A fossilized ventral ganglion reveals a chaetognath affinity for Cambrian nectocaridids.

Abstract

Nectocaridids are enigmatic Palaeozoic animals with a controversial phylogenetic position. Previous hypotheses have placed them in their own phylum, chordates, molluscs (specifically cephalopods), or radiodont panarthropods. We describe here a nectocaridid, <i>Nektognathus evasmithae</i> gen. et sp. nov. from the early Cambrian (~519 million years) Sirius Passet Lagerstätte of North Greenland. Key specimens preserve paired, phosphatized arcuate structures consistent with preservation of a ventral ganglion, a feature characteristic of extant and fossil chaetognaths, including the amiskwiid <i>Timorebestia koprii</i> also from Sirius Passet. <i>Nektognathus</i> shares a gnathostomulid-like jaw apparatus, lateral fins, subterminal anus, and large antennae with <i>Timorebestia</i> and <i>Amiskwia</i>, placing nectocaridids in the chaetognath stem lineage. The complex sensory anatomy of nectocaridids, which is partially shared with other extinct amiskwiids, highlights a more dynamic predatory lifestyle much higher in the trophic food chain during early chaetognath evolution.