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Endocranial structures of Chiroptera (Mammalia) : contribution from fossils

Abstract : The brain is the integrative center of all incoming and outgoing signals of the body. As such, the study of the brain may help to infer the paleobiology of extinct species and to discuss the evolutionary history of eco-functional traits. Bats are outstanding by two extreme traits: active flight and echolocation. Actually, the megabats Pteropodidae are unable to echolocate. In the current phylogenetic framework, this family is sister-taxon to a “microbat” clade. Questions therefore regard the sequence of acquisition of the echolocation (common acquisition for bats and secondary loss in megabats, or convergent acquisitions between “microbat” clades) and of the active flight (flight or echolocation first, or concurrent acquisition of both). Evolution of brain and endocranial characteristics have little been studied in bats. Chiropteran “fossil brains”, accessible as casts of the endocranial cavity (‘endocasts’), have been described during the first half of the XXth century, but with limited techniques; no recent work tackled the question of the evolution of the brain morphology in bats. Recent studies rather focus on the brain mass with extant taxa, proposing a general reduction of the bat brain mass through time. The aim of this thesis is therefore to study the bat endocranial cast using modern techniques, both qualitatively and quantitatively. The study of four fossil endocasts of the family Hipposideridae allows to evaluate the potentialities of the study of the endocranial cast in bats. A first nomenclature of the morphology of the endocast is provided. These comparisons, together with analyses of the encephalization quotient variation through time and of the relative masses of the olfactory bulbs and of the paraflocculi, suggest a general conservatism of brain morphology and size in Hipposideridae. The second part of this thesis aims to generalize the endocast nomenclature to the whole Chiroptera order, the quantitative metrics commonly used in modern paleoneurology are also described and discussed. I propose to use a recently developed phylogenetic comparative method designed to benefit from fossil taxa inclusion. The third part focus on Yinpterochiroptera, dichotomy between a “microbats” Rhinolophoidea, and megabats Pteropodidae. I define discrete anatomical characters of the endocast, and find them moderately congruent with the phylogenetic framework, with potential cases of selection through time. Reconstruction of the evolution of the endocranial shape in the major yinpterochiropteran clades, highlights major differences between rhinolophoids and pteropodids and between rhinolophoid families. Analyses of the evolution of relative neural masses drastically change when fossils are included. A general increase of relative brain size is found, contradicting neontological scenarios; on the other hand, olfactory bulbs and paraflocculi relative masses decrease with time. Previously proposed ecological correlates to these relative neural masses are weakly supported, suggesting more complexity than a direct causality. The fourth part regards the whole Chiroptera order level. I describe the first endocranial cast of a stem bat species; it is a typical bat in its morphology, supporting the hypothesis of a common acquisition of echolocation in bats. Its inclusion to the ordinal picture generally confirms hypotheses proposed in the third part, both regarding morphological and relative neural masses evolution. As a general summary, this work constitutes a first step towards bats endocasts study. The correspondences between the endocast morphology and the disposition of soft tissues in the braincase remain to be assessed, other methodologies can be applied (and compared), and inclusion of other fossil taxa should greatly enhance the very first picture of the evolution of the bat brain proposed here.
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Submitted on : Tuesday, July 12, 2022 - 11:35:13 AM
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Jacob Maugoust. Endocranial structures of Chiroptera (Mammalia) : contribution from fossils. Paleontology. Université Montpellier, 2021. English. ⟨NNT : 2021MONTG084⟩. ⟨tel-03720878⟩

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