title: From body scale ontogeny to species ontogeny: Histological and morphological assessment of the Late Devonian acanthodian Triazeugacanthus affinis from Miguasha, Canada.
abstract: Growth series of Palaeozoic fishes are rare because of the fragility of larval and juvenile specimens owing to their weak mineralisation and the scarcity of articulated specimens. This rarity makes it difficult to describe early vertebrate growth patterns and processes in extinct taxa. Indeed, only a few growth series of complete Palaeozoic fishes are available; however, they allow the growth of isolated elements to be described and individual growth from these isolated elements to be inferred. In addition, isolated and in situ scales are generally abundant and well-preserved, and bring information on (1) their morphology and structure relevant to phylogenetic relationships and (2) individual growth patterns and processes relative to species ontogeny. The Late Devonian acanthodian Triazeugacanthus affinis from the Miguasha Fossil-Lagerstätte preserves one of the best known fossilised ontogenies of early vertebrates because of the exceptional preservation, the large size range, and the abundance of complete specimens. Here, we present morphological, histological, and chemical data on scales from juvenile and adult specimens (scales not being formed in larvae). Histologically, Triazeugacanthus scales are composed of a basal layer of acellular bone housing Sharpey's fibers, a mid-layer of mesodentine, and a superficial layer of ganoine. Developmentally, scales grow first through concentric addition of mesodentine and bone around a central primordium and then through superposition of ganoine layers. Ontogenetically, scales form first in the region below the dorsal fin spine, then squamation spreads anteriorly and posteriorly, and on fin webs. Phylogenetically, Triazeugacanthus scales show similarities with acanthodians (e.g. "box-in-box" growth), chondrichthyans (e.g. squamation pattern), and actinopterygians (e.g. ganoine). Scale histology and growth are interpreted in the light of a new phylogenetic analysis of gnathostomes supporting acanthodians as stem chondrichthyans.
authors.affiliation: Université du Québec à Rimouski, Rimouski, Québec G5L 3A1, Canada.
authors.forename: Marion
authors.identifier: http://orcid.org/0000-0002-7305-0485
authors.initials: M
authors.lastname: Chevrinais
authors.affiliation: UMR 7138-Evolution Paris-Seine, IBPS, Université Pierre et Marie Curie, Paris, France.
authors.forename: Jean-Yves
authors.initials: JY
authors.lastname: Sire
authors.affiliation: Université du Québec à Rimouski, Rimouski, Québec G5L 3A1, Canada.
authors.forename: Richard
authors.initials: R
authors.lastname: Cloutier
mesh_terms: D000818:Animals; D002170:Canada; D004817:Epidermis / Q000033:anatomy & histology; D005399:Fishes / Q000033:anatomy & histology* / Q000145:classification; D005580:Fossils; D010802:Phylogeny; D013131:Spine / Q000033:anatomy & histology