Abstract
Diprotodon Owen, was one of the first fossil mammals described from Australia and has the distinction of being the largest ever marsupial. However, until recently its taxonomy was unclear and knowledge of its continental distribution, palaeoecology and evolution was poorly known. This paper describes cranial elements from an unusually small‐bodied Diprotodon from the Early Pleistocene Nelson Bay Formation, Portland, Australia. It is intermediate in size between the smaller‐bodied Pliocene ancestor, Euryzygoma dunense de Vis 1895 and the larger‐bodied Late Pleistocene D. optatum Owen, . However, it is morphologically inseparable from Late Pleistocene Diprotodon and is here referred to as D. ?optatum . A temporal morphocline most parsimoniously explains the medium body size of the Nelson Bay taxon; thus we provide the first evidence of a transitional form within Diprotodon . The cause(s) of gigantism of the E. dunense – D. ?optatum – D. optatum lineage through the late Cainozoic is unclear but most likely involved the physiological response to changes in the physical and/or biotic environment. Although the size changes within the lineage may have been initially advantageous, they were most likely disadvantageous when the optimum body size was obtained. Our results suggest that Diprotodon was at its largest ever body size during the Late Pleistocene – the time period that coincided with its extinction. Copyright © 2009 John Wiley & Sons, Ltd.