Possible drill holes and pseudoborings in obolid shells from the Cambrian/Ordovician boundary beds of Estonia and the uppermost Cambrian of NW Russia

ABSTRACT

Rare Oichnus simplex drill holes occur in mature obolid shells from the Cambrian/Ordovician boundary beds of northern Estonia (Iru and Ülgase) and the uppermost Cambrian of NW Russia (Lava River). The drill holes are significantly more common in the central rather than the marginal regions of the obolid valves. Drilling predators attacked Ungula ingrica in the Kallavere Formation and Ungula convexa in the Ladoga Formation. Failed predatory attacks on obolids were relatively common in the latest Cambrian-earliest Tremadocian of Estonia. Presumably drilling predators at Lava River and Iru differed from those at the Ülgase as indicated by significant differences in drill hole sizes at these locations. Most likely some types of worms preyed on obolids in the latest Cambrian-earliest Tremadocian of Estonia and latest Cambrian of NW Russia. The predation rate (6% to 9%) of studied obolids indicates that they likely had an epibenthic life mode. In addition to Oichnus drill holes in the obolids, there are also common pseudoborings caused by mineral dissolution.

KEYWORDS:

• Brachiopods
• apatite
• predation
• bioerosion
• trace fossils
• Furongian
• Tremadocian
• Baltica

Acknowledgments

Financial support to O.V. and U.T. was provided by a Paleontological Society Sepkoski Grant and Estonian Research Council grants (IUT20-34 and PRG836). Financial support to L. E. H. was provided by the Swedish Research Council (VR Project no. 2018-03390). We are grateful to G. Baranov, Department of Geology, Tallinn University of Technology for digital photographing of the specimen and H. Suomalainen for collecting samples from Ülgase mine. This paper is a contribution to the IGCP project 653 ‘The Onset of the Great Ordovician Biodiversification Event’.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the Eesti Teadusagentuur [IUT 20-34 and PRG 836]; Vetenskapsrådet [2018-03390].