Rhynie chert

The Rhynie chert is a Lower Devonian sedimentary deposit exhibiting extraordinary fossil detail or completeness (a Lagerstätte). It is exposed near the village of Rhynie, Aberdeenshire, Scotland; a second unit, the Windyfield chert, is located some 700 m away. The Rhynie chert contains exceptionally preserved plant, fungus, lichen and animal material preserved in place by an overlying volcanic deposit. The bulk of the Devonian fossil bed consists of primitive plants (which had water-conducting cells and sporangia, but no true leaves), along with arthropods, lichens, algae and fungi. This fossil bed is remarkable for two reasons. First, the age of the site (Pragian, Early Devonian, formed about ) places it at an early stage in the colonisation of land. Second, these cherts are famous for their exceptional state of ultrastructural preservation, with individual cell walls easily visible in polished specimens. Stomata have been counted and lignin remnants detected in the plant material, and the breathing apparatus of trigonotarbids—of the class Arachnida—(known as book lungs) can be seen in cross-sections. Fungal hyphae can be seen entering plant material, acting as decomposers and mycorrhizal symbionts. The bed is under at least 1 metre of overburden, in a small field near the village of Rhynie, so is effectively inaccessible to collectors; besides which, the site is a Site of Special Scientific Interest. A second unit, the Windyfield chert, is some 700 m from the Rhynie. The Rhynie chert extends for at least 80 m along strike and 90 m down-dip. The chert was discovered by William Mackie while mapping the western margin of the Rhynie basin in 1910–1913. Trenches were cut into the chert at the end of this period, and Robert Kidston and William Henry Lang worked furiously to describe the plant fossils between 1917 and 1921. The arthropods were examined soon afterwards by different workers. Interest in the chert then waned until the field was reinvigorated by Alexander Geoffrey Lyon in the late 1950s, and new material was collected by further trenching from 1963 to 1971.