Bilateral Jaw Elements in Amiskwia sagittiformis Bridge the Morphological Gap between Gnathiferans and Chaetognaths

Jakob Vinther*, Luke Parry

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

6 Citations (Scopus)
76 Downloads (Pure)

Abstract

Amiskwia sagittiformis Walcott 1911 is an iconic soft-bodied taxon from the Burgess Shale [1–3]. It was originally interpreted as a chaetognath [1], but it was later interpreted as a pelagic nemertean [2] or considered of uncertain affinity [3]. Part of this ambiguity is due to direct comparisons with members of the crown groups of extant phyla [4] 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 [5], which is congruent with a shared unique Hox gene that suggests a close relationship between Gnathifera and Chaetognatha [6]. Molecular phylogenetic studies have recently found gnathiferans to be a deep branch of Spiralia and Chaetognaths either a sister group to Spiralia [7] 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.

Original languageEnglish
Pages (from-to)881-888.e1
Number of pages9
JournalCurrent Biology
Volume29
Issue number5
Early online date21 Feb 2019
DOIs
Publication statusPublished - 4 Mar 2019

Keywords

  • Chaetognatha
  • burgess shale
  • Cambrian Explosion
  • gnathifera
  • spiralia
  • lophotrochozoa

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