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, Nektognathus evasmithae gen. et sp. nov. from the early Cambrian (similar to 519 million years) Sirius Passet Lagerstatte 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 Timorebestia koprii also from Sirius Passet. Nektognathus shares a gnathostomulid-like jaw apparatus, lateral fins, subterminal anus, and large antennae with Timorebestia and Amiskwia, 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.