dinosaurs weren’t already stressed then some of them might have survived and even prospered until the present. Of course, it can be argued that this particular experiment has already been run – birds are nothing more than feathered, flying dwarf dinosaurs and are far more diverse than their extinct relatives. It could even be argued that the asteroid was just a blip in dinosaur evolution rather than a catastrophe: after all, the explosive radiation of avian dinosaurs was largely a post-impact event, even if the extinction took out all of their larger (and I’d suggest more exciting) non-avian relatives.

Archibald, J. D. et al. 2010. Cretaceous extinctions: multiple causes. Science 328, 973.

Brusatte, S. L. et al. 2014. The extinction of the dinosaurs. Biological Reviews. doi:10.1111/brv.12128

Schulte, P. et al. 2010. The Chicxulub asteroid impact and mass extinction at the Cretaceous-Paleogene boundary. Science 327, 1214­–1218. doi; 10.1126/science.1177265

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