Careful amendment of morphological data sets improves phylogenetic frameworks: re-evaluating placement of the fossil <i>Amiskwia sagittiformis</i>

The Cambrian fossil <i>Amiskwia sagittiformis</i> has puzzled palaeontologists for more than a century, but recent re-investigation of its morphology suggested a close relationship with the bilaterian clade Gnathifera, comprising Rotifera, Gnathostomulida and Micrognathozoa. Since <i>Amiskwia</i> has already been considered closely related to Chaetognatha, this new interpretation of its morphology supports recent molecular and developmental studies finding a close phylogenetic relationship between Chaetognatha and Gnathifera. The recent re-description of the jaw apparatus of <i>Amiskwia</i> with the first phylogenetic analysis by Vinther &amp; Parry (2019) found a rather surprising topology with <i>Amiskwia</i> and Chaetognatha being sister groups and nested inside Gnathifera. Furthermore, a subsequent paper from Caron &amp; Cheung (2019) re-described the jaws of <i>Amiskwia</i> as very similar to the configuration found in Gnathostomulida, but did not include any phylogenetic analysis. Here we test the topology of Vinther &amp; Parry with various parsimony and Bayesian analyses, taking into account the new description of Caron &amp; Cheung with careful re-amendment of the matrix of Vinther &amp; Parry. According to our results and the recent findings on the molecular phylogeny of their extant members, we suggest a new systematization of these taxa. We recovered <i>Amiskwia</i> as a stem-group chaetognath within a clade Cucullophora nov., rejecting the Caron &amp; Cheung hypothesis of its close affinity to Gnathostomulida, and found a more consensual topology with monophyletic Gnathifera sister group to Cucullophora, all together forming the clade Chaetognathifera. Furthermore, Rotifera + Micrognathozoa form a clade named Gynognathifera nov. We discuss the characters supporting each clade and the reasons that account for the topology found by Vinther &amp; Parry. Finally, we hope that this carefully amended matrix focused on Gnathifera, <i>Amiskwia</i> and Chaetognatha, together with a formal classification and robust phylogeny, will be of use for future studies on the palaeontology and morphology of these clades.