Elegestolepis and its kin, the earliest monodontode chondrichthyans

Chondrichthyan-like scales with simple, single 20 odontode crowns, reminiscent of those of euselachians, have been reported from Silurian strata in a number of previous studies. These specimens comprise the genera Elegestolepis (from Siberia, Mongolia and Tuva) and Kannathalepis (from the Canadian Arctic), and have been considered to exhibit contrasting patterns of ontogenetic development. A study of elegestolepid microremains from the Chargat Formation of Mongolia (Llandovery–lower Wenlock) and the Baital Formation of Tuva (Wenlock–Ludlow) has been undertaken using SEM and micro-CT to examine scale canal system and hard tissue structure. These investigations revealed scales at different stages of development, whose morphogenesis is characterized by growth (elongation) of the crown odontode and formation of neck canals. This ontogenetic pattern (Elegestolepis-type morphogenesis) is also recognized in Kannathalepis and the Lower Devonian species Ellesmereia schultzei, and forms the basis for the unification of these taxa into a new chondrichthyan Order Elegestolepidida ordo nov. Similarities in crown vascularization (branching pulp, single neck canal) shared by Elegestolepis, Ellesmereia and Deltalepis gen. nov. (D. magna sp. nov. and D. parva sp. nov. erected here in for Mongolian specimens) require the erection of the Family Elegestolepididae fam. nov. that is distinguished from the mono-generic Kannathalepididae (non-branching pulp, multiple neck canals).