A Lower Cretaceous chondrichthyan dermal denticle assemblage and its bearing on placoid scale diversity and histology

Abstract

Dermal denticles of Mesozoic chondrichthyans are rarely documented, which tremendously limits the knowledge of different morphotypes during this time and for the whole clade. Here we describe an extraordinarily diverse assemblage of dermal denticles from deposits of Austria, comprising 17 different morphotypes. Based on the functional types of the denticles and their morphological similarities with both extinct and extant representatives, we attempt an order-level taxonomic attribution of the morphotypes recovered. The herein recorded occurrences of Hexanchiformes, Squaliformes, Squatiniformes, Heterodontiformes, the family Protospinacidae (Squalomorphii incertae sedis), and probably Rajiformes or Chimaeriformes increase previous tooth-based taxic diversity estimates of Valanginian cartilaginous fishes from Austria from four to at least nine orders. Additionally, we discuss the absence of denticles of cladodontomorph chondrichthyans, whose teeth were previously reported from the same deposits. Etched sections and light microscope analysis were carried out using the most abundant morphotypes with the aim of exploring the histology of chondrichthyan dermal denticles. Our results confirm previous studies in reporting a single crystallite enameloid, but we also report for the first time the presence of bundled enameloid in dermal scales, a condition so far restricted to the tooth enameloid of some chondrichthyans. Additionally, a putative new dentine type is observed, leading to the introduction of the descriptive term ‘nepedentine’ for the internal, dense, and poorly supplied dentine core by vascular channels.