DEVELOPMENTAL LINK BETWEEN THE BRAIN AND SKULL SHAPES THE EVOLUTION OF THE REPTILIAN HEAD

In the last decades, exceptionally preserved dinosaur fossils clarified the evolutionary origin of modern birds and the morphological transition leading to the avian condition, including the enlargement of the brain and the transformation of the surrounding skull. While the evolution of the nervous and skeletal systems was intensively studied, their phenotypic integration remains unclear due to biases in the fossil record. To overcome this, development of extant organisms can be used to clarify the relationship between the brain and skull across Archosauria with an evo-devo approach. μCT scan and shape analyses were used to quantify evolutionary changes in these two systems. Euparkeria, a Triassic pivotal taxon closely related to crown Archosauria, shows the first identifiable instance of an “archosaur type” endocast with considerable remaining cerebral flexure. Major innovation of the brain appears in theropods on the line to birds, with an enlargement of the forebrain and cerebellum and lateroventralization of the optic lobes. The elaboration of the brain in theropods led to considerable modification of the surrounding bones, especially in the temporal region. Although the developmental influence of the brain on the surrounding skull has been established, the pattern of formation of cranial elements around this organ remains poorly understood. High resolution cell-level imaging techniques —confocal microscopy and immunofluorescence — was used to show that mesenchymal cells condense early in organogenesis between the regions of the developing brain with a conservative pattern shared among modern reptiles. Morphological innovations characterizing the braincase in different taxa appear in later stages through development. A one-to-one correspondence between the frontal and the parietal on the one hand, and the forebrain and midbrain on the other, is recovered through development. Such relationship is conserved through the evolution of reptiles. Birds show a delayed genesis of the cranial roof in comparison to reptiles: this owes to positive allometry of the brain in birds in contrast to negative allometry in reptiles. Various characters of the skull are therefore non-independent and the evolution of the dorsal dermatocranium is tightly linked to the evolution and development of the brain.