Is Onychodontida (Osteichthyes, Sarcopterygii) monophyletic? Assessing discordant phylogenies with quantitative comparative cladistics

Abstract

Onychodontida (Osteichthyes, Sarcopterygii) was an extinct group of Devonian predatory marine fishes, representing an early branch in sarcopterygian evolution, and currently considered closely related to coelacanths (Actinistia). Due to their limited fossil record, the relationships of onychodonts within sarcopterygians, and whether all taxa traditionally considered as onychodonts form a clade, are still unclear. Here we review the most recent phylogenetic analyses by comparing their data matrices using recent tools and methodology in quantitative comparative cladistics in order to evaluate the source of discrepancies in the different datasets and provide possible practical solutions to test the monophyly of Onychodontida. These discrepancies range from the ambiguous formulation of character statements and character states to the poor preservation of certain fossils, which make interpretation of character states difficult. Understudied but highly complete fossil specimens are also a source of missing data that have an impact in discording tree topologies. In-depth analysis and description of these specimens is needed to improve the resolution of future phylogenetic analyses. Finally, we propose a formal stem-based phylogenetic defi nition for Onychodontida.