Thursday, 10 February 2011

More on Tyrants

No furcula............
If you are into tyrannosaurs then Thomas Carr is a name you will be familiar with. If it isn’t then you have some serious reading and studying to catch up on. Indeed, Tom Holtz declared on the DML, not too long ago, that reading Carr’s work is more or less a prerequisite for any student of Tyrannosauroidea.

 And Carr is a busy man. Not content with announcing Bistahieversor sealeyi in 2010 (with long term colleague Thomas Williamson and others), he has also recently announced yet another tyrannosauroid, this time the wonderfully named Teratophoneus curriei.

Carr also gave a presentation at SVP last year regarding ontogenetic variation in Tyrannosaurus rex. Because of the boom in specimens recovered over the last twenty years, there are now examples of Tyrannosaurus that are beginning to fill the ontogenetic gap between the juvenile and adult stage.

This has now highlighted issues similar to those that have caused such debate amongst ceratopsians workers in recent times – namely that the differences between different growth stages can vary greatly and this has led to a number of taxa being mistakenly named.

Using an extraordinarily detailed cladistic analysis, utilising the details from twenty specimens, Carr was able to map out a growth stage of fifteen steps. The only gaps in the sequence were in the juvenile to sub-adult stage but this simply reflects the paucity of recovered specimens and the fact that these animals appear to put on a massive spurt of growth during this exact period of ontogeny. T.rex displays multiple ontogenetic stage variations throughout and appears to confirm that, indeed, there was only one species of Tyrannosaurus in the Maastrichtian.

Also at SVP, Mark Loewen et al presented their findings regarding tyrannosaurid evolution and regional endemism within the clade and took the opportunity to report on yet another new tyrannosaur – this time from the Wahweap Formation of Utah.

When you look at the new taxon, my first thoughts are Teratophoneus. Both have extremely similar morphological patterns including a short robust maxilla and a reduced dental count. Both specimens are animals with powerful, short skulls that are broad and deep. Both specimens are Late Campanian and both appear to be basal tyrannosaurinae.

Running a phylogenetic analysis that tested relationships amongst tyrannosaurids, and to see how the new Wahweap taxon slotted into this group, revealed the apparent existence of a clade of tyrannosaurids in the south of the Western Interior corridor and Loewen et al interpret this data as suggesting a western North American origin for Tyrannosauridae.

Bistahieversor, Teratophoneus and the Wahweap taxon suggest, again, that there appears to be an unusually large quantity of taxa ranging from the south to the north of the Western Interior Basin. Maastrichtian tyrannosaurids Tyrannosaurus and Tarbosaurus settle into the southern clade, emphasising the split between themselves and Campanian tyrannosaurids from the north although where that leaves Daspletosaurus torosus, as a northern Campanian tyrannosaurine, appears unclear to me.

However, as I often allude, there are multiple daspletosaur taxa yet to be described in the literature, and these may demonstrate that both tyrannosaurine evolution and migration was extraordinarily rapid. Still, this work continues to highlight the wonderful uncertainty of tyrannosaurid research and emphasises the continual shifts in thought patterns and theory when it comes to the investigating the evolution of tyrannosaurids.

Also at SVP, there were a couple of tyrannosaur related posters of interest. Shychoski et al looked at how the arctometatarsus of tyrannosaurids enabled enhanced locomotive agility within the group. Using varied techniques including micrography, bone histology and computer aided tomography on a tyrannosaurid metatarsal three, revealed a complex arrangement of ligaments that both strengthened the foot and was capable of dispersing high torsion loads that may be incurred when actively hunting.


The arctometatarsalian condition

An analysis of the tyrannosaurid arctometatarsus which compared comparative agility within the group also confirmed that they could turn rapidly and brake hard making them extremely agile. Indeed, they were far more agile than other theropods with a similar body mass. By inference alone, this combination of high manoeuvrability and the specialised arctometatarsalian condition suggests, yet again, that tyrannosaurs were swift agile hunters which came to dominate the predatory niches within the Late Cretaceous.

Williams et al demonstrated how a new specimen of Tyrannosaurus from the Hell Creek Formation in Montana has provided evidence which demonstrates that the small forelimbs, so synonymous within Tyrannosauridae, were actually larger, and thus more useful, within juveniles. The specimen has a relatively complete forelimb with scapulocoracoid, humerus, ulna and unguals present – such an important find.

The humerus is long and slender in comparison to the adult animals and, coupled with extremely large unguals, demonstrates that the entire forelimb was considerably longer than in the adult morph and highlights that the reduction in forelimb size commenced throughout ontogeny. This confirms earlier work that assumed reduction in forelimb size occurred as the skull became bigger, deeper and more robust, thus emphasising the development of jaw power.

This also, interestingly enough, suggests that juvenile Tyrannosaurus may have had different prey requirements at this stage, perhaps a form of niche partitioning, and they may also have exhibited different behavioural patterns. The authors also highlight the similar trend, in a phylogenetic context, in basal tyrannosauroids such as Guanlong and Dilong which also displayed large forelimbs and manus and point out that rare intermediate specimens, such as Dryptosaurus, display a humerus that is reduced in size, thus indicating that the forelimb began to reduce in size at the proximal end first during tyrannosauroid evolution.


Finally, and back to Thomas Carr, Carthage College indicate that Dr.Carr is currently working on The Tyrant Lizards; The Reference Volume of Tyrannosauroidea. This textbook, aimed primarily at students and vertebrate palaeontologists, is extremely high on my wanted list and will probably be my new bible. You can count on the fact that I’ll be keeping a lookout for any news on publication and will relay any news as it becomes available.


References

Carr, T.D.,2010 Ontogenetic variation in Tyrannosaurus rex: results from a numerical cladistic analysis. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2010, 70A.

Loewen, M, J. Sertich, R. Irmis, and S.D. Sampson 2010 Tyrannosaurid evolution and intracontinental endemism in Laramidia: New evidence from the Campanian Wahweap Formation. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2010, 123A.

Shychoski, L., E. Snively and M. Burns 2010 Manoeuvred out of a corner: ligament entheses of the arctometatarsus enhanced tyrannosaurid agility. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2010, 165A.

Williams, S., S. Brusatte, J. Matthews and P. Currie 2010. A new juvenile Tyrannosaurus and a reassessment of ontogenetic and phylogenetic changes in tyrannosauroid forelimb proportions. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2010, 187A.

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