Wednesday, 22 June 2011

More Tyrannosauridae


From Tsuihiji et al 2011

It cannot have escaped anybody’s attention that there have been a few new tyrannosaur papers this year, two of which I’ll discuss here. The first of these is Tsuihiji et als’ paper regarding a magnificent juvenile specimen of Tarbosaurus bataar from the Nemegt Formation in Mongolia.

I have to say that I really enjoyed this paper and I liked the way it was laid out. Firstly, the identification of the specimen as a juvenile Tarbosaurus had to be established and this was done by comparing it with two other skulls of Tarbosaurus that were adult animals. This direct observation was supplemented by state-of-the-art CT imaging that was overseen by the Witmer Lab.

It was obvious, from day one, that this was a tyrannosaurid and the specimen (Cat. No. MPC-D 107/7) clearly demonstrates numerous synapomorphies that support this affinity. But which tyrannosaur was it? The Nemegt appears to have three confirmed taxa – Tarbosaurus bataar (Maleev, 1955a), Alioramus remotus (Kurzanov, 1976) and A. altai (Brusatte et al, 2009). However, this specimen is from the Bugin Tsav locality and only Tarbosaurus is recovered from here.

Other features such as alveoli count in the juvenile are different from Alioramus and features, that are diagnostic of Tarbosaurus, such as a caudal surangular foramen that is smaller in relation to other tyrannosaurids, are present in the specimen. Combined with the fact that there are several autapomorphies found in Alioramus that are not found in MPC-D 107/7 more or less assures that the animal is indeed a juvenile Tarbosaurus. Estimation of age at death appears to be in the region of two to three years old.

A detailed description of the skull follows and, from a personal point of view, I found it to be one of the better descriptions I’ve read. The spotlight is on ontogenetic differences between MPC-D 107/7 and adult animals and comparisons made using previously described detail including Carr’s craniofacial ontogeny of Tyrannosauridae (1999).

This demonstrated immediately that Tarbosaurus and tyrannosaurids from North America shared a plethora of cranial morphologies during ontogeny. The only significant difference appears to be in the aspect of the antorbital fenestra which does not change greatly in shape in Tarbosaurus but does in their North American counterparts.

Interestingly, the skull lacks specific tyrannosaurid adaptations that allow for the bearing of great stress and torsional forces that are encountered during feeding. In addition, the teeth are much thinner labiolingually and are not as strong and this leads to the conclusion that dietary requirements for this juvenile were different from adults and that they progressively changed throughout ontogeny – not the first time this has been suggested and not just for tyrannosaurids either.

Taxanomic issues raised by the specimen again bring to the fore the status of Nanotyrannus lancensis – the feature this time being maxillary tooth counts and variability within taxa therein. Nanotyrannus has 14 or 15 maxillary teeth whilst Tyrannosaurus has 11 or 12 and here there is a clear difference of opinion as to whether tooth counts vary throughout ontogeny, which suggests Nanotyrannus is a juvenile T.rex (Carr 1999), or whether there is no clear pattern in tyrannosaurids, suggesting that Nanotyrannus is a valid taxon (Currie 2003a, 2003b).

Interestingly, I find both arguments neither correct nor incorrect since they both have merit but, for me, because of the likelihood that individual ontogenetic differences within taxa are highly likely, then maxillary tooth counts cannot be considered as taxonomically diagnostic within Tyrannosauridae – not yet anyway.

Other implications from this study imply more variation in ontogenetic change in tyrannosaurines than previously thought suggesting that tyrannosaurids, as a whole, did not go through a “typical” morphological change as they grew. Also, similarities between Shanshanosaurus huoyanshanensis and MPC-D 107/7 reveal that the former is almost certainly another Tarbosaurus bataar. The status of Raptorex kriegsteini is also considered and after running a phylogenetic analysis, which included MPC-D 107/7, the authors are non-committal although it seems likely that R.kriegsteini is not a juvenile Tarbosaurus. Watch this space.

As I said earlier, I really enjoyed this paper and look forward to the description of the postcrania of this wonderful specimen at some time in the future.


From Tsuihiji et al 2011

 Perhaps not as spectacular, but just as fascinating, is the new tyrannosaurine Zhuchengtyrannus magnus, described by Dave Hone et al in Cretaceous Research. The material described is an associated right maxilla and left dentary from Campanian deposits exposed in the Zhucheng quarries of Shandong Province, China and represents a large theropod on a par with Tarbosaurus – perhaps even bigger.

The maxilla displays a maxillary fenestra that is diagnostic of tyrannosaurines but demonstrates sufficient morphological differences from other tyrannosaurines to warrant the raising of a new taxon and includes some unique characteristics.

Although the available material is of limited value to perform a phylogenetic analysis, the specimens, never the less, enable a few assumptions to be made. Firstly, this animal appears to be an adult based on the lack of juvenile characteristics and, of course, there is the size of the bones which are comparable to both Tyrannosaurus and Tarbosaurus. However, Zhuchengtyrannus can be clearly distinguished from Tarbosaurus due to several different characteristics.

