Abstract

As part of the transition from Holostei to Teleostei, †Pachycormiformes represent a key group of fishes. However, the anatomy and phylogenetic relationships of the group in the context of the neopterygians are far from being understood. In this contribution we describe a new pachycormiform, †Kaykay lafken gen. et sp. nov., from the Upper Jurassic of Argentina. We made an exhaustive review of morphological characters of holostean and teleostean fishes and explore through a cladistic analysis the phylogenetic relationships of the new species. †Kaykay gen. nov. is retrieved among †Pachycormiformes as being the sister taxon of the macrocarnivorous clade composed of †Orthocormus and †Hypsocormus. Among †Pachycormiformes the pattern of relationships mostly agrees with previous hypotheses, although our study highlights the still poor knowledge of the anatomy of this group. According to our results †Saurostomus is the sister taxon of other toothed pachycormids. †Orthocormus species are recovered as a monophyletic group. The unsolved position of †Pachycormus and †Sauropsis might be a consequence of poor preservation, revealing a lack of understanding of their anatomy. Our phylogenetic analysis also confirms the rapid radiation of holosteans and teleosteomorphs in the Early Triassic and the radiation of pholidophoriforms in the Middle Triassic. †Aspidorhynchoidei radiate in the Early Jurassic. The large ghost ranges (e.g. between †Aspidorhynchoidei and Teleosteomorpha) evidence biases in the fossil record.