Sunday, 22 December 2013

Tyrannosaurs at Christmas




Despite SVP being understandably mammal-centric this year, there was still plenty of dinosaur related presentations and posters to keep us all happy. As usual, I was focussing mainly on marine reptiles (not a lot this year unfortunately) and theropod research – particularly tyrannosaurids and, in this particular instance, there were a few really interesting pieces of research on show.
Regular readers are aware that I have often discussed the importance of the tyrannosaurid Daspletosaurus and how continuing research into this animal is important – especially when it comes to tyrannosaurine evolution and the dispersal of Tyrannosauridae throughout North America. We have known for some time now there are multiple species of Daspletosaurus and here, for the first time, Tetsuto Miyashita, of the University of Alberta, and colleagues presented their research into one of these new tyrannosaurids.
The new taxon is represented by multiple skulls and skeletons and displays several characters that are distinctive from Daspletosaurus torosus. These include features in the premaxilla, nasal, postorbital boss, and lacrimal (amongst others). Remains are found in both the Dinosaur Park Formation and the temporaneous uppermost Oldman Formation and are thus more recent than D. torosus.
The authors’ phylogenetic analysis reveals that this taxon, as well as Daspletosaurus sp. from the Two Medicine Formation, forms a clade which is comprised of tyrannosaurine tyrannosaurids that are more derived than those from Campanian deposits in Utah and probably New Mexico. Interestingly, this analysis also recovered Bistahieversor as a tyrannosaurid which is just what the comprehensive analysis recently documented in the Lythronax paper (Loewen et al 2013) also found.
Here it is interesting that the authors favour Daspletosaurus as being derived with both Tarbosaurus and Tyrannosaurus but Loewen et al regard Daspletosaurus as a basal northern clade and separate from the southern Laramidian radiation of tyrannosaurids. They favour a southern distribution for more derived tyrannosaurids and suggest that both Tyrannosaurus and the Asian Tarbosaurus evolved from this dispersal event in the south.
This is intriguing because there are obvious differences between these two analyses and it raises yet more questions than there are answers. Personally, I remain open minded about tyrannosaurid dispersal events and, make no mistake, there were probably several. But perhaps there is now more evidence about Daspletosaurus being “northern” and it comes in the form of another unexpected new tyrannosaurid announced at SVP.
Anthony Fiorillo and Ron Tykoski, both of the Perot Museum of Nature & Science in Dallas, presented their research on skeletal remains of large theropods in the Upper Cretaceous Prince Creek Formation of Alaska. The Prince Creek crops out along the bluffs of the Colville River and the sedimentology reveals deposits that were laid down in a wet/dry seasonal environment. There are several sites of which one, the Kikak-Tegoseak Quarry, has yielded some extremely interesting elements from a tyrannosaurid.
One of these, DMNH 21461, is a skull roof displaying a bifurcated sagittal crest and would seem to indicate a total skull length of around 0.6 metres. Other elements include a partial dentary and are indicative of a derived tyrannosaurine and are quite diagnostic indicating that a new taxon is involved. The authors are naturally enough a little wary at the moment about confirming or naming the new taxon and, when questioned about the height of the sagittal crest by Thomas Carr, declined to answer in public. That is fair enough but my observations of the fossils on display, and the sagittal crest in particular, suggest that this may yet be another species of Daspletosaurus. And there is more.
The Liscomb Bonebed, another site in the Prince Creek, has also yielded tyrannosaurid elements and these display an unusual condition for their size. These include specimens DMNH 2012-25-31 (a neural arch), DMNH 2012-25-30 (a partial radius) and DMNH 2012-22-20 (a pedal phalanx) and  their overall morphology is indicative that they represent an adult tyrannosaurid – but this particular individual is only about 40% the size of what a conventional large adult tyrannosaurid would be.
This should actually not be a surprise considering the conditions that would have been endured in the palaeoenvironment of northern Alaska back in the Late Cretaceous and dwarfism would have been a logical response when resources are limited. As research progresses it will be interesting to know how these tyrannosaurids relate to each other and, as I mused earlier, how they will further enlighten our knowledge of the tyrannosaurid radiation throughout the Late Cretaceous.
Thomas Carr’s contribution to SVP this year was a poster looking at the sister group relationship between Daspletosaurus and Tyrannosaurus. Just in case you are unaware, Carr is still working on a quite significant monograph on Daspletosaurus which was, when we were chatting at SVP, over 1000 pages long – a significant contribution. So it was not surprising to see this poster featuring his research into testing the evolutionary lineage of these tyrannosaurids and whether a case for anagenesis can be made.
A comprehensive cladistic analysis was undertaken to compare both phylogenetic and ontogentic sequences and patterns. Unsurprisingly, perhaps, there are several factors that are suggestive of an anagenetic tyrannosaurine lineage but, equally, there is that question of the dispersal of Tyrannosaurinae in the first place. As Carr points out, Tyrannosaurus is phylogenetically removed from Laramidian species by some Asian taxa in the first place and the fact that, as I have already alluded to, there were several dispersal events between Asia and North America and then the case for anagenesis in Laramidian tyrannosaurines becomes very complex indeed .
Interesting again is how this analysis tends to support that of Myashita et al from earlier and contrasts with that of Loewen et al which favours a southern radiation for Laramidian tyrannosaurines. The origins of these formidable tyrannosaurids looks set to be undetermined for some time to come.
Finally, and moving away from tyrannosaurids, Joseph Sertich, of the Denver Museum of Science & Nature, and his colleagues unveiled another giant theropod but this time in the form of a primitive abelisaurid. This specimen has proven to be remarkably popular and there were lots of extremely interested workers at SVP and, again, in the palaeoworld since the meeting.
The specimens derive from the Lapurr sandstone in the Turkana Basin of Kenya and are rocks of Maastrichtian age. Elements include some from the skull and those from both axial and appendicular skeleton. Unfortunately, a lot of this material is scrappy and poorly preserved but is diagnostic enough to be able to refer these specimens to the Abelisauridae.
However, the size of the animal is readily evident and is confidently predicted to be in the region of eleven metres long – maybe more but there appears to be no significant cranial ornamentation on this taxa unlike other abelisaurids. This large theropod was part  of a diverse fauna that is now coming to life as more fossils are found and include sauropods, ornithopods, theropods, pterosaurs and crocodiles and, despite most of the fossils that have so far been recovered being scrappy, there is every likelihood of yet more surprises to come from this intriguing part of Africa.
Footnote
Well it is the time of year when the season of goodwill should be the order of the day. It has been a very busy year for me and, as a result, I have not been able to post as often as I would like but, rest assured, I will always endeavour to post as often as I can. Between the amount of work I have and also looking after the interests of the group (which is important for all of us) time is of a premium just now - but we will get there in the end.
Thanks for all your support over the last year and now it just remains for me to say that, to those of you who celebrate it, I hope you all have a happy and peaceful Christmas. Enjoy!  

