Volume of the crocodilian brain and endocast during ontogeny

Abstract

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.

Endocranial Casts of Pre-Mammalian Therapsids Reveal an Unexpected Neurological Diversity at the Deep Evolutionary Root of Mammals

“…This is supported by the estimates of brain volume, which suggest that the brain occupies only 32-35% of the volume of the braincase in dinocephalians ( Table 1 ). This small brain volume over braincase volume ratio is comparable to that of the largest modern crocodilians [Hurlburt et al, 2013;Jirak and Janacek, 2017]. In the burnetiamorphs and Herpetoskylax , the cerebral hemispheres on the forebrain and the cerebellum on the hindbrain are more distinct than in Moschops ( Fig.…”

“…These authors used only 50% of the endocranial volume for the most "reptilian-like" endocasts, and up to 90% for the most "mammalian-like" specimens; however, this method strongly relies on one's interpretation of the specimens. In addition, it does not account for the fact that the ratio between brain and endocranial volume varies considerably in modern species, mostly because of absolute brain size [Quiroga, 1980;Hurlburt et al, 2013;Benoit, 2015;Jirak and Janacek, 2017]. Here, we preferred to use published data on the difference between endocranial and brain volume in modern reptiles (i.e., non-avian sauropsids) in order to draw a regression line that was used to convert endocranial volume into brain volume (see online supplementary Data 1; for all online suppl.…”

“…11 ). These very small values are thus better interpreted as secondary reduction, maybe driven by the evolution of a large body size [Hurlburt et al, 2013;Jirak and Janacek, 2017] and the necessity to protect the central nervous system [Benoit et al, 2017b]. Secondary simplification of brain anatomy and reduction of EQs have been documented in several clades, such as lissamphibians, chiropterans, and sirenians [O'Shea and Reep, 1990;Roth et al, 1997;Nieuwenhuys et al 1998;Safi et al, 2005;Benoit et al, 2013].…”

Sensory Evolution and Ecology of Early Turtles Revealed by Digital Endocranial Reconstructions

“…Outlines were smoothed ten times to eliminate pixel noise, and 23 Fourier harmonics were found to describe the outlines of all sampled taxa sufficiently (average Fourier power > 99%) (see also Supplementary Material). In addition to the reconstructed endocasts, further outlines of 52 taxa were collected from the literature (Hopson, 1979;Franzosa, 2004;Neenan and Scheyer, 2012;Bona and Paulina-Carabajal, 2013;Carabajal et al, 2013;George and Holliday, 2013;Herrera et al, 2013;Holloway et al, 2013;Lautenschlager and Butler, 2016;von Baczko and Desojo, 2016;Jirak and Janacek, 2017;Laaß et al, 2017;Paulina-Carabajal et al, 2017;Pierce et al, 2017; Digimorph 1 ) for different turtle, archosauromorph, lepidosauromorph and other amniote taxa (for list of taxa see Table S1). These outlines were redrawn in Adobe Illustrator to ensure sufficient resolution for the digitization process.…”

Endocranial anatomy and life habits of the Early Triassic archosauriformProterosuchus fergusi

“…The endocranial reconstructions of P. fergusi are generally linear in shape and organization (Fig. ), similar to that of modern‐day adult crocodylians (Jirak & Janacek ). The brain cavity of P. fergusi is straighter than many archosauriforms including erythrosuchids (Gower & Sennikov ), phytosaurs (Holloway et al .…”