Did Feathers Enable Global Domination by Dinosaurs?

When Yutyrannus was unleashed on the world in April there was a flood of publicity in both the worldwide media as a whole as well as copious amounts of coverage in the palaeoworld – and rightly so. This was the first time that a large tyrannosauroid had been discovered with good evidence of a feathered covering and naturally this has led to speculation that even tyrannosaurids such as Tyrannosaurus may have been feathered.

My initial reaction to this was well publicised and generated a lot of coverage. However, some people grabbed the wrong end of the stick and assumed I was some kind of “feather denier” but this was soon sorted out. My adversity to feathered tyrannosaurids is purely a cosmetic issue and has nothing to do with science and, ultimately, this is the most important thing. And I actually have Andrey Atuchin’s awesome rendition of Yutyrannus as my desktop so I am not THAT averse to feathers!

So then, Yutyrannus, and what an intriguing discovery this has turned out to be. Like I said, there have been multiple articles regarding this animal even though it is still yet to be formally described and so there is only a limited amount of data to work with. But until the proper paper is published (perhaps a year and more?) we can all speculate a little as to its taxanomic affinities.

Firstly, and slightly problematical, is the provenance of these specimens. To remind you, there are three virtually complete specimens representing an adult, sub-adult and a juvenile that were apparently recovered from the Lower Cretaceous Yixian Formation of Liaoning Province in China. “Apparently” because the remains were purchased from a fossil dealer and, although the sediments the bones are encased in are almost certainly from the Yixian, there still has to be an element of caution. It has been recorded that the dealer also confirmed that these specimens were recovered from the same quarry (well he would) and were cut into smaller pieces to aid handling and transportation. So not ideal provenance – that’s for sure.

But the specimens themselves are superb regardless. The unusual fenestrated and rugose midline crest is the most prominent feature of the skull and is reminiscent of those found in Guanlong and Concavenator. On the face of it, Yutyrannus appears to be a typical basal tyrannosauroid based upon several characteristics in the cranium and especially the large three-fingered manus and yet there are other characters you would expect to find in more derived tyrannosauroids. The most obvious of these is the typically large and deep skull and there are additional features in the maxilla, lacrimal, squamosal and quadratojugal (amongst others) that lend sway to its tyrannosauroid affinities.

Interestingly, and as pointed out when the announcement was made in Nature, there does seem to be something carcharodontosaurian in nature about Yutyrannus. Indeed, Yutyrannus and Concavenator are remarkably similar. However, Andrea Cau did some sterling work in trying to nestle Yutyrannus in Carcharodontosauria but, regardless of the permutations, Yutyrannus sits nicely in Tyrannosauroidea. And Andrea points out that the carcharodontosaurian resemblance may simply be a case of convergent evolution.

A final point to make here is that despite the obvious completeness and preservation of the specimens there must be allowance made for the fact that the specimens are crushed flat and there is a possibility that one or two features are not as they seem and MAY be artefacts of preservation. In addition, and as mentioned earlier, we can but hope that there has been no interference with the specimen to “improve” it. I know this seems unlikely but remember Archaeoraptor? This is why it is so important to wait for the full paper.

Obviously the most interesting thing about Yutyrannus is the preservation of filamentous structures which are obviously analogous to feathers. They are big – around 15cm to 20cm long so are not to be sniffed at.  Despite the lack of coverage in certain areas (commonplace in the Yixian and other parts of the Jehol Group) it does appear likely that Yutyrannus would have been more or less fully covered.

Why Yutyrannus was feathered remains a matter of conjecture. Apart from the usual suggestions such as temperature control, sexual display and intraspecific communication, the authors suggest that Yutyrannus was perhaps adapted to exist in a cold climate and, indeed, this theory may have some merit. The Early Cretaceous of western Liaoning was a time of cold temperatures with a mean average temperature of 10° Celsius and a feathery integument would have been very useful and kept the animal insulated and warm.

