Postotic laterosensory canal and pterotic branch homology in catfishes

Morphology of the postotic laterosensory canal was surveyed across loricarioid and outgroup catfishes in order to resolve conflicting statements regarding homology and phylogenetic significance of intrinsic character variation. A pterotic branch is widespread among catfishes and has been identified as a synapomorphy for siluriforms, but its presence in loricarioid catfishes has been disputed. In contrast to previous statements that absence of a pterotic branch is synapomorphic for loricarioids, we confirm the presence of a pterotic branch in Nematogenys inermis and other trichomycterids, callichthyids, and loricariids. The pterotic branch is secondarily absent in scoloplacids and astroblepids. We present criteria for establishing homology of the pterotic branch and review character state optimization schemes on the currently accepted phylogeny. The postotic region of loricariids is further specialized in having an expanded swimbladder capsule that incorporates the trunk lateral line canal and has a lateral opening covered by a greatly expanded pterotic complex. The trunk lateral line enters the swimbladder capsule mesial to the pterotic lateral wall and passes anteromedially as a fleshy tube before forming the postotic canal in the pterotic, a morphology reported previously for a single loricariid representative. Variation in the relative extent and topographic position of postotic canal branches and other morphologies is diagnostic of certain loricariid taxa, suggesting a rich character complex of potential utility in phylogeny reconstruction.