Understanding complex situations and planning difficult actions require a brain of appropriate size. Animal encephalisation provides an indirect information about these abilities. The brain is entirely composed of soft tissue and, as such, rarely fossilises. As a consequence, the brain proportions and morphology of some extinct vertebrates are usually only inferred from their neurocranial endocasts. However, because the morphological configuration of the brain is not fully reflected in the endocast, knowledge of the brain/endocast relationship is essential (especially the ratio of brain volume to endocast volume or the equivalent proportion of interstitial tissue) for studying the endocasts of extinct animals. Here we assess the encephalic volume and structure of modern crocodilians. The results we obtained using ex vivo magnetic resonance imaging reveal how the endoneurocranial cavity and brain compartments of crocodilians change configuration during ontogeny. We conclude that the endocasts of adult crocodilians are elongated and expanded while their brains are more linearly organised. The highest proportion of brain tissue to endocast volume is in the prosencephalon at over 50% in all but the largest animals, whereas the proportion in other brain segments is under 50% in all but the smallest animals and embryos. Our results may enrich the field of palaeontological study by offering more precise phylogenetic interpretations of the neuroanatomic characteristics of extinct vertebrates at various ontogenetic stages.

Department of Biomathematics Institute of Physiology Czech Academy of Sciences Prague Czech Republic

Institute of Biophysics and Informatics 1st Medicine Faculty Charles University Prague Czech Republic

MR Unit Department of Diagnostic and Interventional Radiology Institute for Clinical and Experimental Medicine Prague Czech Republic

Zobrazit více v PubMed

Healy SD, Rowe C. A critique of comparative studies of brain size. Proc R Soc Lond B Biol Sci. 2007; 274: 453–464. PubMed PMC

Brasier MD, Norman DB, Liu AG, Cotton LJ, Hiscocks JEH, Garwood RJ, et al. Remarkable preservation of brain tissues in an early Cretaceous iguanodontian dinosaur. Geological Society, Special Publications. 2016; doi: 10.1144/SP448.3 DOI

Hopson JA. Paleoneurology In: Biology of the Reptilia, New York: Academic Press, 1979.

Rogers SW. Allosaurus, crocodiles, and birds: evolutionary clues from spiral computed tomography of an endocast. Anat Rec. 1999; 257: 162–173. PubMed

Mace GM, Harvey PH, Clutton-Brock TH. Brain size and ecology in small mammals. J Zool. 1981; 193: 333–354. PubMed PMC

Nesbitt SJ. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History. 2011; 352: 1–292. doi: 10.1206/352.1 DOI

Ferguson MWJ. Reproductive biology and embryology of the crocodilians In: Gans C, Billett F, Maderson PFA, editors. Biology of the reptilia, vol 14, development A. New York: John Wiley & Sons; 1985; pp 329–491.

Jirak D, Janacek J, Kear BP. A combined MR and CT study for precise quantitative analysis of the avian brain. Sci Rep. 2015; 4: 16002 doi: 10.1038/srep16002 PubMed DOI PMC

Jerison HJ. Evolution of the Brain and Intelligence. New York: Academic Press, 1973.

Larsson HCE, Sereno PC, Wilson JA. Forebrain enlargement among nonavian theropod dinosaurs. J Vert Paleontol. 2000; 20: 615–618.

Corfield JR, Wild JM, Cowan BR, Parsons S, Kubke MF. MRI of postmortem specimens of endangered species for comparative brain anatomy. Nature Protoc. 2008; 3: 597–605. PubMed

De Groof G, George I, Touj S, Stacho M, Jonckers E, Cousillas H, et al. A three-dimensional digital atlas of the starling brain. Brain Struct Funct. 2016; 221: 1899–1909. doi: 10.1007/s00429-015-1011-1 PubMed DOI

Kotrotsou A, Bennett DA, Schneider JA, Dawe RJ, Golak T, Leurgans SE, et al. Ex vivo MR volumetry of human brain hemispheres. Magn Reson Med. 2014; 71: 364–374. doi: 10.1002/mrm.24661 PubMed DOI PMC

Hurlburt GR, Ridgely RC, Witmer LM. Relative size of brain and cerebrum in tyrannosaurid dinosaurs: an analysis using brain-endocast quantitative relationships in extant alligators In: Parrish JM, Henderson M, Currie PJ, Koppelhus E, editors. Origin, Systematics, and Paleobiology of the Tyrannosauridae. Northern Illinois University Press; 2013; pp.134.

Kawabe S, Matsuda S, Tsunekawa N, Endo H. Ontogenetic Shape Change in the Chicken Brain: Implications for Paleontology. PLoS ONE. 2015; 10(6): e0129939 doi: 10.1371/journal.pone.0129939 PubMed DOI PMC

Bever GS, Brusatte SL, Balanoff AM, Norell MA. Variation, variability, and the origin of the avian endocranium: insights from the anatomy of Alioramus altai (Theropoda: Tyrannosauroidea). PLoS ONE. 2011; 6(8): e23393 doi: 10.1371/journal.pone.0023393 PubMed DOI PMC

Bever GS, Brusatte SL, Carr TD, Xu X, Balanoff AM, Norell MA, et al. The braincase anatomy of the Late Cretaceous dinosaur Alioramus (Theropoda: Tyrannosauroidea). Bull Am Mus Nat Hist. 2013; 376: 1–72.

Brusatte SL, Carr TD, Erickson GM, Bever GS, Norell MA. A long-snouted, multihorned tyrannosaurid from the Late Cretaceous of Mongolia. Proc Natl Acad Sci USA. 2009; 106: 17261–17266. doi: 10.1073/pnas.0906911106 PubMed DOI PMC

Erickson GM, Makovicky PJ, Currie PJ, Norell MA, Yerby SA, Brochu CA. Gigantism and comparative life-history parameters of tyrannosaurid dinosaurs. Nature. 2004; 430: 772–775. doi: 10.1038/nature02699 PubMed DOI

Xu X, Norell MA, Kuang X, Wang X, Zhao Q, Jia C. Basal tyrannosauroids from China and evidence for protofeathers in tyrannosauroids. Nature. 2004; 431: 680–684. doi: 10.1038/nature02855 PubMed DOI

The evolution of brain neuron numbers in amniotes

Iron-doped calcium phytate nanoparticles as a bio-responsive contrast agent in 1H/31P magnetic resonance imaging

Multiphase progenetic development shaped the brain of flying archosaurs

Synchrotron microtomography of a Nothosaurus marchicus skull informs on nothosaurian physiology and neurosensory adaptations in early Sauropterygia