Comment on 'A standardized terminology of spines in the order Siluriformes (Actinopterygii: Ostariophysi)' by Ballen and de Pinna (2021)

Homology is the foundation of any comparative evolutionary study, and as structures previously considered homologous are found to be convergent, and vice versa, terminology needs to be changed to reflect homology. The dorsal- and pectoral fin-spines of catfishes (order Siluriformes) are morphologically diverse and ornamentations adorning the anterior and posterior edges of the spines have regularly been used in systematic studies of the group. Building on Vanscoy et al. (2015), Kubicek et al. (2019) showed that some of the ornamentations of siluriform fin-spines previously considered homologous result from two different developmental pathways, and presented a terminology based on the homology of these structures. In their recent paper, Ballen and de Pinna (2021) proposed a new terminology for various catfish fin-spine structures, including fin-spine ornamentation. However, homology was not considered and, instead, their terminology is based on the general shape of the ornamentation to allow for 'unambiguous use' in various applications, including the placement of fossil fin-spines into well-defined clades of catfishes. We show that the terminology proposed by Ballen and de Pinna (2021) is problematic as it confuses non-homologous conditions into the same state and thus undermines the usefulness of fin-spine ornamentation as a character in comparative evolutionary studies of siluriforms.