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.

A critical appraisal of appendage disparity and homology in fishes

“…latispinosus, which also possesses prepectoral and possibly admedian spines (Burrow et al, 2017;Maisey et al, 2017), and W. priscus (Potvin-Leduc, 2017). If acanthodians are added to the total group chondrichthyans as suggested by recent investigations (e.g., Brazeau & de Winter, 2015;Burrow et al, 2016;Chevrinais et al, 2017;Long et al, 2015;Qiao et al, 2016), the presence of prepelvic spines will become a common character of stem chondrichthyans. If acanthodians are added to the total group chondrichthyans as suggested by recent investigations (e.g., Brazeau & de Winter, 2015;Burrow et al, 2016;Chevrinais et al, 2017;Long et al, 2015;Qiao et al, 2016), the presence of prepelvic spines will become a common character of stem chondrichthyans.…”

“…Indeed, climatiiforms were resolved as paraphyletic by Burrow and Turner (2010) and Wilson (2002, 2004). As for acanthodians, they have been resolved as either polyphyletic with some being stem chondrichthyans and others stem osteichthyans (Brazeau, 2009;, or paraphyletic with respect to chondrichthyans (Brazeau & de Winter, 2015;Burrow et al, 2016;Chevrinais, Sire, & Cloutier, 2017;Giles, Friedman, & Brazeau, 2015;Long et al, 2015;Qiao, King, Long, Ahlberg, & Zhu, 2016;Zhu et al, 2013).…”

“…;, or paraphyletic with respect to chondrichthyans(Brazeau & de Winter, 2015;Burrow et al, 2016;Chevrinais, Sire, & Cloutier, 2017;Giles, Friedman, & Brazeau, 2015;Long et al, 2015;Qiao, King, Long, Ahlberg, & Zhu, 2016;Zhu et al, 2013).…”

Morphology and histology of acanthodian fin spines from the late Silurian Ramsåsa E locality, Skåne, Swede

“…Abbreviations: cc, central cavity; k, keel; med.c, median canal; r, ribbing; rib.c, ribbing canals; s, sulcus. squamation in early gnathostomes (e.g., Trinajstic, 2001;Brazeau, 2012;Burrow, 2013;Burrow et al, 2013Burrow et al, , 2015Burrow et al, , 2016Chevrinais et al, 2017) will undoubtedly influence future designations of isolated material. On the other hand, the taxonomical framework based on isolated material that Vergoossen (2004) described from Ramsåsa site E may enable an assignment of the spines treated herein on a higher taxonomic level.…”

“…Enameloid on fossil chondrichthyan fin spines is not very common, even though it is reported from teeth and scales (Zangerl, 1981). This tissue is not present on any acanthodian fin spines, but acanthodian scales are sometimes covered by a hypermineralized cap (Denison, 1979;Chevrinais et al, 2017). However, it is not possible to make any further inferences on this matter with the data at hand.…”