Spat fall and mature growth of large-shell brachiopods (Stricklandia lens lens) from the Solvik Formation (lower Silurian) at Sandvika in the Oslo Region of southern Norway

A population of large-shell brachiopods belonging to Stricklandia lens lens from the lower Silurian Solvik Formation is exposed in a capstone layer covering approximately 300 m2 on steeply dipping strata along the shores of Engervannet at Sandvika in the Oslo Region of southern Norway. Based on sample counts, a density estimated at 250 to 350 articulated shells per square meter resulted from crowding during growth. Given that the former sea bed on which the brachiopods settled can be traced over a lateral distance of 60 m with an average width of 5 m, it is calculated that a monospecific population exceeding 90,000 brachiopods matured from a single, extensive spat fall. Related large-shell brachiopods from the lower Silurian of North America, including Virgiana decussata, Pentamerus oblongus, P. palaformis, and Pentameroides subrectus are commonly preserved in growth position, but observed pavements rarely exceed more than 0.25 m2 in size. Articulated shells within those populations are almost always truncated midway above the beak and hinge area due to post-mortem erosion by submarine scour. In contrast, the horizon with Stricklandia shells from Sandvika is enormous in scope and features many whole individuals pushed into a more recumbent position in the same direction by a marine current. Re-orientation of the steeply inclined limestone layer to a horizontal position suggests that such a current flowed from the present-day southwest to the northeast as it swept over the Silurian sea bed.