Tuesday, June 17, 2008

I'm slow but I get there in the end: Megaraptor in Oz

Typical! I get a really busy two weeks with a an overnight trip and a public holiday (yesterday was Youth Day in South Africa) and in come a bunch of new papers I'd like to blog about. Two of the new papers have already been covered by Laelaps here and here. What can I say? Never read this blog to get the latest info hot off the presses, I'm slow and busy, but when I do blog, I hope I can bring just a little insight that hasn't been posted elsewhere.
About two weeks ago Nate Smith and colleagues released a paper in the Proceedings of the Royal Society, that describes an Early Cretaceous theropod dinosaur ulna from Victoria, Australia that bears a close resemblance to Megaraptor, a large penecontemporaneous [edit: Megaraptor is a Late Cretaceous theropod, very far from being penecontemporaneous with the victorian specimen - oops]theropod from South America. Megaraptor is a bit of an oddity, it is clearly a large predator with a wicked manual claws. It may be related to the spinosaurids according to Smith et al. but it also shows similarities to carcharodontosaurids (Calvo et al. 2004), another group of mostly giant predators with a mostly Gondwanan distribution. Smith et al. point out a number of unusual specific resemblances not seen in other theropods and then suggest that the aussie ulna and Megaraptor are each others closest relatives. Okay, nice to get some ID on another bit of dino from Australia but isn't that rather expected? Afterall Australia is most certainly part of Gondwana and we should expect faunal ties with other Gondwanan continents. Yes but oddly the fauna that inhabited the rift system between the southern margin of Australia and Antarctica (where the ulna in question comes from) has been interpreted as largely laurasian in aspect with ornithomimosaurs, oviraptorosaurs, allosaurids, neoceratopsians and eutherian mammals. However all of these identifications are somewhat dubious, and are mostly based on single bones. So is this latest ulna, you might say, but I would counter that the difference here is that the identification is based on specific derived characteristics rather than a simple phenetic assessment (ie. 'it looks like one') that underlies most of the other identifications. So it is nice to get a solidly Gondwanan taxon in the Early Cretaceous of Australia. Or have we? For the moment lets accept that the aussie ulna and Megaraptor are closely related within Theropoda, might there be other members of this newly recognised clade? Well there is one other large theropod with huge manual claws that was found to be a close relative of the spinosaurids in a large cladistic analysis (Rauhut 2003), a poorly known creature named Chilantaisaurus tashuikouensis Hu, 1964, also of similar age to Megaraptor. However this creature is firmly Laurasian, being found in the Nei Mongol region of China.



The claw of Chilantaisaurus from Hu 1964. Scale in cm.

If Chilantaisaurus were another 'megaraptorid' then the neat biogeographic story of smith et al. is somewhat weakened. Basically these things would be part of a cosmopolitan fauna that got isolated on different continents as the continents broke apart, with differences in distribution being more to do with differential extinction than anything else. Can we tell at the moment? Hell no. Sadly the known bits of Megaraptor havealmost no overlap with known the bits of Chilantaisaurus (just the large manual claw and metatarsal 4 - neither of which are particularly usefull in this case). For all we know they may be only distantly related spinosauroids, or Megaraptor might not even be a spinosauroid at all. We won't know until more complete remains of enigmatic Megaraptor are forthcoming. I hate to end this post with the trite 'we need more fossils' message, which I usually regard as a bit of a cop-out, but in this case it seems apt.

References.

Calvo JO, Porfiri JD, Veralli C, Novas F and Poblete F (2004) Phylogenetic status of Megaraptor namunhuaiquii Novas, based on a new specimen from Neuquen, Patagonia, Argentina. Ameghiniana 41: 565-575.

Hu S-Y (1964) Carnosaur remains from Alashan, Inner Mongolia. Vertebrata PalAsiastica 8: 42-63.

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

3 comments:

Mickey said...

Other elements known in both Chilantaisaurus (IVPP V2884) and Megaraptor are proximal caudal vertebrae (MUCPv 341), femora (UNPSJB-PV 944 and 958), tibia (UNPSJB-PV 958), fibula (UNPSJB-PV 944 and 958), metatarsal II (UNPSJB-PV 944 and 958) and metatarsal III (MCF-PVPH 79), though the listed limb elements are generally fragmentary in known Megaraptor specimens.

Adam Yates said...

Okay, I wasn't aware that there was so much more Megaraptor material (I was working from the Calvo et al. 2004 paper). However this still doesn't look like it will help in sorting out if Chilantaisaurus really is realated to Megaraptor. It would be nice to know if the tibia dispalys the distal ridge that Rauhut used to place Chilantaisaurus close to the spinosaurids.

Mickey said...

Unfortunately, the distal tibia isn't preserved, so the morphology of the astragalar ridge is unknown.