SCALE-BASED PHYLOGENY OF PALEOZOIC CHONDRICHTHYANS

Discoveries of scale remains of putative chondrichthyans from the Ordovician–Silurian have been interpreted as evidence for an early diversification of the Chondrichthyes. Nevertheless, the affinities of these early taxa to high-ranked clades still remain equivocal, despite recent resurgence of phylogenetic studies on primitive gnathostomes. This study reexamines all previously recorded scale types attributed to Paleozoic chondrichthyans and places them within a revised phylogenetic analysis of jawed gnathostomes. The chondrichthyan dermal skeleton is identified to exhibit loss of certain crown-gnathostome plesiomorphies (hard-tissue resorption, enamel and osteon formation) that differentiate it from those of ‘placoderms’ and osteichthyans. Four categories of chondrichthyan scale crown architecture (monodontode, polyodontode non-odontocomplex, monodontocomplex and polyodontocomplex) were documented and these are inferred to form via 13 distinct types of developmental patterns (e.g. the Elegestolepis-, Eugeneodus-, Seretolepis- and Mongolepis- types).

This scale-based phylogeny recovers a monophyletic Chondrichthyes that incorporates all studied putative basal chondrichthyans (e.g. mongolepids and elegestolepids) and ‘acanthodians’ with, both, superpositional and non-superpositional crown growth. In agreement with the scale data, chondrichthyans are resolved as a derived gnathostome clade. This corroborates and builds upon recent suggestions that the micromeric condition of the dermal skeleton shared by traditionally recognized chondrichthyans and ‘acanthodians’ is a synapomorphy of the chondrichthyan total group within jawed gnathostomes.