This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Regular readers might have noticed that I’m not all that keen on covering stories that get massive, global exposure across the blogosphere. Consequently, sexy dinosaur news is mostly ignored here. Sometimes, though, I suppose I have to make an exception. Maybe I have a duty to, since the Tet Zoo audience includes more than an average number of dinosaur specialists (meaning that comments and discussions can often be pretty in-depth; more in-depth than they are elsewhere on the web). Furthermore, Tet Zoo is unlike many other sites that cover Mesozoic dinosaurs in that it appeals to many readers who, while zoologically informed, aren’t Mesozoic specialists and hence don’t necessarily get barraged by the same amount of ‘new dinosaur’ announcements that I (and other dinosaur workers) do. Anyway, enough with the preamble...
Today sees the publication of Yutyrannus huali Xu et al., 2012 in the hallowed pages of Nature. Known from three well-preserved specimens, it’s yet another feathered theropod dinosaur from the Lower Cretaceous Yixian Formation of Liaoning Province, China. But it isn’t a dromaeosaurid, early bird or other maniraptoran – it’s a giant tyrannosauroid.
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That is, a member of the same major group of coelurosaurian theropods as Tyrannosaurus and its close kin, but not a member of Tyrannosauridae (the highly modified, short-armed, two-fingered tyrannosauroid clade that includes Tyrannosaurus, Albertosaurus and so on). Tyrannosaurids are mostly big to very big, but non-tyrannosaurid tyrannosauroids were often small to medium-sized (1-4 m being a very approximate range). Anyone who follows me on twitter will know that I spent a significant portion of my life within recent months thinking and writing about tyrannosauroids and not much else.
Anyway, as a non-tyrannosaurid tyrannosauroid, Yutyrannus is typical in having three-fingered hands, proportionally large bony nostril openings, and distal hindlimb segments that are not especially elongate for the size of the animal (Xu et al. 2012). Diagnostic (= unique) characters include the presence of a peculiar rugose crest along the dorsal midline of the snout and a bony boss on the postorbital bone. We’ll get back to those features in a minute, since there’s something very familiar about them.
The feathers!! The feathers!!
As usual, the main gee-whiz points about Yutyrannus are already being widely discussed. We’ve known for a while (since the publication of Dilong paradoxus in 2004) that at least some tyrannosauroids possess ‘stage 1 feathers’ (Xu et al. 2004). That is, filamentous integumentary structures that seem to be evolutionary precursors to the true, complex feathers that evolved elsewhere within coelurosaurian theropods. Yutyrannus is another feathery/filamenty tyrannosauroid, but it’s remarkable in being huge – it’s about 9 m long, meaning that here is the first GIANT feathery/filamenty tyrannosauroid.
As Xu et al. (2012) note, there are other giant coelurosaurs that would also have been feathered (examples: the huge oviraptorosaur Gigantoraptor, the therizinosaur Therizinosaurus, and perhaps the gargantuan ostrich dinosaur Deinocheirus), but Yutyrannus is the first really big one to actually have the structures preserved. If feathers of this sort were present in Dilong and Yutyrannus, it stands to reason that tyrannosauroids close to these two in the phylogeny would have been feathered as well. The tyrannosauroid I know best – Eotyrannus lengi – is thus depicted as fully fuzzed-up by Xu et al. (2012). I can live with that.
The structures in Yutyrannus are mightily impressive, being tightly massed together and as much as 15 cm long. While none of the three Yutyrannus specimens preserves a complete integumentary covering (if you think that’s weird or suspicious or something, go look at rotting animal carcasses some time), their filamentous structures are preserved in association with the neck, hip region, tail and elsewhere. I thus find it totally reasonable to imagine that the covering was complete across much of the body. These were big, shaggy tyrannosaurs in feathery coats – exactly the sort of thing that artists have been drawing and painting for years now. Huh, stupid artists.
If you’re wondering, claims that the integumentary structures present in coelurosaurian theropods might actually be decayed collagen fibres (yes, collagen fibres - - I’m not kidding!!) are not likely and never have been.
Cold adapted tyrannosaurs?
Regular readers will perhaps know that I (and others) have been saying for a while that Mesozoic Earth was not the global hothouse that many have long assumed. There’s evidence for cool continental interiors and poles during parts of the Jurassic and Cretaceous (Barron & Washington 1982, , Sloan & Barron 1990, Sellwood et al. 1994), sea surface temperatures that were similar to those of the cold, modern north Pacific and Atlantic (Van de Schootbrugge et al. 2000), and some researchers think that there were glaciation events – that’s right, I said glaciation events – late in the Jurassic and early in the Cretaceous (Dromart et al. 2003, McArthur et al. 2007).
Recently, data has been compiled indicating that the Liaoning region was cool during the Early Cretaceous – its average temperature being about 10 degrees C (Amiot et al. 2011). If you insist that dinosaurs were scaly and ‘cold-blooded’, this idea of insulated big theropods running around in cool climates must seem silly, but if you disregard preconceptions and biases, I think we can now take very seriously the idea that insulated, endothermic dinosaurs were inhabiting cool, or cold, habitats.
