Variability in the Evolution of Ornithischian Cranial Morphology

Abstract

Phylogenetic relationships within Dinosauria have been recently called into question, including shifts among major taxonomic groups such as Ornithischia. As phylogenetic hypotheses change, so do hypotheses of morphological character evolution. Ornithischian dinosaurs exhibit a wide variety of cranial morphologies. They range from generalized ancestral morphologies, to derived and varied skull shapes with frills, thickened domes, or complex dental batteries associated with complex jaw joints. Here, I describe the evolution of cranial morphology within Ornithischia using an updated phylogenetic analysis and ancestral character state reconstruction methods to map changes in cranial characters through time. With an updated phylogenetic analysis of Ornithischia, I used both parsimony and maximum likelihood methods for ancestral state reconstructions to determine the changes in cranial characters. The ancestral reconstructions also allowed for a broader view of cranial morphological change through time. These estimates demonstrated that basal ornithischians, including those at the base of major clades (e.g. Thyreophora, Ornithopoda), maintained skull morphologies not unlike other basal dinosaurs outside of Ornithischia. The thyreophoran and neornithischian skulls show similar changes in cranial character evolution: relative reduction of the quadratojugal, general lengthening of the preorbital skull, including modifications to the premaxilla and nasal, an initial reduction in the size, placement, and shape of the antorbital fenestra, and incorporation of the palpebral in the supraorbital margin. While slight changes in the postorbital skull of thyreophorans were noted, the morphology within Neornithischia was markedly different. In addition to well‐known modifications of the parietal and squamosal in marginocephalians, other noted changes include movement of cranial foramina in the occipital and basal regions of the skull. This is in contrast to most ornithopods, whose skull bases and occipital regions vary relatively little from basal neornithischians. Derived ornithopods (e.g., hadrosaurs) demonstrate elaborate preorbital modifications, particularly involving the premaxilla and nasal. The dentition and mandible demonstrate modifications throughout the ornithischian tree. However, dramatic changes are noted among derived ornithopods with the development of massive dental batteries and laterally offset, handle‐shaped coronoid processes. This occurs to a lesser degree in ceratopsians as well. These changes in cranial morphology, while present in all ornithischian clades, varies in terms of rates of change. Within Thyreophora, a group that persisted for nearly 150 million years, the general changes within the skull and mandible are relatively small when compared to ornithopods. Ceratopsians and ornithopods demonstrate the most dramatic changes in overall cranial morphology, but ornithopods show the most remarkable alterations in mandibular anatomy through time. With these results, the analysis presented here may be used to develop new hypotheses for the selective pressures behind the varied evolution of the ornithischian skull.Support or Funding InformationUniversity of Iowa Max and Lorraine Littlefield Fund and T. Anne Cleary International Dissertation Research FellowshipThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.