Ontogeny of the Early Carboniferous acanthodian <Emphasis Type="Italic">Acanthodes lopatini</Emphasis> Rohon

The sequence of ossification of skeletal elements in the growth series of the acanthodian Acanthodes lopatini Rohon from the Lower Tournaisian of the Minusa Trough (Siberia) is described. Four formal ontogenetic stages are recognized. It is shown that even 20?30-mm-long juveniles have well-developed fin spines, scapulocoracoids, mandibular bones, branchiostegal rays, and incipient gill rakers. At this stage, scales cover the posterior part of the body and the orbits are marked by dark “eye stains” of uncertain nature. Further ontogenetic changes are connected with the development of sclerotic ring elements, sensory line system, squamation of the head region and fin webs, perichondral ossifications of the Meckel’s, quadrate, and palatine cartilages, and, finally, mineralization of the hyoid and gill arches. Ontogenetic features of A. lopatini are compared with those in other members of the genus and other acanthodiforms. Some developmental features shared by A. lopatini and Lodeacanthus gaujicus Upeniece suggest that acanthodids could have evolved from mesacanthids.