Tyrannosaurs

coverI had the chance to skim the relatively new Tyrannosaurid Paleobiology, a research compendium edited by Parrish, Molnar, Currie & Koppelhus, and published by Indiana University Press. Impressive book that goes through the variety of research programmes now happening on the biology of tyrannosaurs, highly-recommended if you can chew through the technical nature of the book. This post is just a collection of small facts about tyrannosaurs. Nothing detailed.

Source: Benton
Simplified dinosaur phylogeny. Source: Benton (2009)

Tyrannosaurus rex is probably the most famous dinosaur, thanks to its general fearsome appearance (lessened nowadays by the knowledge it had feathers) and its misplaced casting in Jurassic Park, even though its a Late Cretaceous dinosaur. T. rex is a 12 meter long, 5 ton tyrannosauroid (Brochu, 2003), a group of large, basal coelurosaur theropod dinosaurs (Holtz, 1994) that originated in the Middle Jurassic as part of the general dinosaur radiation of the time (Xu et al., 2006). While T. rex is the most famous, tyrannosaurs were actually fairly diverse (other famous names include Albertosaurus, Daspletosaurus, Tarbosaurus), even including a possible dwarf species, Nanotyrannus lancensis, that could just be a juvenile T. rex (Carr, 1999). Research into tyrannosaurs is now booming thanks to a spate of new species discoveries over the past decade (Brusatte et al., 2010b).

Tyrannosaurs ended up as very large animals like T. rex, but they started off smaller, overshadowed by larger predators in the Jurassic (Benson et al., 2010). In the Early Cretaceous, they experienced a burst of gigantism leading to the famous tyrannosaurs (Brusatte et al., 2010a) that became the dominant carnivores of at least the Cretaceous northern hemisphere.

These giant tyrannosaurs, exemplified by T. rex, had heads larger than 1.5 meters, housing a brain with olfactory bulbs larger than their cerebral hemispheres, granting them an excellent sense of smell (Horner & Dobb, 1997). Their jaws had rather unique teeth: broad and with rounded serrations, generally teeth that were not as good at cutting as we would expect a top predator’s teeth would be, although they would have excelled at keeping up with a large struggling prey, like any of the armoured herbivores, or even other tyrannosaurs (Longrich et al., 2010). This ties into their tremendous bite force, which is estimated to be the largest of all known animals at between 35000 and 57000 N (Bates & Falkingham, 2012). This enormous bite force is enabled by a suite of supportive structures and arrangements in the skull so it doesn’t shatter at every bite.

T. rex is also known for its comically reduced forelimbs. This reduction is rather mysterious, considering that they seems to have still had some sort of function: they were very strong. The function could be something as mundane as getting up from a belly-down sleeping position, or they were used somehow in hunting or scavenging.

One of the classic tyrannosaur controversies is whether they could run. In an innovative study, Hutchinson & Garcia (2002) showed that for a dinosaur as large as Tyrannosaurus to run, the mass of leg muscle needed is so immense that the animal would collapse. Tyrannosaurus could walk, even walk briskly, but running quickly isn’t too viable an option.

This has implications for their hunting strategies. Scavenging is pretty much a given, as it is with all predators, but the relative role of active hunting is under dispute. With such a locomotory capacity, hit-and-run attacks and ambushes, delivering a swift traumatic bite and waiting for weakening by blood loss and shock, are the most plausible attack strategies, rather than actual chasing of prey. Its eyesight is capable of detecting live animals, especially with the binocular vision, the specialisations of the skull for the enormous bite forces hint at active hunting (Rayfield, 2004), and its sense of smell can easily be used for detecting living and dead animals. So a combination of scavenging and ambushing was how it led its life.

References:

Bates KR & Falkingham PL. 2012. Estimating maximum bite performance in Tyrannosaurus rex using multi-body dynamics. Biology Letters 8, 660-664.

Benson RBJ, Carrano MT & Brusatte SL. 2010. A new clade of archaic large-bodied predatory dinosaurs (Theropoda: Allosauroidea) that survived to the latest Mesozoic. Naturwissenschaften 97, 71-78.

Brochu CA. 2003. Osteology of Tyrannosaurus Rex: Insights from a nearly complete Skeleton and High-Resolution Computed Tomographic Analysis of the Skull. Journal of Vertebrate Paleontology 22, Supplement 4, 1-138.

Brusatte SL, Benson RBJ & Xu X. 2010a. The evolution of large-bodied theropod dinosaurs during the Mesozoic in Asia. Journal of Iberian Geology 36, 275-296.

Brusatte SL, Norell MA, Carr TD, Erickson GM, Hutchinson JR, Balanoff AM, Bever GS, Choiniere JN, Makovicky PJ & Xu X. 2010b. Tyrannosaur Paleobiology: New Research on Ancient Exemplar Organisms. Science 329, 1481-1485.

Carr TD. 1999. Craniofacial ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria). Journal of Vertebrate Paleotnology 19, 497-520.

Holtz Jr. TR. 1994 The Phylogenetic Position of the Tyrannosauridae: Implications for Theropod Systematics. Journal of Paleontology 68, 1100-1117.

Horner JR & Dobb E. 1997. Dinosaur Lives: Unearthing an Evolutionary Saga.

Hutchinson JR & Garcia M. 2002. Tyrannosaurus was not a fast runner. Nature 415, 1018-1021.

Longrich NR, Horner JR, Erickson GM & Currie PJ. 2010. Cannibalism in Tyrannosaurus rex. PloS ONE 5, e13419.

Rayfield EJ. 2004. Cranial mechanics and feeding in Tyrannosaurus rex. Proc. R. Soc. B 271, 1451-1459.

Xu X, Clark JM, Forster CA, Norell MA, Rickson GM, Eberth DA, Jia C & Zhao Q. 2006. A basal tyrannosauroid dinosaur from the Late Jurassic of China. Nature 439, 715-718.

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