Amiskwia sagittiformis Walcott 1911 is an iconic soft-bodied taxon from the Burgess Shale [1–3]. It was originally interpreted as a chaetognath , but it was later interpreted as a pelagic nemertean  or considered of uncertain affinity . Part of this ambiguity is due to direct comparisons with members of the crown groups of extant phyla  and a lack of clarity regarding the systematic position of chaetognaths, which would allow for assessing character polarity in the phylum with respect to outgroups. Here, we show that Amiskwia preserves a bilaterally arranged set of head structures visible in relief and high reflectivity. These structures are best interpreted as jaws situated within an expanded pharyngeal complex. Morphological studies have highlighted a likely homology between bilateral and chitinous jaw elements in gnathiferans and chaetognaths , which is congruent with a shared unique Hox gene that suggests a close relationship between Gnathifera and Chaetognatha . Molecular phylogenetic studies have recently found gnathiferans to be a deep branch of Spiralia and Chaetognaths either a sister group to Spiralia  or forming a clade with gnathiferans [6, 8]. Our phylogenetic analyses render Gnathifera paraphyletic with respect to Chaetognatha, and we therefore suggest that Amiskwia is best interpreted as a stem chaetognath, but crown gnathiferan. The interrelationships of chaetognaths, or arrow worms, have been a matter of a long-standing debate. Here, Vinther and Parry demonstrate that the Middle Cambrian Burgess Shale fossil Amiskwia possesses a chaetognath bodyplan, but a jaw apparatus reminiscent of gnathiferans. This suggests that chaetognaths belong to Gnathifera within Spiralia.
- burgess shale
- Cambrian Explosion