Abstract
AbstractPlacoderms (Devonian fossil fishes) are resolved phylogenetically to the base of jawed vertebrates and provide important evidence for evolutionary origins of teeth, particularly with respect to the Arthrodira. The arthrodires represent a derived group of placoderms; the dentition of other more primitive placoderms such as the acanthothoracids is less well known. Articulated acanthothoracid dental plates are rare; x‐ray computed tomography of a single, unique specimen, along with 3D segmentation of bone, oral denticles and vascular spaces, provides intrinsic developmental and topological information relevant to tooth origins. Recently, a disarticulated element was identified as a dental plate of the acanthothoracid Romundina stellina, with synchrotron microtomography providing characters to comment on ongoing debates regarding the evolution of teeth. We used segmental quantitative methods to re‐analyse this data, for comparison to the articulated and unquestionable acanthothoracid dental plates above. We demonstrate substantial differences between these, disputing the identity of the isolated plate of R. stellina as a dental plate, and thus its relevance to questions of tooth evolution.
Highlights
CURRENT jawed vertebrate phylogenies resolve the fossil group ‘Placodermi’ as paraphyletic, with the acanthothoracids as one of the more basal groups among the gnathostomes (Giles et al 2015; Qiao et al 2016; but see King et al 2017) and relevant to the evolution of gnathostome characters, for example, teeth and dentitions
We conclude that the R. stellina specimen is not an oral dental plate but belongs to part of the dermal skeleton (Burrow et al 2016), and that a flat anterior supragnathals (ASGs) aboral surface is characteristic of the acanthothoracids, very different to the arthrodire condition
In CPW.9A, the dental plate has a flatter aboral attachment surface to the perichondral bone of the headshield, compared to the convex basal surface of R. stellina. This flat aboral surface may be typical for acanthothoracid dental plates, and is very different from the strongly concave aboral surface of the early Devonian arthrodiran tooth plate utilized by Burrow et al (2016) in their criticism of the R. stellina plate as an oral plate
Summary
CURRENT jawed vertebrate phylogenies resolve the fossil group ‘Placodermi’ as paraphyletic, with the acanthothoracids as one of the more basal groups among the gnathostomes (Giles et al 2015; Qiao et al 2016; but see King et al 2017) and relevant to the evolution of gnathostome characters, for example, teeth and dentitions. Ru€cklin & Donoghue (2015a) recently compared the published figures of the ASGs of the articulated acanthothoracid (Smith & Johanson 2003a; Goujet & Young 2004) to disarticulated bony plates from a rock residue sample originally associated with the acanthothoracid species Romundina stellina (Ørvig, 1975), identifying among these a putative upper gnathal plate This identification was based on similarity in shape to the articulated. Ru€cklin & Donoghue’s (2015a, 2016) interpretation of the disarticulated R. stellina plate does not inform on the interrelationship of dermal and oral skeletons and their conclusions regarding the evolutionary link between teeth and the dermal ornament (non-independence of these) as based on this plate, can no longer be supported
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
