Locomotor and postural diversity of amniotes : contributions from bone microanatomy and implications for the major postural transitions of the Mesozoic

Extant amniotes show remarkable locomotor and postural diversity. Mammals are mainly quadrupedal, but show a variety of limb postures. Within Reptilia, birds are bipedal with parasagittally locomoting, more or less crouched limbs. Most lepidosaurs are quadrupedal with a sprawling posture, but some are able to run on their hind limbs for short distances. Crocodylians are quadrupedal with a so-called “semi-erect” posture. This locomotor and postural diversity is associated with a marked microanatomical disparity. This is because the bones of the appendicular skeleton support the weight of the body, subjecting them to various stresses that partly shape their external and internal morphology. In this thesis, I present analyses demonstrating that some geometric and compactness parameters measured on amniote humeri and femora, especially in two dimensions, appear to be related to locomotion and posture. Using multivariate statistical methods that account for evolutionary relatedness between taxa (generalised least squares, phylogenetic flexible discriminant analysis, Fourier analysis, ancestral state reconstruction), I develop inference models to assess locomotion and posture in many extinct taxa documenting major episodes of locomotor and postural transitions during the Mesozoic. This work contributes to unravelling the complex interaction between phylogeny, body mass, lifestyle, microanatomy, locomotion and posture in amniotes.