Tuesday, 12 November 2013

Theropoda News

November has already brought us two fascinating discoveries in the world of theropod palaeontology, one concerning a new genus and the other a well-established but little understood one.

Deinocheirus arms at the NHM in London.
Deinocheirus mirificus, the 'weird-looking terrible hand,' has been a bit of a mystery since it was first discovered in Mongolia back in 1965. All that was discovered, save a couple of scraps, was a pair of gigantic arms ending in hugely clawed hands. These mighty forelimbs baffled palaeontologists for years. They were clearly theropodan, but all known large theropods had small forelimbs (the tiny arms of Tyrannosaurus rex being the most obvious and pronounced example). Eventually, it was pointed out that the arms of Deinocheirus bore a significant resemblance to those of the ornithomimosaurs, aka the 'ostrich dinosaurs' - generally large, but not gigantic, fleet-footed theropods.

Since then, there has been a huge wealth of discovery and rethinking regarding theropod diversity, including descriptions of the therizinosaurs, the 'scythe lizards,' similarly large theropods with huge claws on their hands. Unlike the traditional view of theropods being solely carnivorous animals, the therizinosaurs seem to have been adapted for herbivory. Examination of the ornithomimosaurs suggests they too were omnivorous or herbivorous.

That first find was incomplete because the poor old Deinocheirus had been eaten by a Tarbosaurus (aka Tyrannosaurus bataar, the 'Asian T. rex), as evidenced by distinctive teeth marks on scraps of rib bones found. However, Korean palaeontologists have now made a second, much more complete find, which finally gives some clue as to the overall body plan of the beast. And, while the find confirms that Deinocheirus was indeed a gigantic ornithomimosaur, it turns up some unexpected new information. For a start, this animal was definitely largely or solely herbivorous, as evidenced by the presence of gastroliths, stones swallowed to aid the digestion of plant matter. These are common in herbivorous dinosaurs with weak or entirely absent teeth. Not so surprising, that, but a contentious point until now.

The most amazing find is the animal's backbone. The middle verterbrae of Deinocheirus were elongated upwards, creating tall spines, not unlike those of the sail-backed Spinosaurus or the ornithopod Ouranosaurus. This is totally unexpected - no spine-backed coelurosaurs have ever been discovered before. The jury is open as to whether Deinocheirus had a fleshy sail or a hump, although a hump would have been useful for a large animal in an arid environment. It seems that Deinocheirus was not entirely unlike the therizinosaurs: large, heavy-bodied theropods adapted to a slower-paced, herbivorous lifestyle.

Aggravatingly, bone thieves made off with the head and feet of the skeleton, meaning that there is still a great deal unknown about Deinocheirus. Still, this is a huge discovery that illuminates much on this mysterious creature. National Geographic has a decent article on the find, albeit with an outdated reconstruction illustration, while the Paleoexhibit blog goes into more technical detail and has an captivating reconstruction of a feathery, hump-backed Deino.

The new species unveiled this month is Lythronax argestes, or 'southwestern king of gore,' an 80-million-year-old tyrannosaur. Discovered in southern Utah, corresponding to the prehistoric area called Laramidia, the same region that recently produced the slightly younger ceratposian genus Nasutoceratops.

The exciting thing about Lythronax is its relationship to T. rex. The tyrannosaurian family tree has become more elaborate of late, which has both improved our understanding of the evolution of these creatures and raised more questions. Articles are describing Lythronax as the 'grand-uncle' of Tyrannosaurus, ie not a direct ancestor but sharing a recent common ancestor. However, Lythronax shows a number of adaptations that are similar to that of T. rex, particularly in the skull, which is especially large and deep, with forard-facing eyes giving some degree of binocular vision. This is unexpected due to the age of the fossil, fifteen million years older than T. rex, and now the earliest known tyrannosaurid. Not the earliest tyrannosaur, by a long chalk, but the earliest member of the narrower taxonomic family of Tyrannosaurus.

Thus, this larger, bulkier tyrannosaurid build developed twice in fairly quick succession, in an example of quite narrowly-divided convergence. The discovery also unearthed more complete remains of Teratophoneus, a smaller but very deep-snouted tyrannosaurid. It seems that Laramidia was the centre of tyrannosaur diversification in the mid-to-late Cretaceous. Cladistic analysis of the two genera has led to Lythronax being placed close to Tyrannosaurus/Tarbosaurus, and as more derived than Teratophoneus. They are both part of the tyrannosaurine branch of the family tyrannosauridae; the family is divided into the sturdier tyrannosaurines and the more slender albertosaurines.

Naturally, since everyone goes mad for T. rex and anything that may be linked to it, there's been loads of coverage in the press, even though it's not nearly as exciting as the Deinocheirus find. BBC Science News will give you a flavour of this.

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