MARSUPIALIA

Benton, MJ, Donoghue, PCJ, Vinther, J, Asher, RJ, Friedman, M & Near, TJ, 2015, Constraints on the timescale of animal evolutionary history, Palaeontologia Electronica (Florence, Italy) 15 (1), pp. 1-107 : 60

CROWN MARSUPIALIA (58)

Node Calibrated. The opossum-kangaroo split is equivalent to the divergence of the two main crown marsupial clades: Ameridelphia and Australidelphia.

Fossil Taxon and Specimen. Djarthia murgonensis (Queensland Museum, QM F52748; Beck 2012), from the early Eocene of Murgon , Australia .

Phylogenetic Justification. Djarthia possesses a continuous lower ankle joint, a synapomorphy shared with the marsupial clade Australidelphia (Beck, 2012). The status of Djarthia as a crown marsupial is stronger than that for Khasia (Beck et al., 2008; Beck, 2012), and thus it serves as a more definitive record of the latest point by which crown marsupials evolved.

Minimum Age. 47.6 Ma

Soft Maximum Age. 131.3 Ma

Age Justification. The early Eocene locality of Murgon, source of Djarthia , correlate with the Ypresian marine stage (Beck, 2012), the top of which is 47.8 Ma ± 0.2 Myr = 47.6 Ma ( Gradstein et al., 2012), which we use as the paleontological minimum constraint for Marsupialia .

Numerous metatherian remains are known from the Cretaceous of both North America ( Kokopellia ; Cifelli, 1993, 1998) and Mongolia ( Asiatherium ; Szalay and Trofimov, 1996), with the Early Cretaceous Sinodelphys szalayi being the oldest undisputed metatherian (Luo et al., 2003). Hence, we place the soft maximum constraint for Marsupialia at the Liaoning beds that produced Sinodelphys , correlating to the Barremian (Zhou et al., 2003) with a lower bound of 130.8 Ma ± 0.5 Myr = 131.3 Ma.

Discussion. There are other metatherians from the Cretaceous, such as Kokopellia and Sinodelphys , but these fall phylogenetically outside the marsupial crown radiation (e.g., Luo et al., 2002). Indeed, non-marsupial metatherians persist well into the Tertiary and, despite close dental similarities with crown didelphids (e.g., Herpetotherium ), they fall outside Marsupialia when examined in a cladistic context ( Goin et al., 2006; Sánchez et al., 2007; Horovitz et al., 2009; but see Beck, 2012). Hence, the oldest potential marsupials are from the Tiupampa fauna of Bolivia (Muizon and Cifelli, 2001), close to 63 Ma. Specifically, the Tiupampan genus Khasia has been reconstructed as having a possible relationship to microbiotheres ( Goin et al., 2006), a radiation for which the only extant genus is Dromiciops . Importantly, microbiotheres have been phylogenetically linked with Australidelphia based on both morphological and molecular data (e.g., Szalay, 1982; Horovitz et al., 2009; Meredith et al., 2011; Beck, 2012). Other elements of the Tiupampa fauna, including the skeletally well known Pucadelphys and Mayulestes , are not demonstrably part of crown Marsupialia ( Horovitz et al., 2009; Beck, 2012). The early Paleocene taxon Cocatherium from Chubut province, Argentina is also likely outside crown Marsupialia ( Goin et al., 2006) .