Microbiotheria Ameghino, 1889

Beck, Robin M. D., Voss, Robert S. & Jansa, Sharon A., 2022, Craniodental Morphology And Phylogeny Of Marsupials, Bulletin of the American Museum of Natural History 2022 (457), pp. 1-353 : 224

Microbiotheria Ameghino, 1889

CONTENTS: Dromiciops (fig. 43) and † Microbiotherium .

STEM AGE: 45.6 Mya (95% HPD: 41.4–48.8 Mya).

CROWN AGE: 17.3 Mya (95% HPD: 14.0–21.8 Mya).

UNAMBIGUOUS CRANIODENTAL SYNAPOMOR- PHIES: Basisphenoid with a distinct sagittal keel (char. 48: 0→1; ci = 1.000); auditory bulla large,

33 But see Kealy and Beck (2017: 15) regarding anomalous relationships recovered in the molecular analysis of May-Collado et al. (2015).

contacting rostral process of petrosal (char. 55: 1→2; ci = 0.300); caudal and rostral tympanic processes of petrosal seamlessly fused, forming a petrosal plate (char. 68: 0→2; ci = 0.154); and five upper incisors present (char. 103: 2→0; ci = 0.333).

COMMENTS: Of the four unambiguous morphological synapomorphies supporting monophyly of Microbiotheria (equivalent to Microbiotheriidae in our analyses), two occur homoplastically in other marsupial groups and one appears questionable because it is a reversal in the number of upper incisors from three back to the ancestral metatherian complement of five. However, presence of a basiphenoid with a distinct sagittal keel is a distinctive cranial apomorphy that is apparently unique to microbiotherians (Hershkovitz, 1992a; 1999; Giannini et al., 2004; Wible et al., 2018). Although several genera of fossil microbiotherians are known from South America (Marshall, 1982; Goin and Abello, 2013; Goin et al., 2016), all except † Microbiotherium (for which cranial material, including a specimen [MACN A 8505] preserving the part of the basisphenoid sagittal keel, is known; Segall, 1969b; Marshall, 1982) are known from dental remains only.

As already discussed (see Australidelphia above), the oldest definitive member of the order described to date is probably the woodburnodontid † Woodburnodon casei from the Cucullaea I Allomember of the La Meseta Formation, Seymour Island, off the Antarctic Penninsula. Based on current evidence, the age of † W. casei appears to be ~40 Mya (Douglas et al., 2014; Amenábar et al., 2019; Mörs et al., 2020). Most recent studies have concluded that the early or middle Palaeocene † Khasia cordillerensis is not a member of Microbiotheria ( Oliveira and Goin, 2006; Goin et al., 2016; Carneiro et al., 2018; but see Muizon et al., 2018), and putative microbiotherian specimens reported from the early Eocene Las Flores Fauna (Goin, 2003; Zimicz, 2012; Woodburne et al., 2014a; Goin et al., 2016) have yet to be formally described (see Australidelphia above).