Pectoral anatomy of Tribodus limae (Elasmobranchii: Hybodontiformes) from the Lower Cretaceous of northeastern Brazil

Abstract

ABSTRACT New, undescribed material of the hybodont shark Tribodus limae, from the Lower Cretaceous of northeastern Brazil, provides new information on the pectoral endoskeleton in this taxon. This allows for a revision of its pectoral morphology, which may be broadly applicable to other Hybodontoidei. Four new specimens of Tribodus with well-preserved pectoral girdles are described, including the first complete, three-dimensionally preserved hybodont scapulocoracoid and partial pectoral fin. These specimens permit examination of the posterior surface of the scapulocor-acoid, not visible in previously described hybodonts. Overall pectoral morphology of Tribodus is similar to that of other hybodontiforms and neoselachians. However, the new material shows that the pectoral articulation in Tribodus comprises an oblique posterior ridge on the scapulocoracoid, unlike previous reconstructions of pectoral anatomy in Tribodus and other hybodonts based on two-dimensionally preserved specimens. However, other elasmobranchs, including many neoselachians, Orthacanthus, and ctenacanths, also have an oblique posterior articulation, indicating that this feature may be primitive for elasmobranchs. The pectoral articulation of Tribodus differs noticeably from that of Cladoselache and similar Paleozoic chondrichthyans, which have an elongated, laterally positioned articular process, and from the outgroup (osteichthyans, acanthodians and placoderms). Tribodus also differs from the Paleozoic hybodontiform Ony-choselache in which the articular region comprises a laterally positioned glenoid fossa and posterior condyle. Tribodus has an additional diazonal foramen not found in other hybodonts. Other pectoral features of Tribodus are in agreement with previous phylogenetic hypotheses, supporting the position of Tribodus within Hybodontoidei.