1. 1 Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Campus Drive, Saskatoon SK S7N 5E2, Canada
2. 2 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
3. 3 Department of Earth Sciences, Palaeobiology, Uppsala University, Geocentrum, Villav. 16, 75236 Uppsala, Sweden