Fin spine ontogeny in the Devonian shark Wellerodus priscus: Paleo-Evo-Devo insights into early chondrichthyan dermal skeleton development

Background Cartilaginous fishes (chondrichthyans) from the Devonian period (419 to 359 million years, Ma) are primarily known from fossilized isolated dermal elements such as teeth, scales, and fin spines. Although previous studies have described the morphology and the histology of these elements, their developmental patterns remain poorly understood. In this study, we propose to explore developmental patterns of fin spines by integrating analyses of ornamentation, morphology, and histology. Results We present the first description of the developmental patterns for the fin spines of the Middle Devonian shark Wellerodus priscus , from the Cairo Quarry Lagerstätte in New York State, USA. Based on the ornamentation pattern and paleohistological (thin sections and micro-computed tomography) characteristics, we identify two distinct growth axes in the pectoral and dorsal fin spines of W. priscus : a longitudinal (length wise) axis and a transverse (width wise) axis, each associated with specific zones of tissue production. Spine developmental patterns observed in Wellerodus are compared among Paleozoic chondrichthyans and their closest relatives, the “acanthodians” (extinct spiny sharks). Conclusions Our framework offers a new line of evidence for understanding the evolution of fin spine modular development by showing that fin spines in early chondrichthyans grow following two axes. It enhances our comprehension of developmental similarities between early chondrichthyans and more derived Carboniferous, Permian, and even extant chondrichthyans. The identified structural features enable well-supported inferences about developmental modularity. The histo-morphological criteria used to describe this developmental pattern provide characters potentially suitable for phylogenetic analyses.