Melanosclerites from the Öjlemyr Cherts, Gotland

Öjlemyr cherts from the island of Gotland, Sweden, have been known for more than a century and have been stratigraphically determined as Upper Ordovician F1c and F2. These cherts are secondarily-silicified limestone concretions that occur exclusively as glacial erratic boulders (‘geschiebes’). This chert type also contains organic microfossils, of which the so-called melanosclerites have been focussed upon in this study.

The term ‘melanosclerite’ was first introduced by EISENACK (1930) to describe problematic rod-shaped organic microfossils recovered from Ordovician and Silurian glacial erratics from the Baltic region. EISENACK (1930) differentiated two groups of melanosclerites: the first group referring to the main ‘skeleton’, the second representing appendices extending from this. An appendix consists of two main parts, a long proximal section and a rod, ball or tine-shaped distal part. In well-preserved material the wall of the melanosclerite test is smooth, opaque and dark-brown coloured, ranging in size between 60–2000 μm. Melanosclerites can be found in marine sediments from Cambrian to Devonian age. The systematic position and natural relationship of the melanosclerites is still unknown.