Friday, 13 April 2012

Papers in Review

A look at a few recently published papers now. Just lately I appear to have been quite lucky with my choice of posts since, on a couple of occasions, no sooner have I published or am about to publish than, lo and behold, another subject related paper is released. Sometimes this appears to vindicate or agree with what you believe and sometimes it makes you completely re-evaluate and reassess your line of thinking. Either way, it is another indication of how fast things move in today’s palaeoworld and it is always, but always interesting.

Not too long ago, I looked at how dinosaurs may have slept. This post surprised me in as much that it has been one of my most read of late and a recent, but brief, paper by Frederico Agnolin and Augustin G. Martinelli describes a specimen of Guaibasaurus candelariensis that is again suggestive of an avian-like resting posture in dinosaurs.
Guaibasaurus was a basal saurischian from the Late Triassic and the referred specimen was found in the Caturrita Formation in an abandoned quarry in the Linha Sao Luis locality in Brazil. The specimen is essentially complete but it is unfortunately missing the skull, neck and some forelimb elements. The authors report that the specimen is preserved tightly articulated and the elements display no evidence of post depositional transport or erosion. Indeed, the only erosion evident is recent as the fossil became exposed on the surface.
This specimen of Guaibasaurus is preserved with flexed forelimbs, the hind limbs are flexed below the body and the base of the neck appears to be curved around to the side – exactly what you would expect in Aves. This is significant since other dinosaur specimens recovered in the supposed avian resting position ie Mei and Saurornithoides are derived coelurosaurian theropods and Guaibasaurus suggests that this posture evolved at the stem level of basal Theropoda and maybe even earlier.
Back in November last year I highlighted research by Michael D’Emic et al that reported on remains of Acrocanthosaurus atokensis recovered from the Lower Cretaceous Cloverly Formation for the first time. The paper was published on line during March and makes for interesting reading. The referred specimen in the paper was successfully identified as Acrocanthosaurus atokensis and is significant since it is a juvenile and also the first representative of the taxon in the north.

It is intriguing that, throughout the Lower Cretaceous of North America, only Acrocanthosaurus is representative of large bodied Theropoda  and, despite fragmentary remains of large theropods from other formations such as the Patuxent and Cedar Mountain, nothing could be labelled as satisfactorily diagnostic to erect further large theropod  taxa throughout the Aptian/Albian time period.
Although there isn’t a great volume of collected material, there are enough characteristics displayed, especially the femoral head, which supports the referral of the material to Acrocanthosaurus. This specimen, again, suggests that, despite many vertebrate fossil localities from the Lower Cretaceous of North America, there is still currently only one large theropod species represented   throughout a large geographic range and the authors highlight the similarities, therein, with the Late Cretaceous Maastrichtian fauna which also has the one large theropod – Tyrannosaurus rex.
Acrocanthosaurus femora from D'Emic et al 2012

The authors also performed bone histology analysis which confirmed the specimen as a juvenile and revealed that the specimen grew at fast avian-like rates – comparable with tyrannosaurids. The specimen, and other elements examined, suggests a span approaching 20 to 25 years for an Acrocanthosaurus to reach adult size.  This again highlights comparison with Tyrannosaurus in as much that both taxa were large, displayed rapid growth and ranged across a large area.
Miriam Reichel, of the University of Alberta, has published more about her continual research into tyrannosaurid teeth and this latest paper is a hi-tech treat which is fully indicative of today’s use of digital technology in palaeontology. This time, focussing on how carinal variation, both anterior and posterior, display different angles that may be used for functional and/or taxonomic analysis.
Carinal angle has always been difficult to measure but, by using a Microscribe® Digitizer, Reichel was able to successfully interpret the angles between carinae by using the tip of the tooth as a “centroid” - a central point between the carinae - which, in turn, highlighted variation in the teeth. Five tyrannosaurid taxa were analyzed and only teeth in situ – that is teeth still in the jaw - were measured accordingly.  This preliminary analysis provided some interesting data.
Perhaps the most obvious result is that there is a significant amount of carinal angle variation in tyrannosaurid teeth – and this was expected.  This was also the case when comparing teeth from different parts of the jaw although there is variation in the acuteness of angles depending on the location of the tooth set.
This also highlights the previously held notion by some workers that different teeth perform different functions. This may be obvious when comparing teeth from the premaxillary with the others in the jaws but are maybe not so obvious between the other teeth. Tyrannosaurid teeth vary in size, shape and placement and this all contributes to the variation in carinal angles but, despite this, the angles do display expected tendencies in accordance with their function and appear to confirm the earlier hypotheses.
This methodology has great promise and the author admits that a greater sampling of in situ specimens in the future will add strongly to the dataset. The study also adds to the continuing fascination with tyrannosaur tooth morphology and perhaps, more significantly, confirms the presence of heterodonty in Tyrannosauridae.
I have only skimmed the depth of this paper and have simplified a few of the conclusions but you have to be very impressed with the depth and quality of Reichel’s work. She has featured on this blog before with her work on Albertosaurus sarcophagus and I look forward to more of her work with tyrannosaurids in the future.
Federico Agnolin & Agustín G. Martinelli (2012): Guaibasaurus candelariensis (Dinosauria,Saurischia) and the early origin of avian-like resting posture, Alcheringa: An Australasian Journal of Palaeontology, DOI:10.1080/03115518.2012.634203 

D'Emic, Michael D., Melstrom, Keegan M., Eddy, Drew R., Paleobiology and geographic range of the large-bodied Cretaceous theropod dinosaur Acrocanthosaurus atokensis, Palaeogeography, Palaeoclimatology, Palaeoecology (2012), doi:10.1016/j.palaeo.2012.03.003.

Reichel, M. 2012. The variation of angles between anterior and posterior carinae of tyrannosaurid teeth.Canadian Journal of Earth Sciences, 2012, 49(3):477-491, 10.1139/e11-068. 


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