Small faecal pellets in Ordovician shelly fossils from Estonia, Baltoscandia

Coprolites (fossil faeces) constitute a group of soft sediment trace fossils that provide useful palaeoecological and sedimentological information, but have generally low preservational potential. In this paper we report abundant occurrence and high diversity of small faecal pellets preserved inside different shelly fossils from Middle and Upper Ordovician carbonates of the Baltoscandian palaeobasin. The material contains ca 180 body fossils with faecal pellets from 40 localities, corresponding to a range of shallow-marine environments from cool-water carbonate ramp to tropical open shelf settings. Stratigraphically the finds range from the Volkhov to Pirgu regional stages (Dapingian to uppermost Katian). The pellets are elliptical or rod-shaped, 0.1–1.8 mm long and 0.08–0.75 mm in diameter, with the length/diameter ratio ranging from less than 2 to ca 6. They occur in shells of gastropods, bivalves, cephalopods, brachiopods, echinoderms and trilobites and represent two ichnospecies, Coprulus oblongus and Coprulus bacilliformis, and some intermediate forms belonging to the same ichnogenus. Additionally, two compound traces were identified: Tubularina (pellets inside small burrows with circular cross section) and Alcyonidiopsis (pellets inside ribbon-shaped burrows). The pellets were produced when the empty shells were located on the seafloor, or possibly during shallow burial in the oxic zone. The preservation of faecal pellets is due to an interaction of several factors, notably protection by the shells and rapid mineralization. The origin of trace makers remains speculative, but polychaete worms having compatible size and body plan and living representatives who produce similar faecal pellets are among the most likely groups. Possibly organisms with different feeding strategies were involved in producing the faecal pellets. Systematic examination of shelly fossils from selected localities showed that up to about half of the shells may contain pellets, which indicates great abundance and diversity of pellet-producing organisms in the Ordovician Baltoscandian basin. Our material also shows that the trace maker of Arachnostega was not related to the faecal pellets inside the shells.