Animal, vegetable or mineral? : unusual structures from the Dartmouth Group at Wembury Point, South Devon

The rocks that crop out on the foreshore at Wembury Point, near Plymouth belong to the Early Devonian Dartmouth Group. The rocks mainly comprise interbedded silty mudstones and siltstones of lacustrine or distal fluvial origin, and have yielded few fossils: mainly fish fragments along with very sparse examples of the gastropod Bellerophon. However, at Wembury Point, a number of unusual structures in a rosette form are preserved along bedding planes. The preservation of the structures is unusually good and can be seen to take a radial habit with spokes reaching outwards from a central point. Between the spokes connecting ladder like structures are seen. Although these features resemble fossils from organic forms, such as jellyfish or plants, no fossils of similar morphology have been recorded from the Devonian. Detailed studies of the structures themselves, and the surrounding sediments, suggest that these features were most likely to have formed as mineral growths in a continental evaporite sequence.