Geometric morphometrics in macroevolution: morphological diversity of the skull in modern avian forms in contrast to some theropod dinosaurs

Abstract

Birds are the most diversified forms of modern terrestrial vertebrates. They are archosaurs, and are related to Theropoda within Dinosauria. Even though bird morphology has been a subject of interest for centuries, most studies have focused solely on discrete morphological characters, leaving a gap in the understanding of morphological organization and integration in macroevolution. We present this chapter to exemplify a quantitative exploration of the macroevolutionary trends of skull morphological diversity in theropod dinosaurs including modern birds. Using a sample with taxa representative of all of known modern avian Orders, skull disparity is described over a morphospace modeled from shape variables obtained by General Procrustes methods. High taxonomical categories imply large-scale morphological difference, thus landmarks were selected according to evolutionary homologous and developmental conservative areas. Morphological diversity of the skull relies mainly on craniofacial variation, and can be found within the first four dimension of a PCA. Modern avian forms have a strikingly localized, but not morphologically independent, variation at the rostrum. Craniofacial variation unfolds in a range of structurally straight-flexed appearance dependant on the covariation between the rostrum and braincase, structurally mediated by the antorbital cavity. Extinct theropods occupy a region within morphospace resembling an extreme straight skull. Major macroevolutionary changes are likely associated to expansion of the braincase.