Synchrotron-aided reconstruction of the prioniodinin multielement conodont apparatus (Hadrodontina) from the Lower Triassic of China

Abstract Although natural assemblages that preserve natural associations of elements in conodont apparatus are extremely rare, they are of fundamental importance in conodont paleobiology, e.g. apparatus architecture, functional morphology, suprageneric classification and evolution. We herein report three exceptionally preserved natural assemblages from the uppermost Helongshan Formation of South Majiashan section, Chaohu City of Anhui Province, China. They were scanned using synchrotron X-ray microtomography and the three-dimensional apparatus of the end-Smithian (Olenekian, Early Triassic) Hadrodontina aequabilis was reconstructed. Our result confirms that the prioniodinin Hadrodontina has a septimembrate apparatus composed of 15 elements (angulate P1 and P2, breviform digyrate M, triramous S0, extensiform digyrate S1 and S2, and bipennate S3/4 elements). Within prioniodinin apparatuses that were reconstructed based on conodont natural assemblages, Hadrodontina shares more features with those of Hibbardella (Late Devonian) and Ellisonia (end-Permian), but differs in its angulate P 2 element having no distinct cusp and twisting distally, and its extensiform digyrate S1 having an inwardly bending short anterior lateral process. Element positions of the Hadrodontina apparatus near to closure are revealed, where at the front lies an axial S0 element, and in rostro-caudal order the paired M elements are located between S0 and S1–4 elements and then the S1–4 elements are successively abaxially flanked the posterior process of the S0 element. Geometric analysis demonstrated that the relative motions of the elements in Hadrodontina apparatus most probably conform to those of “single pulley hypothesis” previously proposed, but mainly differs in its extensiform digyrate S1 and S2 elements having reverse motions of subhelicoidal trajectories to channel food toward the pharynx. Thus, the Hadrodontina apparatus is among the most completely characterized conodonts and serves as a template for prioniodinin multielement reconstruction. This study provides significant information on dynamic reconstruction and the phylogeny of the conodont apparatuses within the Prioniodinina.