Advances in dating and systematics have prompted a revision of monotreme evolution to refine the timing of adaptative trends affecting body size and craniodental morphology. The oldest known monotreme, Teinolophos trusleri, is restricted to uppermost Barremian deposits of the Strzelecki Group in Victoria, Australia. Its body mass is estimated at ∼40 g, making it the smallest known monotreme. Teinolophos trusleri likely possessed an electro-sensitive and/or mechano-sensitive ‘bill’ or ‘beak’, which we suggest evolved for insectivory in seasonally dark Early Cretaceous polar forests. During the early Albian–mid-Cenomanian, monotremes diversified in Australia and evolved body masses greater than 4 kg, becoming amongst the largest Mesozoic mammals. A gap of 35 million years subsequently separates the youngest Mesozoic monotremes from the oldest Cenozoic monotreme, Monotrematum sudamericanum, which is a Paleocene stem ornithorhynchid from southern South America. We also hypothesize that tachyglossids originated in Melanesia, perhaps on the emergent Vogelkop landmass, and then dispersed to Australia during the Pliocene-Pleistocene. Finally, we present a classification of Monotremata to include five families—Teinolophidae fam. nov., Kollikodontidae, Steropodontidae, Ornithorhynchidae, and Tachyglossidae. We also propose a new genus, Murrayglossus gen. nov. for a gigantic Pleistocene echidna from southwestern Western Australia.
Timothy F. Flannery [email@example.com], Kristofer M. Helgen [Kris.Helgen@Australian.Museum], Australian Museum, 1 William St Sydney 2000, Australia; Thomas H. Rich [firstname.lastname@example.org], Tim Ziegler [email@example.com], Museums Victoria, PO Box 666, Melbourne, Victoria 3001, Australia; Patricia Vickers-Rich [firstname.lastname@example.org; email@example.com], School of Earth, Atmosphere and Environment, Monash University, Victoria 3800, Australia; Swinburne University of Science and Technology, Department of Chemistry and Biotechnology, Hawthorn, Victoria 3122, Australia; Elizabeth Grace Veatch [firstname.lastname@example.org], National Museum of Natural History, Smithsonian Institution, Washington, DC 20013, USA.