SYDNEY, June 15 -- For most of the time scientists have studied how Australia filled with marsupials, the story has run in a more or less straight line: a founding group arrived, then branched into the kangaroos, koalas, possums and carnivores alive today. A handful of teeth pulled from the limestone of north-west Queensland now suggests that line was never straight at all.

Writing in the Journal of Paleontology, University of New South Wales palaeontologist Tim Churchill and colleagues describe three small, insect-eating species recovered from the Riversleigh World Heritage Area, a fossil site so rich it has rewritten Australian natural history several times over. The animals were not closely related to the other marsupials sharing thei...