The authors would like to thank N. Schovsbo, Geological Museum, The Natural History Museum of Denmark, University of Copenhagen, Denmark, for his very helpful revision and comments on the manuscript. Furthermore, Ø. Hammer, Geological Museum, University of Oslo, has helped with analytical suggestions. C. Scotese, PALEOMAP kindly allowed two of his palaeoreconstructions to be reproduced. We thank two anonymous reviewers for their constructive and useful comments. This paper is a contribution to IGCP 503.