1. Department of Ecology and Evolutionary Biology; University of Toronto; 25 Willcocks St. Toronto ON M5S 3B2 Canada
2. Palaeobiology Programme; Department of Earth Sciences; Uppsala University; Villavägen 16 SE-752 36 Uppsala Sweden
3. Subdepartment of Evolution and Development; Department of Organismal Biology; Uppsala University; Norbyvägen 18A SE-752 36 Uppsala Sweden
4. Department of Natural History; Royal Ontario Museum; 100 Queen's Park Toronto ON M5S 2C6 Canada
5. Royal Tyrrell Museum of Palaeontology; PO Box 7500 Drumheller AB T0J 0Y0 Canada
6. Department of Paleobiology; Smithsonian Institution; National Museum of Natural History; PO Box 37012 MRC 121 20013-7012 Washington DC USA