How smart was T. rex? Testing claims of exceptional cognition in dinosaurs and the application of neuron count estimates in palaeontological research

Recent years have seen increasing scientific interest in whether neuron counts can act as correlates of diverse biological phenomena. Lately, [Herculano-Houzel (2023)][1] argued that fossil endocasts and comparative neurological data from extant sauropsids allow to reconstruct telencephalic neuron counts in Mesozoic dinosaurs and pterosaurs, which might act as proxies for behaviors and life history traits in these animals. According to this analysis, large theropods such as Tyrannosaurus rex were long-lived, exceptionally intelligent animals equipped with “macaque- or baboon-like cognition” whereas sauropods as well as most ornithischian dinosaurs would have displayed significantly smaller brains and an ectothermic physiology. Besides challenging established views on Mesozoic dinosaur biology, these claims raise questions on whether neuron count estimates could benefit research on fossil animals in general. Here, we address these findings by revisiting Herculano-Houzel’s (2023) work, identifying several crucial shortcomings regarding analysis and interpretation. We present revised estimates of encephalization and telencephalic neuron counts in dinosaurs, which we derive from phylogenetically informed modeling and an amended dataset of endocranial measurements. For large-bodied theropods in particular, we recover significantly lower neuron counts than previously proposed. Furthermore, we review the suitability of neurological variables such as neuron numbers and relative brain size to predict cognitive complexity, metabolic rate and life history traits in dinosaurs, coming to the conclusion that they are flawed proxies of these biological phenomena. Instead of relying on such neurological estimates when reconstructing Mesozoic dinosaur biology, we argue that integrative studies are needed to approach this complex subject. ### Competing Interest Statement The authors have declared no competing interest. * AMNH : American Museum of Natural History, New York City, New York, United States BMNH / NHMUK : Natural History Museum, London, UK BSP : Bayerische Staatssammlung für Paläontologie und historische Geologie, Munich, Germany BYU : Brigham Young University, Earth Science Museum, Provo, Utah, United States CAPPA/UFSM : Centro de Apoio à Pesquisa Paleontológica da Quarta Colônia / Universidade Federal de Santa Maria, São João do Polêsine, Rio Grande do Sul, Brazil. CM : Carnegie Museum of Natural History, Pittsburgh, Pennsylvania CMN : Canadian Museum of Nature, Ottawa, Ontario, Canada DINO : Dinosaur National Monument, Jensen, Utah, United States FIP : Florida Institute of Paleontology, Palm Beach, Florida, United States FMNH : Field Museum of Natural History, Chicago, Illinois, United States FPDM : Fukui Prefectural Dinosaur Museum, Fukui, Japan HMN / MB.R. : Museum für Naturkunde, Berlin, Germany IGM : Mongolian Institute of Geology, Ulaan Bator, Mongolia IRSNB / RBINS : Institut Royal des Sciences Naturelles de Belgique, Brussels, Belgium IVPP : Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, China KUVP : Kansas University Natural History Museum, Lawrence, Kansas, United States MACN : Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Buenos Aires, Argentina MPC-D : Institute of Paleontology and Geology, Mongolian Academy of Sciences, Ulaan Bator, Mongolia MUCPv-CH : Museo de la Universidad Nacional del Comahue, colección del Museo Ernesto Bachmann, Villa El Chocón, Argentina MOR : Museum of the Rockies, Bozeman, Montana, United States NMC : Canadian Museum of Nature, Ottawa, Canada NCSM : North Carolina Museum of Natural Sciences, Raleigh, North Carolina, United States OMNH : Sam Noble Museum at the University of Oklahoma, Norman, Oklahoma, United States PIN : Paleontological Institute, Russian Academy of Sciences, Moscow, Russia PKUP : Peking University Paleontological Collections, Beijing, China ROM : Royal Ontario Museum, Toronto, Ontario, Canada RTMP/TMP : Royal Tyrrell Museum of Palaeontology, Drumheller, Alberta, Canada SGM : Ministere de l’Energie et des Mines, Rabat, Morocco USNM : Smithsonian National Museum of Natural History, Washington, D.C., United States UUVP : University of Utah, Salt Lake City, Utah, United States YPM : Yale Peabody Museum, New Haven Connecticut, United States [1]: #ref-75