A true caudal fin or not? New insights in the evolution of the gadiform caudal fin

Abstract

The distinctive caudal-fin skeleton of gadiforms has puzzled scientists for a long time, because of its many differences in comparison to other teleosts. Contradicting hypotheses interpreted this structure as (i) a highly derived teleostean caudal fin, (ii) a new formation with parts from the caudal, dorsal and anal fins, a so called pseudocaudal, or (iii) a complete evolutionary novelty, a so called neocaudal. To shed light on to this issue, the caudal-fin ontogeny of Lota lota was studied in detail. It differs from the development in non-gadiform teleostean taxa, e.g. by absence of a distinct and early notochord flexion. However, there are also many similarities with other teleosts, e.g. the caudal fin develops before the dorsal and anal fins. Furthermore, the morphology of adult caudal fins of all major gadiform families were studied and reviewed. Our results, in combination with the latest molecular phylogenies, allowed us to discuss the evolution of the gadiform caudal fin and resolve the origin of this highly debated character complex. Although their caudal-fin skeleton shows several derived apomorphies, the gadiform caudal fin is homologous to the caudal fins of other teleosts, without principal inclusion of dorsal- or anal-fin elements.