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| Irritator challengeri Martill, Cruickshank, Frey, Small & Clarke, 1996 in Schade, Rauhut, Foth, Moleman et Evers, 2023. DOI: 10.26879/1242 |
ABSTRACT
Although originally described almost three decades ago, the holotype of Irritator challengeri from the Lower Cretaceous Romualdo Formation of Brazil still represents the most complete spinosaurid skull known to science. Here, we present a detailed description of the skull of Irritator based on digital reconstructions from medical and micro computed tomography (µCT) data. Segmentation reveals the near-complete palatal complex and braincase, an unusual morphology of the retroarticular process, a large, ventrally inclined surangular shelf and the tooth replacement pattern. The digitally reconstructed skull anatomy indicates a robust dentition, a field of binocular vision in front of the skull with an inclined snout orientation, a relatively weak but fast bite, as well as laterally spreading and rotating lower jaw rami during jaw opening. We modified an existing phylogenetic matrix of Tetanurae to account for new observations on the morphology of Irritator and analysed this using parsimony and Bayesian methods. Results support Spinosauridae as members of Megalosauroidea and recover a monophyletic Carnosauria (Megalosauroidea + Allosauroidea). Parsimony analysis recovers Monolophosaurus nested within Megalosauroidea as sister taxon to spinosaurids, but this is not supported by the Bayesian analysis. Bayesian time-calibration and evolutionary rate analysis indicate that spinosaurid evolution happened fast, despite a long ghost lineage of at least 35 million years. High evolutionary rates over a prolonged time can explain the highly derived skull morphology of spinosaurids. This study provides an in-depth look into the evolution of spinosaurid skull anatomy and refines our understanding of these specialized Mesozoic predators.
Keywords: Dinosaur; Theropod; Spinosaurid; Cretaceous; Mesozoic; Brazil
Theropoda Marsh, 1881
Tetanurae Gauthier, 1986
Megalosauroidea Fitzinger, 1843; sensu Carrano et al. (2012)
Spinosauridae Stromer, 1915
Spinosaurinae (Stromer, 1915); sensu Sereno et al. (1998)
Irritator challengeri Martill, Cruickshank, Frey, Small and Clarke, 1996
Holotype. SMNS 58022, largely complete skull, missing most of the premaxillae, anterior ends of the maxillae, and anteriormost parts of both mandibles.
Locality and horizon. Near Buxexé, close to Santana do Cariri, Ceará State, northeastern Brazil (see Sues et al., 2002: 535). Lower part of the Romualdo Formation (Santana Formation of some authors; see discussion in Arai and Assine, 2020) of the Santana Group, late Aptian (Arai and Assine, 2020).
Marco Schade, Oliver W. M. Rauhut, Christian Foth, Olof Moleman, and Serjoscha W. Evers. 2023. A Reappraisal of the Cranial and Mandibular Osteology of the spinosaurid Irritator challengeri (Dinosauria: Theropoda). Palaeontologia Electronica. 26(2): a17.DOI: 10.26879/1242
palaeo-electronica.org/content/2023/3821-the-osteology-of-irritator