I have always been fascinated by thought of how did feathered dinosaurs preen? One assumes that they did preen otherwise the feathers would have soon have become dishevelled and of no use. Most (but not all) extant dinosaurs have a gland – the uropygial gland – which is situated at the base of the tail and this produces, unsurprisingly, uropygial oil which the bird collects with its beak and distributes throughout its feathers to help keep them supple and strong.

The birds supplement this by grooming with a foot claw and, indeed, some birds display a comb-like structure that enables them to do this more efficiently. Dust baths also help condition the feathers and help remove parasites, excessive uropygia oil and also helps to dry wet feathers. It would be fascinating to see large theropods preening and grooming and one can only imagine what a spectacle a dust bath may have looked like! And there may be a further importance attached to preening.

Some extant birds, especially those that are monogamous, will often preen each other’s feathers (allopreening) and this is quite often part of a courtship ritual and is interpreted as strengthening the bond between such birds. This leads us nicely to the latest feathered dinosaur to hit the headlines – the stunning little theropod that is Sciurumimus albersdoerferi.

Like Yutyrannus, Sciurumimus has arrived on the scene like a bullet from a gun. We first got a sneak peak of the fossil last year and, indeed, I featured it here on the night the story broke. There has been plenty of coverage regarding this amazing fossil all over the blogosphere and on the web and I won’t be going over it again except for the fact that this taxon has been recovered in phylogenetic analyses as a megalosauroid.

Megalosauroids are an ancient group situated at the base of Theropoda and Dinosauria as a whole. The fact that this taxon is not a coelurosaur is also significant. This suggests that there is a strong possibility that most (some say all) theropods may have been feathered to at least some extent and that because the feathered ancestor of animals such as Sciurumimus must have been at the very base of Dinosauria then, by implication, there is even the possibility that all dinosaurs may have been feathered to a degree.

Of course, this requires an element of speculation and ultimately more fossils to be found before general acceptance can be assumed but it is a remarkable and challenging thought never the less. I tend to believe that feathers evolved several times within dinosaurs throughout their long existence and that they evolved for different reasons.   And maybe now, with the discoveries of animals such as Yutyrannus and Sciurumimus, there is a more profound and challenging notion to consider.

The feather may have been the single most important factor in the global domination of the world by dinosaurs. This one astonishing evolutionary step may have helped them regulate body temperature and aid in species recognition. If that was the case then it follows that feathers would have been used to aid natural selection – perhaps the male with the biggest or brightest plumage was most likely to be the most vigorous or strongest. The same feathers would then have probably been used for courtship ritual. We now enter an ever more complex world.

As feathers evolved and became more derived then preening would have been essential. It then follows that preening may have become part of courtship or a bonding ritual. Since preening and grooming is suggestive of an increasingly complex interaction between species this suggests that perhaps dinosaurs living in socially interactive groups were more complex than first thought. Indeed grooming in primates also dictates hierarchy although, granted, you cannot compare primates with archosaurs - but it does raise intriguing ideas.

If dinosaurs evolved in such a fashion then we should not be surprised at their worldwide success for over 150 million years.  Perhaps dinosaurs were highly evolved creatures that lived in complex social groups with what may have been advanced intelligence for such a time as the Mesozoic. Sciurumimus, by its very existence, with feathers in place may be indicative of just that. Perhaps tyrannosaurs hunting in packs are not so farfetched after all. 

References

Rauhut, O. W. M.; Foth, C.; Tischlinger, H.; Norell, M. A. (2012). "Exceptionally preserved juvenile megalosauroid theropod dinosaur with filamentous integument from the Late Jurassic of Germany". Proceedings of the National Academy of Sciences. doi:10.1073/pnas.1203238109

Xu, Wang, Zhang, Ma, Zing, Sullivan, Hu, Cheng & Wang. 2012. A gigantic feathered dinosaur from the Lower Cretaceous of China. Nature doi:10.1038/nature10906