Theropod dinosaurs had primate‐like numbers of telencephalic neurons.

Understanding the neuronal composition of the brains of dinosaurs and other fossil amniotes would offer fundamental insight into their behavioral and cognitive capabilities, but brain tissue is only rarely fossilized. However, when the bony brain case is preserved, the volume and therefore mass of the brain can be estimated with computer tomography; and if the scaling relationship between brain mass and numbers of neurons for the clade is known, that relationship can be applied to estimate the neuronal composition of the brain. Using a recently published database of numbers of neurons in the telencephalon of extant sauropsids (birds, squamates, and testudines), here I show that the neuronal scaling rules that apply to these animals can be used to infer the numbers of neurons that composed the telencephalon of dinosaur, pterosaur, and other fossil sauropsid species. The key to inferring numbers of telencephalic neurons in these species is first using the relationship between their estimated brain and body mass to determine whether bird‐like (endothermic) or squamate‐like (ectothermic) rules apply to each fossil sauropsid species. This procedure shows that the notion of "mesothermy" in dinosaurs is an artifact due to the mixing of animals with bird‐like and squamate‐like scaling, and indicates that theropods such as Tyrannosaurus and Allosaurus were endotherms with baboon‐ and monkey‐like numbers of telencephalic neurons, respectively, which would make these animals not only giant but also long‐lived and endowed with flexible cognition, and thus even more magnificent predators than previously thought. [ABSTRACT FROM AUTHOR]