Early Ordovician molluscs with an exceptional preservation from the Timan-Pechora Basin of Russia

In the Timan-Pechora Basin of northeast Baltica, the Bol’shepul’skaya-1 borehole penetrated coarse-grained sandstones unconformably overlying Riphean metamorphic rocks at a depth of 1587 m. The limestone lenses, 50 meters above these unfossiliferous sandstones and grey mudstones, yield abundant monoplacophoran molluscs with preserved colour pattern. Higher in the drill-hole are siltstones intercalated with limestones with rare acrotretid brachiopods and conodonts that suggest a late Tremadocian age for these strata.

Collection of monoplacophoran molluscs is represented by more than fifteen specimens with preserved colour pattern. Several specimens show muscle scars. The shells are obviously been sorted and transported but not far from the habitat due to an excellent preservation. Most of shells have a rather uniform size about 1 cm, though several, cut by borehole, shells exceeded two centimetres in size.

Muscle scars arranged in circum-apical ring, with the two smallest pair of rounded scars on the supra-apical slope closer to the apex. The third to fifth (from the medium line of the supra-apical slope) pairs are tightly fused. The sixth to eighth elongated pairs are approximately the same size.

The colour pattern consists of eight well preserved pairs of radial brown bands. These bands vary in breadth. Two pairs of narrow bands on the supra-apical slope of the shell corresponds with the least muscle scars that situated closer to the apex. A pair of the widest bands obviously related to the muscle scars of fused retractors. The sub-apical area consists a wide light-brown sector with two pare of darker radial narrow strips that probably correspond with a sub-apical muscle field that is often obscured in a coarsely preserved fossil material.