Possible metazoan egg fossils from the Darriwilian (Middle Ordovician) of Baltoscandia

The Baltoscandian lower Palaeozoic succession is well known for the abundance, diversity and excellent preservation of various groups of microfossils, such as acritarchs, chitinozoans, scolecodonts and conodonts. This paper describes a new enigmatic Middle Ordovician organic-walled microfossil Vikisphaera kundanagen. et sp. nov., characterized by a dark brown to black hollow spherical shell, 90–200 μm in diameter, with an equatorial groove and lack of openings. The specimens of Vikisphaera have been collected from several localities across Baltoscandia including Estonia, Sweden, Latvia and northwestern Russia, and their stratigraphic range is restricted to the Kunda Regional Stage, lower Darriwilian. The wide geographical distribution and short temporal range of Vikisphaera imply its biostratigraphic utility. The shape and characteristics of the shell wall of Vikisphaera are different from those of prasinophytes and other organic-walled microfossils common in lower Palaeozoic rocks. The new microfossil resembles egg shells of some invertebrates, such as arthropods, suggesting that it could represent egg capsules of a group of marine metazoans, possibly soft-bodied creatures with low fossilization potential. However, before the metazoan origin of Vikisphaera is confirmed, it can be considered as an acritarch – an organic-walled microfossil of unknown affinity.