The Ovalentaria is a taxon of teleosts which has only recently been proposed on the basis of molecular-genetic analyses. Multiple, previously widely-separated fish families are assembled with this taxon. For the first time, the Ovalentaria are analysed using a comparative morphological approach. The caudal fin skeleton of 355 species covering all 48 ovalentarian families are examined in cleared and stained specimen, µCT datasets and x-ray images. A total of 38 morphological characters are evaluated and used for ancestral character state reconstructions and phylogenetic analyses. Results provided phylogenetic hypotheses for the relationships of ovalentarian taxa and a grundplan of the caudal fin skeleton of the last common ancestor of all Ovalentaria. An evolutionary trend towards the reduction of skeletal elements in the caudal fin can be observed. Furthermore, a connection between the evolution of the caudal skeleton and the modes of locomotion found in ovalentarian taxa is discussed. The phylogenetic analyses provided convincing topologies that are highly in agreement with molecular-genetic based hypotheses.