Ursactis

Ursactis is an extinct genus of Cambrian polychaete annelid.^[1] It was found in the Wuliuan Stage of the Burgess Shale, although most specimens come from a Burgess Shale locality known as the Tokumm Creek in northern Kootenay National Park.^[1] Ursactis is the most abundant Cambrian polychaete found as of 2023.^[1]

Ursactis Temporal range: Wuliuan PreꞒ Ꞓ O S D C P T J K Pg N ↓
Scientific classification
Kingdom:	Animalia
Phylum:	Annelida
Class:	Polychaeta
Order:	incertae sedis
Genus:	†Ursactis Osawa, Caron & Gaines, 2023
Species:	†U. comosa
Binomial name
†Ursactis comosa Osawa, Caron & Gaines, 2023

Description

Ursactis is a small species, 3–15 millimetres (0.1–0.6 in) long, bearing a pair of large palps. It has between 8 and 10 segments, a smaller number than other polychaetes.^[1] Because specimens with 10 segments vary in size significantly and the length of each segment increases with the body length, researchers think Ursactis grew its segments rather adding new ones.^[1] Meanwhile, modern polychaetes add new segments throughout their life. This suggests that annelids began to start adding new segments during their life by the mid-Cambrian.^[1]

See also

• 2023 in paleontology