A Review of Tooth Implantation Among Rhynchocephalians (Lepidosauria)
Acrodont dental implantation is widely considered an important character for referring fossil material to Rhynchocephalia. Under its purest definition, acrodonty involves teeth being attached to the crest of the marginal bones without roots. A similar mode of tooth attachment is known in a variety of other reptile groups including some squamates and procolophonids. There is a lack of consensus on the definition of acrodont, how best to characterize tooth implantation, and the relationship between implantation and tooth replacement. Rhynchocephalians already are known to demonstrate variation in their mode of tooth attachment. Unambiguous acrodonty associated with little or no tooth replacement has been associated with Sphenodon, but it appears to have been the most widespread condition for much of the Mesozoic. A form of pleurodonty, where teeth are attached to the inside of the jaw bone with shallow roots, appears to be the plesiomorphic condition for both Lepidosauria and Rhynchocephalia. Jaws with anterior pleurodont teeth and posterior acrodont teeth appear to have been common for early rhynchocephalians in the Triassic, and Ankylosphenodon from the early Cretaceous of Mexico demonstrates that at least some later rhynchocephalians possessed continually replacing dentition, but identification of this trait requires inspection of internal anatomy. When cross-sections of teeth are unavailable or the lingual view of jaws is obscured, one cannot be 100% confident of acrodont implantation, and “acrodonty” should not be used as a single character to refer incomplete jaw material to Rhynchocephalia. Tooth implantation is a component that was highly variable in a once-diverse reptile group. La implantación dental acrodonta, es considerada como una característica importante para asignar material fósil al grupo Rhynchocephalia. La acrodoncia propiamente dicha, se refiere a la implantación de los dientes sin raíz, en la cresta de los huesos marginales. Este modo de implantación es conocido en otros grupos de reptiles, como escamados y procolofónidos. No existe un consenso en la definición del término acrodonte, además falta información sobre los rasgos que caracterizan este modo de implantación y su relación con los dientes de reemplazo. Los rincocéfalos exhiben una diversidad de modos de implantación de los dientes. Acrodoncia total, donde hay poco o ningún reemplazo de dientes, se ha definido en Sphenodon, pero también ha sido la condición predominante en el Mesozoico. Una forma de pleurodoncia, donde los dientes se unen a la parte interna de los huesos marginales, es la condición plesiomórfica en Lepidosauria y Rhynchocephalia. En algunos rincocéfalos del Triásico se desarrolla la combinación de dientes pleurodontes anteriores y acrodontes posteriores en la mandíbula. Ankylosphenodon del Cretácico temprano de Méjico, muestra remplazo continuo de dientes, aunque la identificación de esta condición requiere que se revise la anatomía interna. En casos donde no hay disponibilidad de cortes transversales de los dientes, o cuando la vista lingual de las mandíbulas está cubierta, no es posible estar seguro de la implantación acrodonte. También, la condición acrodonte no debe ser el único caracter para referir material incompleto de mandíbulas al grupo Rhynchocephalia. La implantación de los dientes es un componente de este grupo de reptiles que fue diverso en el pasado.Abstract
Resumen
Cross sections of partial dentary bones of rhynchocephalians. (A) Lingual side of right dentary of Gephyrosaurus (SEE1); (B) lingual side of left dentary of Gephyrosaurus (SEE2); (C) lingual side of Planocephalosaurus (SEE3); (D) lingual side of left dentary of Clevosaurus (SEE4). Scale bar equals 2 mm. Tomographic measurement of specimens were conducted using microCT station Scanco μCT 40 (Scanco Medical AG, Bassersdorf, Switzerland). The samples were fixed into a transparent polyacrylic cylindrical sample holder of 10.2 mm diameter. The X-ray tube was set at acceleration voltage of 70 kV and current of 160 μA. At an integration time of 3 s, 1000 projections were taken over 180°. The tomographic reconstruction including beam hardening correction was performed by Scanco software. CT data set was obtained at a linear isotropic voxel size of 6 μm. The 3D visualizations of the CT data were generated with the use of Drishti v2.6.3 (Australian National University, ANUSF VizLab), and cross sections were generated with the use of Avizo 8.0.1 (VSG Inc., Burlington, Massachusetts). Specimen acronym SEE represents Susan E. Evans Collection.
Supertree representing the compatible nodes of two recent Rhynchocephalia trees (Jones et al., 2013; Apesteguia and Carballido, 2014). The broad picture shows basal branches with a combination of pleurodont and acrodont dentition, and most derived branches with just acrodont dentition. Two taxa display two additional changes, ankylothecodonty (Ankylosphenodon) and toothless (Sapheosaurus).
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