The Osteology and Systematics of the Enigmatic Australian Oligo-Miocene Metatherian Yalkaparidon (Yalkaparidontidae; Yalkaparidontia; ?Australidelphia; Marsupialia)

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We provide the first detailed description of the osteology of the enigmatic Oligo-Miocene Australian metatherian Yalkaparidon. This taxon exhibits a number of unusual craniodental apomorphies but appears to be plesiomorphic within Metatheria in retaining four molars, rather than three as previously reported. We demonstrate that the only known skull of Yalkaparidon almost certainly represents a single individual. We also tentatively refer a number of isolated tarsals to the genus. Maximum parsimony analyses of a 258 character morphological matrix (with information from the tarsals described here either included or excluded) place Yalkaparidon within the superordinal clade Australidelphia, but Bayesian analyses of the same matrix are less well resolved, placing Yalkaparidon within Marsupialia but without unequivocally supporting australidelphian affinities. Bayesian analyses of a total evidence matrix that combines the morphological data with 9 kb of sequence data from five nuclear protein-coding genes (APOB, BRCA1, IRBP, RAG1 and VWF), 78 indels, and 53 retroposon insertion characters are similarly poorly resolved and do not clarify the supraordinal relationships of Yalkaparidon beyond suggesting that it is probably a member of Marsupialia. However, if the tarsal remains are correctly attributed to Yalkaparidon, then membership of Australidelphia seems likely, as these specimens exhibit characteristic australidelphian apomorphies. We conclude that the ordinal status of Yalkaparidon remains justified based on current evidence, and we present a revised diagnosis for Yalkaparidontia. We maintain the two currently recognized species, Y. coheni and Y. jonesi, but present revised specific diagnoses. We suggest a revised phylogenetic definition for Marsupialia, and provide phylogenetic definitions for Eomarsupialia (the clade comprising all extant Australian marsupial orders) and for the clade comprising Dasyuromorphia, Peramelemorphia, and Notoryctemorphia to the exclusion of Diprotodontia; we propose the name Agreodontia for the latter clade.