The Gonorynchiformes are a small order of fish comprising only 38 species (Fricke et al., 2022). However, within the last two decades, gonorynchiforms were studied extensively because of their phylogenetic position within otomorphs (e.g., Britz & Moritz, 2007;Coburn & Chai, 2003;Grande & Arratia, 2010). The Gonorynchiformes represents the sister taxon of the Otophysi, which are a very diverse group comprising two-thirds of the worldwide freshwater species (

Mormyridae are well known and intensively studied for their weak electric organ discharges, which facilitate communication and orientation. The Gemminger bones of Mormyridae are located next to the electrical organ in the caudal peduncle; however, they have not attracted much interest until recently. Therefore, we investigated the diversity of Gemminger bones in mormyrids and studied their ontogenetic development in Mormyrus rume proboscirostris. Gemminger bones are paired, thin, elongated ossifications lying on the dorsal and ventral sides of the caudal peduncle, and usually reach anterior well below the dorsal and anal fin bases. Ontogeny revealed that they are not intermuscular ossifications, as suspected based on the anatomical position of this structure and the systematic position of the mormyrids. Instead, they are membrane ossifications that originate from the fin stays of the dorsal and anal fins.