As noted in the paper, perhaps the most interesting detail is the fact that there have been remains of a second tyrannosaurid recovered from the same quarry. When you add Tarbosaurus into the mix, then this demonstrates the presence of three large tyrannosaurid theropods living contemporaneously throughout the Campanian.

This is yet another example of an ancient ecosystem with multiple large predators coexisting and the authors highlight other formations that yield such animals such as the Morrison, Two Medicine, Dinosaur Park and Kem Kem Formations. This is what makes the latest Maastrichtian of North America so interesting if, indeed, Tyrannosaurus is the only large bodied theropod in residence.

The implications of this are continually fascinating. How did so many large theropods coexist at the same times? Niche partitioning maybe? Perhaps – and what are the physiological implications for all this, especially if you believe in endothermic dinosaurs? Bob Bakker (1972) postulated that predator-prey ratios were indicative of whether a specific ecosystem was endothermic or ectothermic ie low ratio of predatory animals to prey animals was typically an endothermic community whilst a high ratio of predators to prey animals was indicative of an ectothermic fauna.

Although predator-prey ratios have come under considerable scrutiny over the years and, in some circumstances, considered a flawed analytical tool, it would be interesting to see more research using them, taking into account the faunal increase and turnover in some formations, although sampling bias is often one of the biggest objections to predator-prey analysis. Perhaps it may support theories, such as Scott Sampson’s (2010), that dinosaurs may have been something in between cold blooded and warm blooded – mesothermic he described them.

Whatever the outcome, specimens such as the tyrannosaurids described above can, not only add to our understanding about these animals, but also may help in expanding our knowledge regarding Dinosauria as whole.

Footnote

Make a point of tuning into the Discovery Channel at 9 o’clock GMT on Sunday night for Phil Currie’s latest theories about gregarious tyrannosaurs in Dino Gangs. For a sneak preview, take a look here.

References

Bakker, R.T., 1972. Anatomical and ecological evidence of endothermy in dinosaurs. Nature 238:81-85.

Brusatte, S.L., Carr, T.D., Erickson, B.R., Bever, G.S., Norell, M.A., 2009. A longsnouted, multihorned tyrannosaurid from the late Cretaceous of Mongolia. 1Proceedings of the National Academy of Sciences 106, 17261:17266.

Carr, T.D., 1999. Craniofacial ontogeny in Tyrannosauridae (Dinosauria: Theropoda). Journal of Vertebrae Paleontology 19, 497:520.

Currie, P. J. 2003a. Cranial anatomy of tyrannosaurid dinosaurs from the Late Cretaceous of Alberta, Canada. Acta Palaeontologica Polonica 48:191–226.

Currie, P. J. 2003b. Allometric growth in tyrannosaurids (Dinosauria: Theropoda) from the Upper Cretaceous of North America and Asia. Canadian Journal of Earth Sciences 40:651–665.

Hone, D.W.E., et al., A new, large tyrannosaurine theropod from the Upper Cretaceous of China, Cretaceous Research (2011), doi:10.1016/j.cretres.2011.03.005

Kurzanov, S.M., 1976. A new late Cretaceous carnosaur from Nogon-Tsav Mongolia. Sovmestnaâ Sovetsko-Mongolskaâ Paleontologiceskaâ Ekspeditciâ. Trudy 3, 93e104 (in Russian).

Maleev, E. A. 1955a. [A gigantic carnivorous dinosaur of Mongolia]. Doklady Akademii Nauk SSSR 104:634–637. [Russian]

Sampson, S.D., 2009. The Goldilocks Hypothesis. In: Sampson, S.D., Dinosaur Odyssey. Fossil Threads in the Web of Life. University of California Press, pp. 175-191.

Tsuihiji, Takanobu , Watabe, Mahito , Tsogtbaatar, Khishigjav , Tsubamoto, Takehisa , Barsbold, Rinchen , Suzuki, Shigeru , Lee, Andrew H. , Ridgely, Ryan C. , Kawahara, Yasuhiro and Witmer, Lawrence M.(2011) 'Cranial osteology of a juvenile specimen of Tarbosaurus bataar (Theropoda, Tyrannosauridae) from the Nemegt Formation (Upper Cretaceous) of Bugin Tsav, Mongolia', Journal of Vertebrate Paleontology, 31: 3, 497 — 517 DOI: 10.1080/02724634.2011.557116

1 comments:

qilong said...

One of the more interesting things to come up in this debate was actually Scannella and Horner's arguments of anagensis, whereby the lineages are differentiated into biologically unique "ends." These ends tend to correspond to stratigraphy and may also to geography. So when the Zhucheng, Bugin Tsav and Nemegt localities are correctly placed in sequence, they may tend to distinguish groups of taxa that are nonetheless distinct from one another. What follows is nomenclature.

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