Xu et al. (2012) suggest that Yutyrannus was ‘cold adapted’, and that the tyrannosauroids of warmer climes were scaly-skinned and not insulated in the same way. That might be correct, but it’s worth pointing out that we actually know little or nothing about the integumentary covering of tyrannosauroids outside of the Liaoning taxa. There are a few patches of scaly skin alluded to here and there, but nothing substantial, so far. I’m with Andrea Cau (and others) on the idea that ‘fuzziness’ was a gradational thing in archosaurs, with fuzz and feathering mostly (but not wholly) grading out as body size increased. That’s nicely illustrated in Andrea's diagram below: the redder parts of the diagram are the more scaly/less fuzzy ones, while the purple parts are fuzziest/featheryest (the skeletal reconstructions were produced by Greg Paul, Jaime Headden, Scott Hartman, Marco Auditore and Lukas Panzarin). For a sharper version go here on Theropoda. And I have no idea if "featheryest" is a word.
Yutyrannus the... carcharodontosaurian?
I must confess to being somewhat sceptical of the tyrannosauroid identification for Yutyrannus. I reviewed this paper (to those who don’t know: I did my PhD thesis on basal tyrannosauroids), and noted immediately that Yutyrannus actually resembles carcharodontosaurian allosauroids in some respects. Those of you who know theropods, or who have very good memories, will recall the brouhaha back in 2010 when the carcharodontosaurian Concavenator was described from Lower Cretaceous rocks of Spain (Ortega et al. 2010). Concavenator is an awesome fossil, but what really guaranteed its appearance in the top-tier glamour mags is the claim that raised bumps on its ulnae are quill node homologues – that is, evidence for proto-feathers on its arms. I was sceptical of this at the time (the ‘quill node homologues’ look like bumps on an intermuscular line to me), and am sceptical of it still.
Anyway, it’s somewhat ironic – given its alleged featheryness – that Concavenator should raise its head, so to speak, in the discussion here. But I think that Concavenator and Yutyrannus are somewhat alike. The skulls of both animals are particularly similar. Both possess a rugose dorsal crest along each nasal bone, decorated on its lateral surface by a series of subcircular (?pneumatic) recesses, both have a rounded posterodorsal boss (possibly with a concavity on its lateral surface) on the postorbital bone, and, in both, a projection from the postorbital bar invades the orbit.
Other elements of these taxa are also similar. Concavenator is superficially tyrannosauroid-like in possessing a small anterodorsal concavity on the anterior lobe of the ilium (also present in Yutyrannus) and the shaft of its ischium appears more slender than the shaft of the pubis (a feature noted in the Yutyrannus paper to be a tyrannosauroid-like character).
While the authors took account of the carcharodontosaurian-like features of Yutyrannus and modified their phylogenetic analysis accordingly, they stuck to their guns about it being a tyrannosauroid. I agree that Yutyrannus is tyrannosauroid-like in many features but, I dunno, I think it needs to be included within a larger and more comprehensive data set. Why didn’t I do this myself when I was reviewing the paper? Several reasons. One being that – so long as a given proposal isn’t fatally flawed or shot full of holes – you can’t stop authors from coming up with their own favoured phylogenetic hypothesis, even if you disgree with it. I should note that my suggestion here is just a suggestion, not an outright challenge, and I'm not all that confident about being right. I'm just sceptical.
Anyway, none of this stops Yutyrannus from being an awesome dinosaur discovery that will get lots of attention in future publications. And it raises lots of questions. Most evidence indicates that the tyrannosauroids of the Early Cretaceous were generally small and ecologically ‘inferior’ to contemporaneous megalosauroids and allosauroids. If Yutyrannus is a tyrannosauroid, does it show that things were more complex, and that big tyrannosauroids lived alongside big megalosauroids and/or allosauroids in some or many places? Or is it that tyrannosauroids were ‘controlling’ regions (like - the cooler regions?) where those other groups were absent? Were tyrannosaurids perhaps better adapted for cool/cold climates than other big-bodied theropod lineages, and could this have been key to tyrannosauroid success? Were the cranial bossess and nasal crest we see in Yutyrannus sexually selected characters, and is their presence in more than one individual an indication of mutual sexual selection? (Hone et al. 2012). Perhaps most interestingly of all, will more big, fuzzy dinosaurs be announced in future? Place your bets.
For previous Tet Zoo articles on some of the subjects mentioned here, see...
Tyrant dinosaurs were not a Northern Hemisphere speciality: they also colonised Australia!
Concavenator: an incredible allosauroid with a weird sail (or hump)... and proto-feathers?
Did dinosaurs and pterosaurs practise mutual sexual selection?
Refs - -
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Barron, E. J. & Washington, W. M. 1982. Cretaceous climate: a comparison of atmospheric simulations with the geological record. Palaeogeography, Palaeoclimatology, Palaeoecology 40, 103-133.
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