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Golden graptolites

Aberystwyth University in Wales, UK, is nicely located in the middle of a huge fossil-rich Palaeozoic basin. But I struggled as a poor undergraduate student with no car, to get to the best fossils sites in remote locations. So when Dr Denis Bates and Dr Dave Loydell took me and the first-year undergraduate class on our first fieldwork trip to the Rheidol Gorge, the fact that it was within biking distance from Aberystwyth marked it out as somewhere with special potential for me.

When we arrived at the dark, cleaved shale outcrops, deep in the beautiful green mossy gorge, Denis and Dave picked up a few specimens (David Attenborough style) as they explained about graptolites being branches of coral-like polyps called thecae which floated in the Silurian oceans some 440 million years ago. In locations first described by the great Welsh geologist Owen Thomas Jones (known as O T Jones) we found specimens of Monograptus triangulatus, which according to the monographs and range charts we had, proved this to be the Aeronian stage of the Early Llandovery. But unlike most locations where graptolites can be found as flat films, only visible when the light catches them from a certain angle, the specimens in the Rheidol Gorge are perfect 3D replicas of the original beasts in spellbinding pyrite gold.

Mesograptus magnus and Monograptus triangulatus (left) and Petalolithus (right) from the O T Jones locations.

The pyrite fills the thecae completely, and must have been precipitated before the sediment suffered any appreciable compaction, when the graptolites were buried by perhaps only a few centimetres of mud. The organic material of the graptolite has been carbonized, and is still present. On the inside of the graptolite thecae, the pyrite bears moulds of fibrils which were the overlapping bandage-like secretions that the organism used to construct its protective body walls.

O T Jones identified the potential importance of the Rheidol Gorge graptolite locations while mapping the Plynlimon–Ponterwyd area in the 1900s. Sixty years later, he returned there with members of the geology department — and he had total recall of the exposures. In the photograph he is shown in the Plynlimon area, discussing the geology with staff member Nancy Kirk (herself a legendary figure in Welsh geology and in research on graptolites). Detailed sedimentological and mapping work by Richard Cave from the British Geological Survey and graptolite biostratigraphy by David Loydell show how the outcrops within the Rheidol Gorge fit into the wider Silurian turbidite system of mid-Wales.

 O T Jones and Nancy Kirk in the Plynlimon area Mid Wales, circa 1960. Credit: A S G Jones.

O T Jones and Nancy Kirk in the Plynlimon area Mid Wales, circa 1960. Credit: A S G Jones.

I returned to the gorge many times that year, but it was not until the following summer while researching my undergraduate thesis that I found and identified all the recognized species listed by Jones and subsequent researchers. To me the most beautiful forms were the broader genera Diplograptus and Petalolithus, where two strands of the rhabdosome (graptolite body) have their thecae zipped together. The few specimens I found of the delicate Rastrites — that look like fragile strips of eyelashes — always made me think of the spiky Hallucigenia creature from the Burgess Shale. In its way the Rheidol Gorge is a similarly special locality where the preservation of the fossils is truly unique.

Further reading

Cullum, A and D Loydell (2011). The Rhuddanian/Aeronian transition in the Rheidol Gorge, Mid Wales. Proceedings of the Yorkshire Geological Society. 58 (4), 261–266. ISSN 0044-0604, DOI 10.1144/pygs.58.4.299.

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