Timorebestia

Timorebestia koprii is an extinct species of stem-group chaetognath (arrow worm relative) that lived about 520 million years ago, in the Cambrian. Its fossils are known from the Sirius Passet Lagerstätte in Greenland, and it was first described in 2024.

Description
Timorebestia has a wide body marginally surrounded by continuous rayed fins: a pair of lateral fins and a rounded caudal fin. The anterior region is a short head, bearing a pair of antennae as long as half of the body length. The mouth opened below the head, with internal jaw apparatus consisting paired subtriangular elements, blunt anterior elements, and an unpaired anterior, possibly ventral or basal plate. Rows of longitudinal and transverse muscles surround the wide trunk region. The digestive tract terminates at the base of caudal fin. It also has a pair of well-defined ventral ganglia that identifies it as a chaetognath, but was probably more basal to modern arrow worms than Amiskwia.



As of January 2024, 13 known specimens have been assigned to this species, ranging from 2.2 cm to more than 20 cm in body length (the largest specimen had a body length of ~206 mm in addition to 92-mm-long antennae).

Paleoecology
Compared to extant chaetognaths, which are planktonic and rarely grow beyond 10 cm, Timorebestia was much larger, up to 20 cm (30 cm when antennae included), and occupied a higher trophic level. This suggests that back in the Cambrian, chaetognaths constituted an important group of nektonic predators alongside various panarthropod groups like the radiodonts. It lived alongside other nektonic Sirius Passet fauna such as Kerygmachela, Kiisortoqia, Pauloterminus, Isoxys and so on. Some specimens with gut contents suggest it prey on the small and abundant arthropod Isoxys volucris. Timorebestia is thought to be one of the earliest examples of an apex predator.

Phylogeny
The following dendrogram shows the evolutionary relationships of Timorebestia as in Park et al. 2024.


 * A yet-undescribed chaetognath, as of January 2024, from Sirius Passet

Etymology
The genus name is derived from the Latin words timor (fear) and bestia (beast) which translates to "fear-causing beast". The species name is derived from the Korea Polar Research Institute (KOPRI), who has helped with the expeditions to the lagerstätte.