References

Carr, T. 2013. Using ontogeny and phylogeny to test hypotheses of anagenesis in the vertebrate fossil record: a case study of the sister group relationship between Daspletosaurus and Tyrannosaurus (Dinosauria, Coelurosauria). Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2013, pp101.
Fiorillo, A & Tykoski, R. 2013. Distribution and polar palaeoenvironments of large theropod skeletal remains from the Prince Creek Formation (Early-Late Maastrichtian) of Northern Alaska. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2013, pp127.
Loewen MA, Irmis RB, Sertich JJW, Currie PJ, Sampson SD (2013) Tyrant Dinosaur Evolution Tracks the Rise and Fall of Late Cretaceous Oceans. PLoS ONE 8(11): e79420. doi:10.1371/journal.pone.0079420
 Miyashita, T., Currie, P. & Paulina Carabajal, A. 2013 A new species of Daspletosaurus (Theropoda: Tyrannosauridae) from the Campanian of southern Alberta represented by a growth series of well-preserved skulls and skeletons. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2013, pp178.
Sertich, J., O’Connor, P., Seiffert, E. & Manthi, F. 2013. A giant abelisaurid theropod from the latest Cretaceous of Northern Turkana, Kenya. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2013, pp211.

5 comments:

Andrea Cau said...

There is a lot of work on Daspletosaurus! In fact, I've preferred not to enter it (them, if multiple species) into my analysis pending the huge amount of information from all these new studies.

Alessio said...

Interesting as always, didn't know there were so many species of Daspletosaurus!
Happy Holidays, by the way :)

Mark Wildman said...

Yes - there are others still to come as well......

Anonymous said...

Thanks for commenting on all the wonderful work being done on Tyrannsaurids, and theropods in general.

Mark Wildman said...

Thank you for the kind words!

Post a Comment