Tyrant dinosaurs – properly called tyrannosauroids – are most usually associated with the Late Cretaceous of North America. Of course, if you know anything about dinosaurs you’ll also know that many tyrants were Asian. So, the most familiar tyrants – the big, short-armed kinds like Tyrannosaurus, Tarbosaurus, Albertosaurus and Daspletosaurus (all of which belong to the best-known tyrant clade, Tyrannosauridae) – were all animals of Laurasia, the northern landmass that split up during the Cretaceous to form North America and Eurasia [image above provided by Roger Benson; read on for explanation].
The history and diversity of early, non-tyrannosaurid tyrants is still poorly known, but in recent years a number of such forms have been discovered in Europe and Asia. Eotyrannus lengi (named in 2001) is from the Lower Cretaceous of southern England; little Aviatyrannis jurassica (named in 2003) is from the Upper Jurassic of Portugal; small, feathered Dilong paradoxus (named in 2004) is from the Lower Cretaceous of Liaoning Province in China; crested Guanlong wucaii (named in 2006) is from the Upper Jurassic of the Junggar Basin in China; Stokesosaurus langhami (named in 2008) is from the Upper Jurassic of southern England; Xiongguanlong baimoensis (named in 2009) is from the Lower Cretaceous of Gansu in China; Raptorex kriegsteini (also named in 2009) is inferred to be from the Lower Cretaceous of somewhere in north-eastern China*; and Sinotyrannus kazuoensis (also named in 2009) is also from the Lower Cretaceous of Liaoning Province [adjacent image shows life restoration of Raptorex by Nobu Tamura, from wikipedia].
* There is some scepticism about the claim that Raptorex came from Liaoning Province, as it was recovered from a commercial source.
Proceratosaurus bradleyi [shown here, image © NHM] from the Middle Jurassic of England has also been recently identified as an early tyrannosauroid closely related to Guanlong (Rauhut et al. 2010). The name Proceratosauridae can be used for the Proceratosaurus + Guanlong clade (I’m pleased to say that I found Proceratosaurus to be a tyrannosauroid in my 2006 Ph.D. thesis… though shame I still haven’t published).
The presence of Proceratosaurus in the Jurassic of England, Aviatyrannis in the Jurassic of Portugal, Stokesosaurus in the Jurassic of the USA and England, and Guanlong in the Jurassic of China indicates that tyrants were distributed across Laurasia by the Late Jurassic.
This distribution raises an interesting question: seeing as tyrants were living across Laurasia during the Jurassic, why didn’t they colonise the southern continents? During the Cretaceous, several dinosaur groups (such as macronarian sauropods and allosauroid theropods) occurred in North America and Eurasia in the north, and in South America, Africa and Australia in the south. This pattern indicates that these groups were able to colonise both the northern (Laurasian) and southern (Gondwanan) landmasses during the Jurassic, prior to their Cretaceous separation. So, the deep Jurassic origin of tyrants means that we should predict their presence on the Gondwanan continents.
Today sees the publication of a paper by Roger Benson and colleagues on the first Australian tyrant dinosaur (Benson et al. 2010). Thanks to this find, we now know that tyrants did, indeed, get deep into the south. The specimen is, unfortunately, not a complete skeleton or even a partial one: it’s a single, 30-cm-long pubic bone, currently known only by its catalogue number, NMV P186046 [shown here, image provided by Roger Benson]. It’s from the famous Dinosaur Cove site in Victoria, and its detailed anatomy demonstrates its tyrannosauroid identity. Additional clues show whereabouts the specimen lies within the tyrannosauroid radiation.
The pubic boot – the projecting, foot-shaped bony mass present at the lower end of the bone – is narrow from side to side. This means that the bone belongs to a coelurosaurian theropod (other theropods have broader pubic boots). Among coelurosaurs, a particularly large pubic boot (like that present in NMV P186046) is seen in tyrannosauroids, as is a prominent, flange-like ridge (the pubic tubercle) at the upper end of the bone. Other characteristic tyrannosauroid features are present on the bone too. The proposed tyrannosauroid identification of the specimen is therefore pretty secure: no other group of theropods possesses this combination of unusual features.
NMV P186046 is Aptian in age (that is, it’s from the late part of the Early Cretaceous), so it’s somewhat younger than all of the Jurassic tyrants, and also younger than the non-tyrannosaurids Dilong, Eotyrannus, Sinotyrannus and Raptorex. Interestingly, its anatomy suggests that it was more closely related to tyrannosaurids than were any other currently known tyrants (the pubic flange, for example, is present in tyrannosaurid tyrants, but not in Raptorex or other non-tyrannosaurid tyrants). It can therefore be inferred that any characters shared by Raptorex and Tyrannosauridae (see Sereno et al. 2009) were also present in the Victorian tyrant: it would have had relatively short arms and hands (in contrast to Eotyrannus and some other early tyrants), and it would have had robust jaws and generally been tyrannosaurid-like in overall proportions (Benson et al. 2010) [cladogram below from Benson et al. (2010)].
