title: Jaws for a spiral-tooth whorl: CT images reveal novel adaptation and phylogeny in fossil Helicoprion.
abstract: New CT scans of the spiral-tooth fossil, Helicoprion, resolve a longstanding mystery concerning the form and phylogeny of this ancient cartilaginous fish. We present the first three-dimensional images that show the tooth whorl occupying the entire mandibular arch, and which is supported along the midline of the lower jaw. Several characters of the upper jaw show that it articulated with the neurocranium in two places and that the hyomandibula was not part of the jaw suspension. These features identify Helicoprion as a member of the stem holocephalan group Euchondrocephali. Our reconstruction illustrates novel adaptations, such as lateral cartilage to buttress the tooth whorl, which accommodated the unusual trait of continuous addition and retention of teeth in a predatory chondrichthyan. Helicoprion exemplifies the climax of stem holocephalan diversification and body size in Late Palaeozoic seas, a role dominated today by sharks and rays.
authors: Department of Geosciences, Idaho State University, Pocatello, ID 83209, USA. tapaleif@isu.edu
authors: Leif
authors: L
authors: Tapanila
authors: Jesse
authors: J
authors: Pruitt
authors: Alan
authors: A
authors: Pradel
authors: Cheryl D
authors: CD
authors: Wilga
authors: Jason B
authors: JB
authors: Ramsay
authors: Robert
authors: R
authors: Schlader
authors: Dominique A
authors: DA
authors: Didier
mesh_terms: D000222:Adaptation, Physiological; D000818:Animals; D005075:Biological Evolution; D049628:Body Size; D005399:Fishes / Q000033:anatomy & histology* / Q000145:classification / Q000502:physiology; D005580:Fossils; D021621:Imaging, Three-Dimensional; D007568:Jaw / Q000033:anatomy & histology* / Q000502:physiology; D008334:Mandible / Q000033:anatomy & histology / Q000502:physiology; D010802:Phylogeny; D013045:Species Specificity; D014057:Tomography, X-Ray Computed / Q000379:methods; D014070:Tooth / Q000033:anatomy & histology* / Q000502:physiology