// Workers AI · dad joke modeWhat did Castle Bank say to the customer? You're withdrawn.

Castle Bank
Stratigraphic range: Middle Ordovician ~ Trem. Flo. D Darriwilian San. Kat. H
Diagrammatic reconstruction of Mieridduryn bonniae, an opabiniid-like panarthropod from Castle Bank
Type	Formation
Unit of	Gilwern Volcanic Formation
Thickness	10 m (33 ft)
Lithology
Primary	Basalt
Other	Dacite, Rhyolite, Mudstone, Sandstone, Conglomerate
Location
Region	Powys
Country	Wales
Type section
Named by	Botting et al.
Year defined	2023

Castle Bank is a Middle Ordovician aged fossiliferous deposit, which is found outcropping within the Gilwern Volcanic Formation of the wider Builth Volcanic Group, near to Llandrindod Wells in Mid Wales. Considered to be a Konservat-Lagerstätte, it contains rare examples of Burgess Shale-type fauna, such as the opabiniid-like panarthropod Mieridduryn bonniae.

Fossil discoveries

When 2020 COVID-19 lockdowns prevented Llandrindod residents and researchers for Amgueddfa Cymru – Museum Wales, Joe Botting and Lucy Muir, from travelling, they reinvestigated a site they had discovered a decade earlier in a nearby sheep field. They had failed to find the site interesting at the time because it was just a 10-meter-wide quarry. More careful examination has revealed an exceptional find among fossils. So far, 170 species have been found, among them more than 30 panarthropods and what resembles a marine relative of insects.

Being from the middle Ordovician, the fossils coincide with the Great Ordovician Biodiversification Event. What makes these fossils different from those found in places such as the Cambrian fossils of the Burgess Shale is their small size. Many forms that have been found in the Castle Bank deposit are of animals who had gone through a miniaturization trend when compared to Cambrian animals found among the Burgess Shale fossils, making the biota in the Castle Bank deposit more similar to modern faunas.

Their reduced size makes it difficult to identify some of the species, as most of these fossils are just between 1 and 5 mm in body size. Nonetheless, the fossils are exceptionally well preserved, with details as small as 10 μm often being visible, including tiny limbs, digestive systems, nerves, and filter-feeding tentacles. Such details also exist among certain Cambrian fossils, but have been absent from Ordovician finds that have been discovered to date.