Complete osteology of bumblebee catfish Microglanis lundbergi uncovers skeletal simplification and previously overlooked morphological diversity in Pseudopimelodidae

Abstract

The complete osteology of Microglanis lundbergi, a pseudopimelodid catfish from the Amazon River (Amazonas, Brazil), is described and compared with representatives of all Pseudopimelodidae genera. This is the first comprehensive skeletal account for a species of Microglanis, encompassing both cranial and postcranial elements. The skeleton is organized into thirteen topographic and functional complexes based on cleared-and-stained and radiographed specimens. Comparative analysis confirms key family-level synapomorphies in M. lundbergi, including an anteriorly V-shaped mesethmoid, absence of the dorsal hypohyal, and a reduced number of infraorbitals. Within Batrochoglaninae, it shares derived features, including the absence of the vomer and a short supraoccipital process that does not contact the nuchal plate. The species also exhibits distinctive autapomorphies, including a triangular opercle with a pointed posterosuperior corner and a pectoral spine with predominantly retrorse serrations. Additional traits, such as a filamentous mesocoracoid, a slender premaxilla lacking a lateral posterior projection, and five branched pectoral-fin rays, support the diagnosis of Microglanis while indicating intrageneric variation. Although small-bodied, M. lundbergi does not qualify as a miniature species sensu stricto but displays skeletal simplifications consistent with size reduction. This study establishes a detailed anatomical reference for future phylogenetic analyses and demonstrates that even within a morphologically conserved genus, fine-scale osteological data can reveal autapomorphies and refine intrafamilial relationships.