Is it possible that the specimen isn’t really a tyrant, but an Australasian ‘tyrant mimic’? When making identifications based on partial specimens, we’ve been wrong before, sometimes very wrong. In the absence of better remains, this possibility will remain, but it should be clear that it’s nothing more than a speculation: at the moment, a tyrannosauroid identity for NMV P186046 is as robust as it could be. The conclusion must be that ‘mid-sized’, Raptorex-like tyrant dinosaurs (total length = about 3 m) invaded the southern continents, and inhabited Australia as well as Asia during the Early Cretaceous.
As usual, this discovery raises new, interesting questions. Tyrants became successful, gigantic arch-predators in the north, yet we have no evidence that this happened in the south. Is this because other big theropods (like megalosauroids and/or allosauroids) held sway in the south, therefore preventing the evolution of giant size in southern tyrants? Or is it that only tyrannosaurid tyrants had the requisite anatomical or behavioural features that ‘allowed’ gigantism? Did this lineage of mid-sized southern tyrants die out long before the close of the Cretaceous, or are there more finds to come? We can’t pretend to know that much about the Cretaceous fossil record of Australia, let alone that of Antarctica or much of Africa, so could mid-sized or even gigantic southern non-tyrannosaurid tyrants await discovery, either in Australia, or in Africa, India, Antarctica or elsewhere in Gondwana?
In fact, while NMV P186046 is the first Gondwanan theropod that we can confidently identify as a tyrant, there are at least a few other southern continent taxa that might, just might, belong to this group too. Santanaraptor placidus and Mirischia asymmetrica, both from the Santana Formation of Brazil, have been suggested to be non-tyrannosaurid tyrants (Holtz 2004, Naish 2006). To be honest, they don’t really look like they might be part of the same clade as Raptorex, NMV P186046 and Tyrannosauridae, so were additional tyrannosauroid lineages present in Gondwana during the Jurassic and Cretaceous?
And, one more thing – – it’s well known these days that Dinosaur Cove was located pretty close to the Cretaceous South Pole [adjacent palaeomap from wikipedia]. Therefore, it was affected by seasonal darkness and cool or cold weather. The dinosaurs and other animals that lived here must have coped with these conditions. What does the presence of a mid-sized tyrant dinosaur in the Dinosaur Cove fauna tell us about tyrannosauroid physiology, behaviour and ecology? We don’t know, obviously, but it’s going to be fun to speculate.
As usual, things have turned out to be more interesting that we might previously have guessed. I leave you with the vision of a short-armed, cold-adapted tyrant dinosaur, chasing an ornithopod through the chilly air of an Australian winter…
For previous articles on tyrannosauroids and other coelurosaurian theropods see…
- Oh no, not another new Wealden theropod!
- 100 years of Tyrannosaurus rex
- Long and Schouten’s Feathered Dinosaurs, a review
- The Crystal Palace monsters, armoured tyrannosaurs and lurking sauropods: a look back at ‘Dinosaurs – A Historical Perspective’ (part I)
- An American tyrant in London
- Of Becklespinax and Valdoraptor
- Feathers and filaments of non-avian dinosaurs, part I
- Feathers and filaments of dinosaurs, part II
Refs – –
Benson, R. B. J., Barrett, P. M., Rich, T. H. & Vickers-Rich, P. 2010. A southern tyrant reptile. Science 327, 1613.
Holtz, T. R. 2004. Tyrannosauroidea. In Weishampel, D. B., Dodson, P. & Osmólska, H. (eds) The Dinosauria, Second Edition. University of California Press (Berkeley), pp. 111-136.
Naish, D. 2006. The osteology and affinities of Eotyrannus lengi and other Lower Cretaceus theropod dinosaurs from England. Unpubished Ph.D. thesis, University of Portsmouth.
Rauhut, O. W. M., Milner, A. C. & Moore-Fay, S. 2010. Cranial osteology and phylogenetic position of the theropod dinosaur Proceratosaurus bradleyi (Woodward, 1910) from the Middle Jurassic of England. Zoological Journal of the Linnean Society 158, 155-195.
Sereno, P. C., Tan, L., Brusatte, S. L., Kriegstein, H. J., Zhao, X. & Cloward, K. 2009. Tyrannosaurid skeletal design first evolved at small body size. Sciencexpress 10.1126/science.1177428