The skeleton adapts to the functional constraints undergone by organisms. Consequently, although bones are also the result of other constraints (historical, developmental, structural), they carry a strong functional signal. Understanding this form-function relationship is important for understanding the evolutionary history of organisms. To analyze and characterize the relationships between the shape of skeletal elements and the functional constraints they have to cope with, the main case studies are convergences. They make it possible to distinguish common adaptive traits, whatever the lineage, which reflect 'obligatory' acquisitions to respond to a specific function, but also various specific adaptations, which highlight how, with distinct heritages, species have succeeded differently in adapting to a particular function.
The convergent adaptation that GRAVIBONE has focused on is the adaptation to support and locomote a heavy weight (more than a tonne), a pattern that has evolved in numerous lineages throughout amniote evolutionary history. The term "graviportality" was introduced to characterize these heavy-bodied animals; these forms require specialized limbs (proportions, bone shape, arrangement between the bones) adapted to support their weight, since body mass (and therefore weight) is multiplied by eight when size is multiplied by two. The adaptive changes associated with graviportality had been little studied, and the definition of graviportality itself was imprecise. While only a few modern forms are well over a tonne (elephants, rhinoceroses and hippopotamuses), the fossil record is much richer in massive taxa that are very diverse, so that convergences can be clearly analyzed.
The ERC-StG GRAVIBONE project thus focused on the adaptation of the long bones of the limbs, at the anatomical and microanatomical levels, to the biomechanical constraints associated with graviportality, and on the way in which the external and internal structures of the bone co-evolve. The aim was to gain a precise understanding of the changes in the structure of the bone as a whole in response to the mechanical constraints imposed by a heavy skeleton in modern and fossil animals with different morphologies and locomotor behaviors, but also to combine anatomical and microanatomical analyses on whole bones with biomechanical modelling in order to characterize in detail form-function relationships to be able to characterize various modes of adaptation to graviportality. The ultimate aim was to model the relationship between bone anatomy, microanatomy and the functional requirements for body support and locomotion in graviportal amniotes, and thus to better understand how bone responds to biomechanical constraints.
