Anatomy of the early Devonian acanthodian Brochoadmones milesi based on nearly complete body fossils, with comments on the evolution and development of paired fins

Abstract

ABSTRACT New anatomical details are described for the acanthodian Brochoadmones milesi based on nearly complete body fossils from Lochkovian rocks at MOTH, Mackenzie Mountains, Northwest Territories, Canada. The body and caudal peduncle are deep, and a prominent nuchal hump is present before the dorsal fin origin. The caudal fin is correspondingly deep and ventrally, the caudal fin lies close to and is partly joined to the slender anal fin. A delicate pectoral fin trails the flattened pectoral-fin spine where previously known specimens showed only a fin spine resembling a bivalve shell. Seen for the first time in any vertebrate, each of the six pairs of prepelvic spines supports a small, scale-covered finlet. Both prepelvic spines and scale-covered finlets increase in size posteriorly. The series of paired prepelvic finlets originates ventral to the branchial chamber and anteroventral to the pectoral fin, and extends posteriorly as far as the pelvic fins. The scales of the body and fins are thin and flat, without obvious evidence of ossified basal tissue or entry point for vascular tissue. The main lateral-line canal passes dorsal to the branchial chamber and terminates at the trailing edge of the caudal fin web. Lateral-line scales are thicker than body scales and show concentric growth zones. Scales from the dorsal midline of the caudal fin are also thicker, showing few superpositional growth zones in the mesodentine of the crown together with what appears to be cellular basal tissue. The structure and position of the pectoral spine and fin, the extremely thin body scales, the slender anal fin, and the prepelvic finlets are all unique and appear to be autapomorphic features compared to those of other acanthodians. Brochoadmones milesi is derived relative to other fishes traditionally classified in the Climatiiformes. Kathemacanthus rosulentus is removed from the Brochoadmonoidei, leaving only B. milesi in a monotypic suborder.