All hail the Southern King of Gore – Lythronax argestes. As names go in palaeontology that’s a superb entry and one that has more than a nice ring to it. While it is true that new dinosaurs are named all the time, new tyrannosaurs are generally considered rather special since they are not common, and the charisma of the group remains undimmed. Thus the naming of Lythronax in the journal PLOS ONE is unsurprisingly popular, but the animal has way more going for it than a cool name.
Lythronax is not known from a large part of the skeleton: there’s most of a very nice skull preserved, but not too much after that. Although estimates are hard to make of size based on less than complete remains, it is clearly a good-sized animal with a skull getting on for a meter in length and the head and body being around 3.5 metres long, standing perhaps two metres at the hips and weighing around 2.5 tonnes. It’s hardly a giant compared with some of its later relatives (Tyrannosaurus could have had a head and body nearly twice this length) it would have been one of the larger carnivores of its time.
The age of the rocks that yielded the animal are from the Late Cretaceous period and around 80m years old, and thus come (in relative times) quite close to the end of the non-avian dinosaurs around 65m years ago. In Asia and North America at this time the tyrannosaurs were the largest and most common of the big carnivorous dinosaurs, but since all predators are relatively rare components of ecosystems we don’t tend to find too many of them, so every find is a welcome addition.
However sexy the tyrannosaurs are, and exciting the name is (and I really like this one, if you’d not already guessed), the interest in the new paper should lie with the accompanying analyses of the evolution of the group. A new family tree of the tyrannosaurs in the paper considers Lythronax to be very close to Tyrannosaurus and its nearest relatives. Like these giants from the end of the Cretaceous, Lythronax has a relatively broad skull with the orbits facing forwards. This is a form otherwise only seen in the very last of the group, so this skull shape may have evolved more than once and was certain around a good 10m years earlier than previously suspected.
That shape of the skull is linked to how they would bite and by extension probably linked to both feeding and killing, so it suggests Lythronax had a style rather different to other tyrannosaurs of its time. What really makes this stand out though is that the positions of some of the other tyrannosaurs have moved quite considerably and suggests a very different pattern of evolution to what has previously been considered.
Previously, analyses suggested that for various species of tyrannosaurs their nearest relatives lived a long way away. So for example one would find two close relatives in an evolutionary branch where one is found in Asia, and the other from North America. For some, it was almost as if there were alternating groups of tyrannosaurs evolving in Asia then North America then back again. The implication was that there was a semi-regular exchange of animals between the two continents (across what is now the Bering Strait) resulting in multiple small radiations in each place.
Exchanges between such land bridges is not uncommon and certainly we know of some dinosaur lines that at various times moved very considerable distances between continents. In the Late Cretaceous the overall faunas of North America and eastern Asia were similar and we do see numerous similar species in each location suggesting they were linked at some points. Clearly it was possible for these animals to move around, but were the tyrannosaurs really moving this much when other groups seemed to be much more stable?
The new analysis however, groups almost all of the Asian tyrannosaur species together and all of the North American ones together. As a result, this implies that rather than regular interchanges between the continents, in fact there were (primarily) just three geographically centered evolutionary radiations of tyrannosaurs – one in Asia, one in the northern part of North America (the northern part of the States and Canada) and towards the southern US, with Lythronax naturally representing part of the southern group. There are still interchanges going on, and in particular the new analysis still has a "last-minute" switch built in as the nearest relatives to Tyrannosaurus itself are actually Asian, but this is rather fewer than before.
As noted by American palaeontologist Tom Holtz of the University of Maryland, “This new discovery also supports the idea that the giant thick-toothed Asian tyrants Zhuchengtyrannus and Tarbosaurus were immigrants from North America rather than their own Asian line, because they share a common origin with Lythronax and Tyrannosaurus.” This hypothesis for relationships and movements is new to the tyrannosaurs, but one that has some precedence and other data behind it.
Other recent analyses of dinosaurs from the same part of Utah have also suggested a similar pattern of local radiations with relatively few moves between areas. If these animals didn’t (or perhaps couldn’t) move much, it would follow that the tyrannosaurs were similarly limited. Of still greater importance is the geography of the region at the time with what is now North America separated into two continents, split by a sea that ran north to south. At this point Lythronax appeared, the sea was at its highest, and thus had the maximum potential to separate out bits of land from one another inland, just as floods and rising sea levels can do today. This peak of sea level may therefore have helped separate out the tyrannosaurs and prompted the local diversification of the north and south groups.
While there is therefore some strong circumstantial evidence backing the new idea, it will take time to see if other data can be found to support it. So the shifting evolutionary relationships of the tyrannosaurs are likely to be controversial in the short term at least. Either way, Lythronax is an interesting and important new find that provides new clues into the early evolution of the tyrannosaurs.
Dr Corwin Sullivan of the Institute of Palaeontology and Palaeoanthropology states that “It’s always great to meet a new Cretaceous tyrant, and Lythronax might just be a particularly noteworthy one. Despite being the oldest known tyrannosaurid, it’s by no means a primitive member of the group, which tells us two interesting things: that tyrannosaurids started their evolutionary radiation sooner than we thought, and that a fair bit of their early record is still missing.”
Holtz agrees, saying “we know little about the diversity of Asian dinosaur faunas of the same age as Lythronax, so there may be similar forms over on the other side of the Pacific.” As ever, an interesting new answer does throw up some new questions, and hopefully some older Asian finds will help solve the great tyrannosaur transfers between the continents tens of millions of years ago.
M.A. Loewen, R. B. Irmis, J. J. W. Sertich, P. J. Currie, & S.D. Sampson. 2013. Tyrant dinosaur evolution tracks the rise and fall of Late Cretaceous oceans. PLoS ONE.
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