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.
Highlights
The correlation between brain size and behavioural complexity remains ambiguous, brain size, brain dimension compartmentalisation and endoneurocranial space are widely used as measures of behavioural complexity and sensory-locomotive capacity in vertebrates [1]
The signal-to-noise ratio (SNR) was calculated using SNR = 0.655ÁS/σ, where S is signal intensity in the region of interest (ROI), σ is the standard deviation of background noise and constant 0.655 reflects the Rician distribution of background noise in a magnitude MR image
The anterior parts of dinosaur endocasts have been shown to mould the developing braincase walls with the midand hind-brain enveloped by extensive blood and lymphatic sinuses [2]. This might imply that the pattern of EV-BV observed in modern crocodilians of conservative evolution is typical of large archosaurs
Summary
The correlation between brain size and behavioural complexity remains ambiguous, brain size, brain dimension compartmentalisation and endoneurocranial space are widely used as measures of behavioural complexity and sensory-locomotive capacity in vertebrates [1]. In the case of extinct animals, it is difficult to determine these characteristics because the brain is entirely composed of soft tissue and, the chances of fossilisation are extremely rare [2]. The brain proportions and morphology of some extinct. Volume of the crocodilian brain and endocast during ontogeny vertebrates are usually inferred from their endocasts only [3,4]. The whole enlargement of the brain and the relative dimensional increases in its major components reflect the functional importance of the organ [5]. The absence of preserved neural tissues in fossils means that the volume and parts of the brain in extinct animals cannot be calculated accurately enough
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