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Surprising things you can learn from unremarkable looking fossils

Posted by Chris Andrew on

Some of the most interesting stories come from the most unremarkable looking fossils..

People always like big spectacular fossils, entire ichthyosaur skeletons and blocks full of ammonites. These are wonderful finds, but you are unlikely to make them unless you can get to good sites regularly and know exactly what you are doing. Do not despair! Small unremarkable looking fossils can tell amazing and interesting stories (often more than some of the spectacular fossils). More years ago than I care to remember I used to collect at Salthill Quarry near Clitheroe in Lancashire. This site is Carboniferous, Visean in age. One of the most common fossils at the site are several species of crinoid. You can find complete heads of the crinoids but more commonly you find sections and ossicles of the stems. These are jumbled up with fragments of brachiopod shells, corals, and other fossils.

Among these debris you find branches of a coral called Cladochonous that have wound around crinoid stems. This would make sense as the crinoid stems would help support the weight of the coral. This was noticed by early collectors, but these noticed something else as well. Some crinoids had strange swellings which formed lumps on the crinoid’s stem.

If you look closely at these swellings you can see the branches of the coral protruding out. It seems that the coral caused a response in the crinoid causing it to try to grow over and smother the coral. Some swollen crinoid stems have no sign of the coral on the outside, but if sectioned they reveal the coral inside.

The conclusion is that Cladochonous is a coral that is parasitic on crinoid stems. This is a wonderful story and remarkable to be able to work out after 340 million years. I certainly did not work this out myself, I read about it on leaflets about the site. This allowed me to find some unremarkable small pieces of carboniferous limestone that tell a remarkable story.

Preparation

The pieces I collected came from an eroded slope. The erosion had softened the matrix and revealed part of the fossil. When originally collected I removed some matrix with a mounted needle to reveal more of the structure. I have since further prepared these using the Microraptor air pen for detail and a bit of air-abrasive. I used aluminium oxide as the abrasive powder, probably a bit harsh (my excuse is lockdown made life difficult!).

While on the subject of crinoids

A second fossil worth mentioning is a small Carboniferous gastropod called Platyceras. It looks a bit like a tall limpet with a slight twist at the top of its shell. Platyceras is thought to have lived on the Calyx of a crinoid and fed on its fecal matter (poo!). This specimen is interesting because it the head of a crinoid (Amphoracrinus gilbertsoni) with a specimen of Platyceras in feeding position (below). 


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