The unbearable uncertainty of panarthropod relationships

Panarthropoda, the clade comprising the phyla Onychophora, Tardigrada and Euarthropoda, encompasses the largest majority of animal biodiversity. The relationships among the phyla are contested and resolution is key to understanding the evolutionary assembly of panarthropod bodyplans. Molecular phylogenetic analyses generally support monophyly of Onychophora and Euarthropoda to the exclusion of Tardigrada (Lobopodia hypothesis), which is also supported by some analyses of morphological data. However, analyses of morphological data have also been interpreted to support monophyly of Tardigrada and Euarthropoda to the exclusion of Onychophora (Tactopoda hypothesis). Support has also been found for a clade of Onychophora and Tardigrada that excludes Euarthropoda (Protarthropoda hypothesis). Here we show, using a diversity of phylogenetic inference methods, that morphological datasets cannot discriminate statistically between the Lobopodia, Tactopoda and Protarthropoda hypotheses. Since the relationships among the living clades of panarthropod phyla cannot be discriminated based on morphological data, we call into question the accuracy of morphology-based phylogenies of Panarthropoda that include fossil species and the evolutionary hypotheses based upon them.

Automated summary

Panarthropoda, including Onychophora, Tardigrada, and Euarthropoda, is a clade with the majority of animal biodiversity. The relationships among these phyla are disputable, and resolving them is essential to understanding the evolutionary assembly of panarthropod bodyplans. Molecular phylogenetic analyses generally support the monophyly of Onychophora and Euarthropoda, excluding Tardigrada (Lobopodia hypothesis). However, morphological data analyses can also be interpreted to support the monophyly of Tardigrada and Euarthropoda, excluding Onychophora (Tactopoda hypothesis). Additionally, there is support for a clade of Onychophora and Tardigrada that excludes Euarthropoda (Protarthropoda hypothesis). Our study shows that morphological datasets cannot statistically distinguish between the Lobopodia, Tactopoda, and Protarthropoda hypotheses, calling into question the accuracy of morphology-based phylogenies and evolutionary hypotheses of Panarthropoda, including fossil species.