Fig. 5.

Long-term decline in geographic distribution and species’ diversity in monotremes and their early descendants. Cretaceous monotremes probably occurred throughout much of eastern Gondwana. By the early Paleocene, ornithorhynchids were geographically as widespread across Gondwana as Patagonia in southern South America. By the late Oligocene/Miocene (25–15 Mya), at least three ornithorhynchids occurred across the continent of Australia but none survived on other continents. Today, the platypus (Ornithorhynchus anatinus) maintains an even more restricted area, the river systems of eastern Australia (modified after Archer 1995; Steropodon image by Peter Schouten; Monotrematum image by James McKinnon—Archer 1995; Obdurodon image by Peter Schouten—Pian et al. 2013; Ornithorhynchus artwork by Rod Scott, Australian Geographic Magazine).