A giant stem-group chaetognath.

Abstract

Chaetognaths, with their characteristic grasping spines, are the oldest known pelagic predators, found in the lowest Cambrian (Terreneuvian). Here, we describe a large stem chaetognath, <i>Timorebestia koprii</i> gen. et sp. nov., from the lower Cambrian Sirius Passet Lagerstätte, which exhibits lateral and caudal fins, a distinct head region with long antennae and a jaw apparatus similar to <i>Amiskwia sagittiformis</i>. <i>Amiskwia</i> has previously been interpreted as a total-group chaetognathiferan, as either a stem-chaetognath or gnathostomulid. We show that <i>T. koprii</i> shares a ventral ganglion with chaetognaths to the exclusion of other animal groups, firmly placing these fossils on the chaetognath stem. The large size (up to 30 cm) and gut contents in <i>T. koprii</i> suggest that early chaetognaths occupied a higher trophic position in pelagic food chains than today.