Document Type

Book Chapter

Publication Date

Winter 2013


Alligators, cerebrum size. dinosaurs, endocasts, relative brain size, Tyrannosauridae.


Brain and cerebrum mass are estimated from endocasts of three tyrannosaurid taxa (Tyrannosaurus rex, Gorgosaurus, and Nanotyrannus) using morphological and quantitative brain-endocast relations in a size series of sexually mature alligators (Alligator mississippiensis). The alligator size series (N = 12) ranged from the smallest sexually mature size to the largest size commonly encountered. Alligator brain mass (MBr) increased regularly with increasing body mass, while the ratio of brain mass to endocast volume (MBr:EV) declined regularly from 67 percent to 32 percent. The ratio of cerebrum mass to cerebrocast was 38 percent in the largest alligators and regularly exceeded the MBr:EV ratio by 5.6 percent. For estimates from endocasts of non-avian dinosaurs of unknown sex, a MBr:EV ratio of 37 percent was used, the mean of the ratio of the largest male and female alligators. A corresponding 42 percent ratio was used for the cerebrum-cerebrocast ratio.

Relative brain size was measured as Encephalization Quotients (EQs) based on brain-body relations in extant non-avian reptiles (REQs) and birds (BEQs). Tyrannosaurus rex has the relatively largest brain of all adult non-avian dinosaurs, excepting certain small maniraptoriforms (Troodon, Bambiraptor, and Ornithomimus), which are well within the extant bird relative brain size range. The relative brain size of T. rex is within the range of extant non-avian reptiles and, at most, 2 standard deviations (SDs) above the mean of non-avian reptile log REQs, which are normally distributed. Gorgosaurus REQs overlapped the lower end of the T. rex. Log BEQs of all theropods, excepting small maniraptoriforms, were well below the range of extant birds. Nanotyrannus log REQs were anomalously high for an adult, but the difference between Nanotyrannus log REQs and T. rex values paralleled the difference between log REQs of the smallest subadult and largest alligators. Nanotyrannus cerebrum:brain ratios were also consistent with those of an older juvenile or youngest subadult. Cerebrocast:endocast ratios of the three T. rex endocasts ranged from 41.1 to 43.5 percent, and cerebrum mass:brain mass (MCb:MBr) ratios range from 47.5 to 49.53 percent, more than the lowest ratios for extant birds (44.6 percent) but very close to ratios (45.9–47.9 percent) typical of the smallest sexually mature alligators. In

Carcharodontosaurus saharicus, these ratios were 37.1 percent and 42.1 percent, respectively, the latter essentially identical to actual MCb:MBr ratios (40.76–42.91 percent) of the two largest alligators. Although the relative brain size of Carcharodontosaurus (SGM-Din 1), was approximately two thirds that of T. rex, the MCb:MBr ratio of the former was only 5.5–7.5 percent less than that of T. rex.


Faculty of Humanities & Social Sciences (FHASS)

First Page


Last Page



Publisher's version

Terms of Use

Terms of Use for Works posted in SOURCE.

Creative Commons License

Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License.

Original Publication Citation

Hurlburt, G. R., Ridgely, R. C. and Witmer, L. M. (2013). Relative size of brain and cerebrum in tyrannosaurus rex: An analysis using brain-endocast quantitative relationships in extant alligators. in J. M. Parrish, M. Henderson, P. J. Currie, and E. Koppelhus (Eds.), Origin, Systematics, and Paleobiology of the Tyrannosauridae (pp. 134-154.) Northern Indiana University Press.