Abstract
AbstractThe ‘acanthodian’ fishes provide key anatomical insights into the deepest branches of the chondrichthyan stem group. We review the anatomy of the acanthodian Vernicomacanthus uncinatus from the Lochkovian (Lower Devonian, 419.2–410.8 Ma) of Scotland based on eight articulated fossils, one of which is newly described. Broadly, the anatomy of V. uncinatus fits with that of contemporaneous acanthodians such as Climatius and Parexus, with a head covered by robust tesserae, an enlarged postorbital scale, an armoured shoulder girdle, and many pairs of ventrolateral spines. However, it departs from this anatomy in key respects. Its pectoral fin spines are obliquely ridged and posteriorly denticulated, similarly to Carboniferous gyracanth stem‐group chondrichthyans. Its scales consist of multiple anteroposteriorly aligned odontodes, similarly to many Palaeozoic ‘sharks’. And its endoskeletal shoulder girdle may have a posterolateral angle, previously observed only in shark‐like chondrichthyans. We propose that the differences between V. uncinatus and its congeneric, V. waynensis, which include potentially phylogenetically significant characters of the shoulder girdle and spines, are sufficient to erect a new genus for V. waynensis: Dobunnacanthus gen. nov. The scales of Vernicomacanthus are identical to those of the ‘shark’ scale genus Altholepis, suggesting that some such scales may instead belong to taxa with acanthodian‐like gross anatomies. Based on these scales we highlight potential patterns in chondrichthyan scale evolution, in particular the axial addition of odontodes. Anatomical similarities between Vernicomacanthus and gyracanths, highlighted by previous authors, may indicate the existence of a grade including these and similar acanthodian‐grade taxa placed relatively crownwards in the chondrichthyan stem‐group.
Highlights
IntroductionO V E R the last decade, evidence has mounted that ‘acanthodians’ (a collection of poorly understood fishes previously grouped with bony fishes, cartilaginous fishes, stemgnathostomes, or all three) are all stem-group chondrichthyans (Zhu et al 2013; Brazeau & Friedman 2014; Coates et al 2018)
O V E R the last decade, evidence has mounted that ‘acanthodians’ are all stem-group chondrichthyans (Zhu et al 2013; Brazeau & Friedman 2014; Coates et al 2018)
We present new data on the pectoral and dermal skeletons of Vernicomacanthus and Dobunnacanthus gen. nov., taxa that lie at a key phylogenetic juncture in ongoing debates about early chondrichthyan evolution
Summary
O V E R the last decade, evidence has mounted that ‘acanthodians’ (a collection of poorly understood fishes previously grouped with bony fishes, cartilaginous fishes, stemgnathostomes, or all three) are all stem-group chondrichthyans (Zhu et al 2013; Brazeau & Friedman 2014; Coates et al 2018). Miles (1973) assigned material from the Welsh borders to V. waynensis that we assign to a new genus in the present paper This reassignment reflects anatomical differences in the shoulder girdle, spines, and size that distinguish the two taxa. Climatiid-like taxa variably appear in phylogenies as either nested close to crown-group chondrichthyans (Giles et al 2015; King et al 2016; Chevrinais et al 2017; Coates et al 2018; Dearden et al 2019), with remaining lightly armoured acanthodians in a more remote position, or as a basal grade of the chondrichthyan total group (Burrow et al 2016; Frey et al 2020), perhaps reflecting their retention of osteichthyan-like characters such as branchiostegal plates
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