The Temporal Region of the Cranium of Loricarioid Catfishes (Teleostei: Siluriformes): Morphological Diversity and Phylogenetic Significance

The morphology of the temporal region of the cranium of the monophyletic loricarioid catfishes was examined in representatives of the families Trichomycteridae, Callichthyidae, Scoloplacidae, Astroblepidae, and Loricariidae, and compared to non-loricarioid outgroup taxa. The results indicate the presence of derived specializations in the morphology of the pterotic bone, swimbladder capsule, lateral-line nerve, and associated structures which have not been previously recognized. Astroblepids, callichthyids, loricariids and scoloplacids differ from trichomycterids and non-loricarioid catfishes in having a reduced number of bones in the temporal region. Loricariids share with astroblepids and some trichomycterids the presence of direct contact between the trunk lateral-line canal and swimbladder capsule chamber; however, only loricariids have the lateral-line canal in direct contact with the swimbladder membrane, resulting in a laterophysic connection in addition to the otophysic connection between inner ear and swimbladder characteristic of otophysan fishes. Derived trichomycterids share with the outgroups Amphiliidae, Plotosidae and Sisoridae a cranial exit of the posterior lateral-line nerve separate from that of the vagus nerve, whereas both nerves exit the cranium via a common foramen in other loricarioids and outgroup catfishes. Callichthyids share with scoloplacids and loricariids a lateral-line nerve traversing the swimbladder capsule. Loricariids are further modified in having a compound pterotic bone that is double-layered and bears the postotic canal on the ventral layer. The dorsal layer is posteriorly expanded to enclose the lateral opening of the swimbladder capsule and an expanded capsule chamber. Based on the previously well-established scheme of interrelationships among loricarioid families, we infer that much of the variation in temporal cranial morphology over the course of loricarioid evolution has involved independent convergence toward increased functional association of primitively separate sensory structures of the octavolateralis system.