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

Abstract

The Cambrian fossil Amiskwia sagittiformis has puzzled palaeontologists for more than a century, but recent reinvestigation of its morphology suggested a close relationship with the bilaterian clade Gnathifera, comprising Rotifera, Gnathostomulida and Micrognathozoa. Since Amiskwia 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 redescription of the jaw apparatus of Amiskwia with the first phylogenetic analysis by Vinther and Parry (2019) found a rather surprising topology with Amiskwia and Chaetognatha being sister groups and nested inside Gnathifera. Furthermore, a following paper from Caron and Cheung (2019) redescribed the jaws of Amiskwia as very similar to the 2 configuration found in Gnathostomulida, but did not carry any phylogenetic analysis.Here we test the topology of Vinther and Parry with various parsimony and Bayesian analyses, taking into account the new description of Caron and Cheung with careful re-amendment of the matrix of Vinther and 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 Amiskwia as a stem group chaetognath within a clade Cucullophora nov., rejecting the Caron and Cheung hypothesis of its close affinity to Gnathostomulida, and found a more consensual topology with monophyletic Gnathifera sister group to Cucullophora, altogether forming the clade Chaetognathifera. Furthermore the clade 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 and Parry. Finally, we hope that this carefully amended matrix focused on Gnathifera, Amiskwia 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.

A giant stem-group chaetognath

“…4 ) in Timorebestia . Amiskwia shares this suite of body plan features with Timorebestia , and these taxa have a combination of characters that are not seen in any group of extant gnathiferans and are hereafter referred to as “amiskwiiforms.” The affinities of amiskwiiforms have been debated ever since Amiskwia was first described ( 18 , 30 , 31 ) and most recently in terms of their position within the Chaetognathifera ( 11 , 12 , 16 ). The unique preservation of a ventral ganglion with lateral neuron somata through phosphatization present in both T. koprii and the more crownward chaetognath taxon from Sirius Passet described here offer further evidence implying closer affinities of amiskwiids to the chaetognaths ( Fig.…”

“…While the absence of grasping spines in Amiskwia led to a rejection of a relationship with chaetognaths, recent studies demonstrated the presence of an internal jaw apparatus ( 11 , 12 ), similar to that possessed by gnathiferans. Recent molecular phylogenetic studies have found that chaetognaths and gnathiferans may form a clade ( 13 – 15 ), Chaetognathifera ( 16 ). The presence of a gnathostomulid-like jaw in Amiskwia has led to competing interpretations of its position in the tree of life, with the lateral fins representing either shared plesiomorphies ( 12 , 16 ) or convergences ( 11 ) with those in chaetognaths.…”

“…Recent molecular phylogenetic studies have found that chaetognaths and gnathiferans may form a clade ( 13 – 15 ), Chaetognathifera ( 16 ). The presence of a gnathostomulid-like jaw in Amiskwia has led to competing interpretations of its position in the tree of life, with the lateral fins representing either shared plesiomorphies ( 12 , 16 ) or convergences ( 11 ) with those in chaetognaths. This phylogenetic position has important implications for understanding the evolution of the four phyla, Chaetognatha, Gnathostomulida, Micrognathozoa, and Rotifera, such as primitive or secondary miniaturization, benthic versus pelagic lifestyle, and ancestral jaw apparatus morphology.…”