Origin and evolution of the integumentary skeleton in non‐tetrapod vertebrates

Abstract

Most non-tetrapod vertebrates develop mineralized extra-oral elements within the integument. Known collectively as the integumentary skeleton, these elements represent the structurally diverse skin-bound contribution to the dermal skeleton. In this review we begin by summarizing what is known about the histological diversity of the four main groups of integumentary skeletal tissues: hypermineralized (capping) tissues; dentine; plywood-like tissues; and bone. For most modern taxa, the integumentary skeleton has undergone widespread reduction and modification often rendering the homology and relationships of these elements confused and uncertain. Fundamentally, however, all integumentary skeletal elements are derived (alone or in combination) from only two types of cell condensations: odontogenic and osteogenic condensations. We review the origin and diversification of the integumentary skeleton in aquatic non-tetrapods (including stem gnathostomes), focusing on tissues derived from odontogenic (hypermineralized tissues, dentines and elasmodine) and osteogenic (bone tissues) cell condensations. The novelty of our new scenario of integumentary skeletal evolution resides in the demonstration that elasmodine, the main component of elasmoid scales, is odontogenic in origin. Based on available data we propose that elasmodine is a form of lamellar dentine. Given its widespread distribution in non-tetrapod lineages we further propose that elasmodine is a very ancient tissue in vertebrates and predict that it will be found in ancestral rhombic scales and cosmoid scales.

Scale morphology and specialized dorsal scales of a new teleosteomorph fish from the Aptian of West Gondwana

“…15a). Ganoine, as in most actinopterygians, is composed of multiple layers (Sire et al, 2009), as can be seen in photographs under normal and polarized light (Fig. 15b and c).…”

“…1 from the La Cantera Formation are of the lepisosteoid type with multiple ganoine layers, canaliculi of Williamson (Schultze, 1966(Schultze, , 2015Sire et al, 2009) in the basal bony layer, and no dentine. The surface of the flank scales is smooth, and microscopically it is bare of tubercles or denticles.…”

Spines of the porcupine fish: Structure, composition, and mechanical properties

“…Sire, et al [46] note that the dermal plates in tetraodontiformes are composed of only bone. However, the order tetraodontiformes is so diverse in morphology that it is unlikely that every modified scale across the order is compositionally identical.…”

Histology of the heterostracan dermal skeleton: Insight into the origin of the vertebrate mineralised skeleton

“…Giles et al (2013) demonstrated that this dermal architecture is also plesiomorphic with respect to placoderms, regardless of whether they are perceived as monophyletic or paraphyletic. Osteichthyans possess comparable dermal skeletal architecture, although their tubercles are capped with ganoine or enamel [ganoine is a hypermineralised tissue developmentally comparable to enamel (Sire et al, 1987;Sire, 1994;Sire et al, 2009)]. Chondrichthyans, the sister group to all other crown-gnathostomes, possess dermal denticles capped with enameloid, thus the phylogenetic distribution of dermal skeletal capping tissues suggests ganoine is apomorphic with respect to osteichthyans.…”