The highly-diverse Middle Ordovician fossil site at Castle Bank, Wales, is directly comparable with the famous Burgess Shale and Chengjiang biotas in paleoenvironment and preservational style. The site is nearly 462 million years old and includes animals with morphologies similar to the iconic Cambrian species Opabinia, Yohoia and Wiwaxia, combined with early examples of more derived groups.
Reconstruction of the Castle Bank community; sponge in foreground is 2 cm tall. Image credit: Botting et al., doi: 10.1038/s41559-023-02038-4.
The Castle Bank fossil site was discovered in 2020 by Amgueddfa Cymru – National Museum Wales researchers Joe Botting and Lucy Muir near Llandrindod, central Wales, the United Kingdom.
During the Ordovician period, the area was a volcanic island complex within the Welsh Basin, which was part of the microcontinent Avalonia that occupied temperate southern latitudes.
The Castle Bank fauna rivals the best Cambrian deposits in abundance, labile tissue preservation and fine detail of the fossils but reveals major changes in ecology.
It includes a diverse suite of soft-bodied and cuticularized organisms (including many major groups), preserved as carbonaceous remains with fine detail of even delicate labile tissue.
Despite most fossils being relatively small, between 1 and 5 mm in body size, the fidelity of preservation allows recognition of extremely fine structures within many specimens, with details as small as 10 μm being commonly visible.
“The estimated total diversity of species recovered so far exceeds 170 species, including 35-40 sponges, over 30 panarthropods and a broad range of other major groups,” the paleontologists said.
“About 30 of these are currently undetermined but recognizably distinct.”
“In some animals, internal organs such as digestive systems and even nerves are preserved, together with the limbs of tiny arthropods and delicate filter-feeding tentacles.”
“Such exquisite detail is known from the best Cambrian faunas, but not previously from the Ordovician.”
“The range of fossils includes several unusual discoveries, from unexpectedly late examples of Cambrian animals looking like opabiniids (weird proto-arthropods with a long proboscis) and wiwaxiids (slug-like mollusks armored with scales), to tantalizing, unexpectedly early fossils that resemble modern goose barnacles, cephalocarid shrimps (which have no fossil record at all) and possibly even a marine relative of insects.”
Middle Ordovician fossils from the Castle Bank site. Image credit: Botting et al., doi: 10.1038/s41559-023-02038-4.
“The Cambrian witnessed the origin of the major animal groups,” they added.
“The Ordovician was a critical time in the history of life as well, with an extraordinary diversification of animals that produced hard skeletons and abundant fossils.”
“In addition, more familiar ecosystems like today’s coral reefs appeared by the end of the Ordovician. Until now, however, a big gap has existed between these two evolutionary events.”
“A new Burgess Shale-type fauna from the middle of this interval will help close this gap by answering questions about the animal shift from Cambrian fauna to Paleozoic fauna and about the shift in ecosystems from the Cambrian style (which were similar across much of the world) to the much more diversified ecology we see today.”
The discovery is reported in a paper in the journal Nature Ecology & Evolution.
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J.P. Botting et al. A Middle Ordovician Burgess Shale-type fauna from Castle Bank, Wales (UK). Nat Ecol Evol, published online May 1, 2023; doi: 10.1038/s41559-023-02038-4
