Symmetroprion spatiosus (Hinde), a jawed polychaete showing preference for reef environments in the Silurian of Gotland

Jawed polychaetes formed an important part of the fauna in the Silurian tropical shelf sea of the Gotland region (Sweden) as shown by the presence of abundant and diversified jaw elements (scolecodonts) in the strata of the island. Fluctuating sea-levels in combination with a variety of bottom sediments produced environments suitable to the benthic annelids. Symmetroprion, a monospecific genus within the Eunicea labidognatha taxa, is recorded hitherto only from the Silurian in the Baltic region. Symmetroprion spatiosus evolved slowly from early Wenlock (Sheinwoodian) through late Ludlow (Ludfordian). It is recorded from 9 of the 13 major stratigraphical units on Gotland. However, it has been found at only about 10% of the studied localities and is not one of the more common species. It is concentrated in the vicinity of some reefs or in areas with isolated stromatoporoids and it has also been found in high abundance in marly pockets in a patch reef and tidal pools. The habitat of S. spatiosus was apparently reefal and lagoonal, where it formed up to 38 % of the fauna.