Morphological variation within an individual Pleistocene Diprotodon optatum Owen, 1838 (Diprotodontinae; Marsupialia): implications for taxonomy within diprotodontoids GILBERT J. PRICE AND IAN H. SOBBE PRICE, G.J. & SOBBE, I.H., March, 2011. Morphological variation within an individual Pleistocene Diprotodon optatum Owen, 1838 (Diprotodontinae; Marsupialia): implications for taxonomy within diprotodontoids. Alcheringa 35, 21–29. ISSN 0311-5518. A comprehensive understanding of morphological and morphometrical variation for systematically important characters is a prerequisite for reliable taxonomic analyses of extinct morphospecies. However, perception of such variation is commonly limited due to a paucity of fossil material that forms the basis of such analyses. Here we report a new record of Pleistocene Diprotodon optatum Owen, 1838 (Diprotodontoidea) represented by near-complete upper tooth rows that exhibit a high degree of individual variation within systematically important upper premolars. The individual possesses features (e.g. development of parastyle, anterocristae and anterior longitudinal groove) that are considered to be close to the polar extremes of premolar variation within the morphospecies, but occurring on either side of the palate. Although such morphologies were previously recognized on the basis of isolated specimens, they have never been observed as having occurred on either side of the tooth row within a single indvidual. The observation of such extreme variation in premolar form within a single individual means that although the diprotodontoid P 3 may serve some systematic importance in distinguishing certain taxa, the weight placed upon perceived differences within any single character of the premolar must be gauged by an understanding of broader variation within the group. Gilbert J. Price [g.price1@uq.edu.au] and Ian H. Sobbe [iansobbe@bigpond.com], Radiogenic Isotope Facility, Centre for Microscopy and Microanalysis, The University of Queensland, St. Lucia, Queensland, 4072, Australia. Received 10.9.2009; revised 13.1.2010; accepted 15.1.2010. Key words: Diprotodon optatum, Diprotodontoidea, morphological variation, premolar, megafauna, Darling Downs, Pleistocene. MANY members of the Diprotodontoidea (now-extinct, middle to late Cenozoic wom- bat-like marsupials) exhibit a high degree of intraspecific variation, particularly with respect to systematically important premo- lar morphology (Glauert 1921, Longman 1921, Stirton et al. 1967, Hand et al. 1993, Black & Archer 1997, Murray et al. 2000, Price 2008, Price & Piper 2009). Such variation has significant implications for the taxonomy of poorly known morphos- pecies. For example, previously there were eight described species of the Pleistocene Diprotodon Owen, 1838 (Mahoney & Ride 1975). Species such as D. optatum Owen, 1838, D. australis Owen, 1844 and D. minor Huxley, 1862, were defined in part, on what were considered to be significant differences in third upper premolar morphology (Hux- ley 1862). Price (2008) recently explored the range of individual morphological and morphometrical variation in several large Diprotodon assemblages. That study was based primarily on isolated maxillary and mandibular fragments, and isolated teeth. The results suggested that the range of ISSN 0311-5518 (print)/ISSN 1752-0754 (online) Ó 2011 Association of Australasian Palaeontologists DOI: 10.1080/03115511003793553 Downloaded By: [University of Queensland] At: 01:32 14 March 2011