The palaeontology and taphonomy of the Soom Shale: An Upper Ordovician lagerstatte, South Africa.

The Soom Shale lagerstatte is unusual in three respects: it is Ordovician in age, a time when there is a dearth of other conservation deposits; it was deposited at a high latitude under glacial influences; and, the mode of preservation of hard and soft tissues is unique. The Soom Shale is a finely-laminated black shale and contains a highly restricted marine biota. Several taxa preserve soft tissues, notably the giant conodont Promissum pulchrum Kovacs which preserves muscle tissues through replacement by clay minerals to the subcellular level, a level of detail which has only previously been reported in phosphatized soft tissues. Replacement of soft tissues by clays occurred either directly, or indirectly through a phosphate precursor. Bottom and pore water conditions were dominantly anoxic-euxinic and without carbonate to buffer the pH, the biominerals aragonite, calcite and apatite were rapidly dissolved. The extremely low pH environment may have also militated against the authigenesis of phosphate indicating that direct replacement of soft tissues by colloidal clays was the most likely taphonomic route. Recalcitrant biomolecules such as chitin and scleratin show either complete replacement or coatings of clay minerals, suggesting the reactivity of the substrate was important in controlling mineralization. Longicone orthocones in the shale are associated with Ungulate brachiopods, ostracodes and cornulitids. Orthocone-epibiont relationships demonstrate that the size distribution of epibionts on the host may determine the timing of epibiont attachment. The orbiculoid epibionts on one orthocone attached whilst the host was alive as did cornulitids, which have grown aligned to a forward head-on orthocone swimming direction. Three orthocones preserve radulae which represent the oldest in the fossil record and have a tooth configuration more comparable with coleoids and ammonoids than with nautiloids