AYSHEAIA PEDUNCULATA IS A STEM-GROUP TARDIGRADE

Tardigrada is a phylum of microscopic invertebrates with some members that are known for their extreme survival capabilities. Together with onychophorans and arthropods, they form the supergroup Panarthropoda. Given this relationship with the two macroscopic phyla, it is hypothesized that the common ancestor of tardigrades underwent miniaturization that involved reduction of trunk segments and body size. This would have happened before the divergence of the crown-group phylum, which is estimated to be around the mid-Cambrian based on molecular clock analyses. However, understanding the evolution of the body plan of this group of animals remains limited due to the lack of recognized macrosocopic stem-group representatives. Among Cambrian fossils, Aysheaia pedunculata is sometimes recovered as a stem-group tardigrade in morphology-based phylogenetic analyses but with low support values. These analyses also appear to be biased towards eutardigrades and underutilize the morphologically diverse marine heterotardigrades – the tardigrade group hypothesized to most likely represent the tardigrade ancestor. In this study, we examined new specimens of A. pedunculata to re-assess its possible relationship with tardigrades. We were able to obtain high-quality images that allowed us to visualize certain morphologies that are similar to tardigrades. In particular, A. pedunculata appear to have claws – a highly conserved trait among tardigrades – that are only found in echiniscoid heterotardigrades. We then updated a published lobopodian-focused matrix to contain tardigrade taxa that evenly represents the morphological diversity observed in the phylum. By using a new hypothesis that considers the cephalic cirri A and primary clava as remnants of the first pair of tardigrade limbs, our phylogenetic reconstructions via maximum parsimony (MP) and Bayesian inference (BI) showed strong supports of A. pedunculata as an outgroup to all tardigrades, indicating a stem-group relationship. Overall, our results supported the miniaturization of the tardigrade ancestor and allowed us to hypothesize on how the tardigrade body plan evolved from an Aysheaia-like shape to the morphologies observe among extant tardigrades.