A revision of Vernicomacanthus Miles with comments on the characters of stem‐group chondrichthyans

Abstract

The '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, 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.

Spiny chondrichthyan from the lower Silurian of South China

“…2x, y and Extended Data Fig. 7) rests on similarities with the dermoskeletal anatomy of climatiids 13,14,27,28 , particularly that of Climatius reticulatus 27,28 . Isolated tooth whorls were also recovered from our Rongxi samples from Shiqian (Extended Data Fig.…”

“…Assignment of Fanjingshania spine/fin spine morphologies to specific body positions is based on close similarities with the spine/fin spine complements of the climatiids (sensu Burrow et al 7 ) Climatius 8-10 and Vernicomacanthus 9,[11][12][13] . Comparison with the head dermoskeleton of the latter two taxa, and that of another climatiid, 11,14 , allowed to identify in Fanjingshania tectal and postorbital tesserae, as well as sclerotic and branchiostegal plates.…”

Bony-fish-like scales in a Silurian maxillate placoderm

“…In addition, Entelognathus shows a complement of fin spines that adds significant details to the evolution of these structures. Entelognathus 9 bears a pair of dermal pectoral girdles with spinal plates, as in other placoderms [72][73][74][75] , stem chondrichthyans [76][77][78][79] , and some early osteichthyans 53,80,81 (Supplementary Fig. 7).…”

“…Entelognathus also has a dorsal fin spine, which is absent in most placoderms 39 but present in some ptyctodonts [82][83][84] . The dorsal fin spine is common in chondrichthyans [76][77][78][79] and some early osteichthyans 53,80,81 (Supplementary Fig. 7).…”

“…4a-c), which is unknown in any other placoderms. The anal fin spine and paired pelvic fin spines were previously thought to be restricted to stem chondrichthyans [76][77][78][79]85,86 and were regarded as two probable synapomorphies of the chondrichthyan total-group (Supplementary Fig. 7).…”