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.
Highlights
Fish fossilised ontogenies are rare, especially in the Palaeozoic record [1] because the preservation of weakly mineralised skeletal elements of immature specimens requires exceptional fossilisation conditions
Some, such as scales, record the ontogeny of the individual and provide a unique opportunity to describe their ontogeny [1, 2]. For which both abundant individual isolated elements and complete specimens are known, are indispensable to describe the relationship between individual growth of isolated elements and species ontogeny [1]
Only a few acanthodian ontogenies based on complete specimens have been discovered [1, 8]: one possible ischnacanthiform [Nerepisacanthus denisoni [3]], two diplacanthiforms [Diplacanthus horridus [9] and Uraniacanthus curtus [10]], one “climatiiform” [Tetanopsyrus breviacanthias [11]], one species of uncertain order Lupopsyrus pygmaeus [12]], and nine acanthodiforms [Triazeugacanthus affinis [13, 14], Lodeacanthus gaujicus [15,16,17,18], Homalacanthus concinnus [9], Acanthodes bridgei [19], A. bronni [20], A. gracilis [21], A. lopatini [22], A. ovensi [23]], and an acanthodiform indet
Summary
Fish fossilised ontogenies are rare, especially in the Palaeozoic record [1] because the preservation of weakly mineralised skeletal elements of immature specimens requires exceptional fossilisation conditions. Complete and articulated fossil fishes are rare in the Palaeozoic record, compared to isolated elements, which are abundant. Among these isolated elements, some, such as scales, record the ontogeny of the individual and provide a unique opportunity to describe their ontogeny [1, 2]. For which both abundant individual isolated elements and complete specimens are known, are indispensable to describe the relationship between individual growth of isolated elements and species ontogeny [1]. Acanthodian species known from complete specimens are relatively rare compared to the number of taxa known solely from isolated scales [5,6,7]. The rarity of complete specimens of acanthodians has been attributed to the micromeric nature of the dermal skeleton as well as the poor ossification of the endoskeleton [17, 25]
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