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

Cartilaginous fishes (chondrichthyans) from the Devonian period (419–359 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 early chondrichthyan fin spines by integrating paleohistological cuts, micro-CT scan, SEM and microscopy analyses. We present the first description of the developmental patterns for the fin spines of the Middle Devonian chondrichthyan Wellerodus priscus , from the Cairo Quarry Lagerstätte in New York State, USA. Based on the ornamentation pattern and paleohistological (thin sections and micro-CT scan) characteristics, we identify a longitudinal (length wise) growth and a transverse (width wise) growth, each associated with specific zones of tissue production in the pectoral and dorsal fin spines of W. priscus . Spine developmental patterns observed in Wellerodus are compared among Paleozoic total-group chondrichthyans composed by the stem-chondrichthyans “acanthodians” and the crown group. Our framework offers a new line of evidence for understanding the evolution of fin spine development by showing that Wellerodus priscus fin spines grow in length and in width. It enhances our comprehension of developmental similarities between early chondrichthyans and more derived Carboniferous, Permian, and even extant chondrichthyans. The histo-morphological criteria used to describe this developmental pattern provide characters potentially suitable for phylogenetic analyses.