Sunday, July 5, 2009
Three new Australian dinosaurs, Australovenator wintonensis at the top, Wintonotitan wattsi in the middle and Diamantinasaurus matildae below. Scale bar equals 1 metre. From Hocknull et al. 2009.
What a year for dinosaur research it has been. We’ve had: the publication of a Cretaceous heterodontosaurid with filamentous integument; a slew of new taxa including a member of the perennially popular tyrannosauroids; a toothless, herbivorous ceratosaur !!! with a bizarre hand (which may or may not shed light on the homology of bird fingers). Now that dinosaur depauperate continent , Australia, that produced naught but a handful of decent dinosaur fossils in all the years I lived there, has thrown up three new taxa. The open access paper is here.
Aussie geo-folklore would lay the blame on its apparent dinosaurlessness on the geological quiescence Australia has experienced since the Mesozoic. With no major mountain building events to thrust up strata of the right age there is very little exposure to search and much of that has just been sitting around since it was deposited getting ever more deeply weathered.
While much of this is true it does not preclude the preservation and excavation of decent dinosaur fossils as this new paper shows. The fossils come from the cattle country of central Queensland. Where the land is as flat as a tack, and almost completely grassed over. Not exactly promising territory for palaeontological exploration. Nevertheless the area is unlerlain by the Winton Formation an sedimentary unit laid down on a floodplain fringing a great epicontinental (‘inland’) sea during the middle part of the Creataceous Period.
So why not dig down to the sediments? That is exactly what the team reporting (Hocknull et al.) these new dinosaurs has done. According to the pHocknull et al. the subcrop of the Winton Formation lay under just 1 m of overlying soil. We’ve cut through thicker piles of overburden in our Elliot Formation, so it is not that the Winton Formation is out of reach. Of course, on such flat soil covered land you are lacking the usual clues like fragments of weathered bone falling downslope to lead you to productive sites, nonetheless chunks of weathered bone in the soil can signal something worthwhile lies below. Furthermore the excellent state of preservation of some of these bones (particularly those of the theropod, Australovenator) show that deep weathering profiles may not be quite the problem they’ve been made out to be.
So what did Hocknull et al. find? Two new sauropods (Wintonotitan wattsi and Diamantinasaurus matildae, both titanosauriforms, and an allosaurid theropod (Australovenator wintonensis). Actually to say that two new sauropods were found is not strictly accurate. The holotype specimen of Wintonotitan had been found decades earlier, described as Austrosaurus sp. (Coombs and Molnar 1981) and even incorporated into a phylogenetic analysis of sauropod relationships (as Austrosaurus, Upchurch et al 2004). The type of Austrosaurus is just a series of beat-up dorsal vertebrae, consisting mostly of the centra alone. It comes from the slightly older Allaru Mudstone and differs slightly from the dorsal vertebra of Wintonotitan, indicating that the two are not synonyms. In anycase the name Austrosaurus is best dropped as a nomen dubium based on inadequate remains. A couple of rather preliminary phylogenetic analysis suggest that both sauropods are somphospondyls (titanosauriforms more closely related to ‘classic’ titanosaurs (exemplified by the armoured Saltasaurus from Argentina) than to Brachiosaurus. This is an unsurprising result as almost all sauropods of this age belong to this group. Wintonotitan was found to be a relatively basal member of the group, outside the Titanosauria proper but certain features such as the plate-like ischium with elongate iliac peduncle, medially shifted deltopectoral crest on the humerus and the eye-shaped pleurocoels in the dorsal vertebrae indicate that Wintonotitan may actually hold more derived position within Titanosauria, like its compatriot Diamantinasaurus.
Anyway it is the carnivore, Australovenator that I want to discuss for the rest of this post. As Hocknull et al. point out the non-avian theropod record from Australia is abysmal so there is little to compare it too. One recently described Australian theropod is NMV P186076, an isolated ulna from the Cretaceous of Dinosaur Cove, Victoria that Smith et al. (2008) found to be closely related to the large tetanuran Megaraptor from Argentina. While Hocknull et al. point out some differences between the ulna of Australovenator and NMV P186076 they don’t make much of the strong similarities that these ulnae display in comparison with other basal tetanuran theropods. These similarities include an hypertrophied, mediolaterally compressed olecranon process and an enlarged, proxiodistally elongated lateral tuberosity, defining a cranial fossa. You can see the similarity in the composite figure I whipped up below.
