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Curr Biol. 2019 Mar 4;29(5):881-888.e1. doi: 10.1016/j.cub.2019.01.052. Epub 2019 Feb 21.

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

Author information

1
School of Earth Sciences, University of Bristol, Life Sciences Building, 24 Tyndall Avenue, Bristol BS8 1TQ, UK; School of Biological Sciences, University of Bristol, Life Sciences Building, 24 Tyndall Avenue, Bristol BS8 1TQ, UK. Electronic address: jakob.vinther@bristol.ac.uk.
2
School of Earth Sciences, University of Bristol, Life Sciences Building, 24 Tyndall Avenue, Bristol BS8 1TQ, UK; Department of Geology and Geophysics, Yale University, 210 Whitney Ave., New Haven, Connecticut 06511, USA.

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.

KEYWORDS:

Burgess Shale; Cambrian explosion; Chaetognatha; Gnathifera; Lophotrochozoa; Spiralia

PMID:
30799238
DOI:
10.1016/j.cub.2019.01.052

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