The revival of a so-called rotten fish: the ontogeny of the Devonian acanthodian Triazeugacanthus

Abstract

Since its original description as a chordate, the Late Devonian Scaumenella mesacanthi has been interpreted alternately as a prochordate, a larval ostracoderm and an immature acanthodian. For the past 30 years, these minute specimens were generally considered as decayed acanthodians, most of them belonging to Triazeugacanthus affinis. Among the abundant material of 'Scaumenella', we identified a size series of 188 specimens of Triazeugacanthus based on otolith characteristics. Despite taphonomic alteration, we describe proportional growth and progressive appearance of skeletal elements through size increase. Three ontogenetic stages are identified based on squamation extent, ossification completion and allometric growth. We demonstrate that what has been interpreted previously as various degrees of decomposition corresponds to ontogenetic changes.

New Insights in the Ontogeny and Taphonomy of the Devonian Acanthodian Triazeugacanthus affinis From the Miguasha Fossil-Lagerstätte, Eastern Canada

“…Three ontogenetic stages (larval, juvenile and adult) have been recognized in Triazeugacanthus based on the degree of squamation, the progression of the ossification and the size of the specimens [21]. Six complete or almost complete specimens, two larvae (Figure 1a-f), two juveniles (Figures 1j-o, and 2a-f) and two adults (Figure 2h-s) and one thin section of the dermal skeleton of a juvenile (Figure 1g-i) Triazeugacanthus preserved in laminites, have been used for the non-destructive scanning electron microscopy (SEM) coupled to the energy dispersive X-ray spectrometry (EDX) (Figures 1 and 2).…”

“…The notochordal elements (visible only in rare specimens) show no sign of mineralization (Figure 3g). During the larval stage, otoliths form shortly after the previous elements ( [21]: Electronic Supplementary Material, Figure S3). These non-bony elements display a strong Ca peak and lack P; these results are consistent with the expected calcium carbonate composition [27] (Figures 1c,f and 3b,d).…”

“…Minor Al and Si contributions are detected and are likely related to the presence of aluminous clay minerals in the surrounding matrix (i.e., illite and chlorite; Figures 3-5). In larval Triazeugacanthus (Figure 1a,b), notochordal elements and eye lenses are among the first elements to develop ( [21]: Electronic Supplementary Material, Figure S3). Notochordal elements and eye lenses display C and O peaks with a minor contribution of Ca, Al and Si in the EDX spectrum (Figure 3a,c,g).…”

“…Among these 14 species, larval and juvenile specimens of the acanthodian Triazeugacanthus affinis have been reinterpreted recently [21]. The exceptional preservation of a large number of Triazeugacanthus within a large size range allowed the description of ontogenetic changes.…”

“…The exceptional preservation of a large number of Triazeugacanthus within a large size range allowed the description of ontogenetic changes. This ontogenetic series is characterized by the simultaneous increase in the number of skeletal structures and the progressive extension of the squamation as a function of animal size [21]; thus, endoskeletal as well as exoskeletal mineralization proceed simultaneously.…”