Ulnae of Australovenator and cf. Megaraptor from Dinosaur Cove. Lateral view on left anterior vie in the middle and proximal (top) view on the right. The larger pale brown bone is Australovenator the smaller dark grey bone is cf. Megaraptor.
A ‘megaraptorid’ identification for Australovenator also gels with its rather large wicked looking thumb claw that is about three times longer than its proximal height. More interesting is the astragalus (main ankle bone) of Australovenator which bears a striking resemblance to another isolated Victorian bone, this time from the Cretaceous deposits near Inverloch, which has been touted as everything from Allosaurus to an abelisaurid. Thus despite the small time gap between the two isolated Victorian theropod bones it is quite possible they came from closely related animals. That three separate occurrences from the middle of the Cretaceous of Australia seem referable to this clade indicates to me that these ‘megaraptorids’ were the dominant large carnivores in the middle Cretaceous of Australia.
‘Megaraptorids’ (if indeed they are a clade) are still rather fragmentarily represented, so the addition of Australovenator is most welcome. It helps pull in some other poorly known theropods into this newly recognized fold. One of these is Fukuiraptor kitadanensis a small possible allosauroid from Japan, that has an astragalus that closely resembles the astragalus from Inverloch and that of Australovenator. Although damaged the ulna also appears to bear an unusually large mediolaterally compressed olecranon process. Chilantaisaurus tashuikouensis is a second possible ‘megaraptorid’ from the Cretaceous of Asia. Last year when Smith et al. published the Dinosaur Cove ulna I suggested that this giant asian theropod was a megaraptorid based on: 1, the enlarged manual ungual 2, an apparent close relationship to spinosaurids found by Rauhut (2003)and 3, a similar position for ‘megaraptorids’ found by Smith et al. (2008). Although the spinosauroid position for ‘megaraptorids’ is looking weaker than their position as a basal radiation of carcharodontosaurid allosauroids, I still think there may be a possibility that Chilantaisaurus is a megaraptorid. Supporting this is the presence of a distal craniomedial ridge of the tibia in Australovenator much like the ridge seen in Chilantaisaurus (admittedly this ridge is also found in Suchomimus and Coelurus, so it is not unique to ‘megaraptorids’). Adding a little strength to this idea is the rather hatchet-like deltopectoral crest of Fukuiraptor which looks like a partially developed version of the strongly hatchet-shaped deltopectoral crest of Chilantaisaurus.
In conclusion the ‘megaraptorids’ might be a cosmopolitan clade of Cretaceous allosauroids that share a hatchet-shaped deltopectoral crest, an unusual ulna morphology with an enlarged, blade-shaped olecranon process, an enlarged thumb claw, a femoral head that is not as strongly elevated as other known carcharodontosaurids, a medial ridge on the distal tibia and a distinctive astragalus with a square shaped ascending process. If this clade really exists we can expect earlier representatives to extend back into the Jurassic to allow them to have achieved their near cosmospolitan distribution. Time, new fossils and further analysis may tell.
Coombs WP Jr., Molnar RE (1981) Sauropoda (Reptilia, Saurischia) from the Cretaceous of Queensland. Memoirs of the Queensland Museum 20: 351-373.
Hocknull, S., White, M., Tischler, T., Cook, A., Calleja, N., Sloan, T., & Elliott, D. (2009). New Mid-Cretaceous (Latest Albian) Dinosaurs from Winton, Queensland, Australia PLoS ONE, 4 (7) DOI: 10.1371/journal.pone.0006190
Rauhut OWM (2003) The interrelationships and evolution of basal theropod dinosaurs. Special Papers in Palaeontology 69: 1-213.
Smith ND, Makovicky PJ, Agnolin FL, Ezcurra MD, Pais DF and Salisbury SW (2008) A Megaraptor -like theropod (Dinosauria: Tetanurae) in Australia: support for faunal exchange across eastern and western Gondwana in the Mid-Cretaceous. Proceedings of the Royal Society B: doi:10.1098/rspb.2008.0504
Upchurch P, Barrett PM and Dodson P (2004) Sauropoda.Pp. 259-322. In Weishampel DB, Dodson P and Osmolska H (eds) The Dinosauria. Second Edition.University of California Press: Berkeley.
Thursday, July 2, 2009
Photos by Matt Bonnan
No time for an in depth post today, so I'm keeping the squamate theme going with a picture from my archives. This is a legless skink (Acontias gracilicauda)that happened to have made it home directly above an early Jurassic sauropod bone bed. So it had to be relocated. There is a moderate diversity of acontine skinks in southern Africa: they are just one of many lizard lineages, apart from snakes, that have beome completely limbless.