New Triassic stem selachimorphs (Chondrichthyes, Elasmobranchii) and their bearing on the evolution of dental enameloid in Neoselachii

ABSTRACT

This study identifies three new neoselachian tooth taxa from the Middle and Upper Triassic. On the basis of morphological and histological characters, Rhomaleodus budurovi, nov. gen. et sp. (Anisian of Bulgaria), is resolved as stem selachimorph, with an additional specimen from the same stratigraphic level assigned to Synechodus sp. Selachimorph grade of enameloid microstructural organization (crystalline bundles) was also identified during examination of the dental tissues of “Polyacrodus” holwellensis (Rhaetian of England), necessitating its transfer to the new genus Duffinselache. The present data reveal several levels of increasing architectural complexity in the arrangement of the enameloid crystalline bundles of stem selachimorphs, which are argued to convey a phylogenetic signal that can be used to establish relationships within the group. It is suggested that the evolution of the hypermineralized enameloid cover of neoselachian teeth progressed from a plesiomorphic single crystalline state, through amalgamation of individual crystals into loosely defined bundles (marking the appearance of Selachimorpha), and their subsequent differentiation into a highly ordered parallel bundles, followed by the development of an inner layer of haphazardly oriented bundles. The superficial shiny-layered enameloid of Neoselachii is interpreted as remnant of a much reduced single crystalline layer.

ACKNOWLEDGMENTS

The authors wish to thank Dr. Lyudmila Petrunova (Geological Institute, Bulgarian Academy of Sciences) for kindly donating the Anisian neoselachian teeth studied in the present work. The initial phase of this scientific investigation was conducted during the “Elasmobrach teeth enameloid microstructure as a taxonomic criterion” training course funded by the European Distributed Institute of Taxonomy.

Handling editor: Charlie Underwood