Thylatheridium cristatum, Reig, 1952

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 : 318-319

† Thylatheridium

SPECIES SCORED: † Thylatheridium cristatum (type species).

GEOLOGICAL PROVENANCE OF SCORED SPECIMENS: Barranca de los Lobos Formation and Chapadmalal Formation (including “Playa Estafeta,” “nivel VI-VII,” “Los Acantilados,” and “Las Palomas”), Mar del Plata, Buenos Aires Province, Argentina.

AGE OF SCORED SPECIMENS: The Barranca de los Lobos Formation is considered to represent the Barrancalobian substage of the Marplatan stage/age (Cione et al., 2015: fig. 2; Beck and Taglioretti, 2020), which is about 2.9–3.3 Mya (Woodburne, 2010: fig. 3; Prevosti and Forasiepi, 2018: table 1.1 ). As noted above (see † Sparassocynus ), the Chapadmalal Formation is thought to fall within the interval from 3.3 to about 5 Mya.

ASSIGNED AGE RANGE: 5.000 –2.900 Mya.

REMARKS: Reig (1952) erected the genus † Thylatheridium and described a single species, † T. cristatum , based on a well-preserved skull (MACN 6442) and additional specimens collected from late Pliocene localities near Chapadmalal, Mar del Plata. Two other species have been named—† T. hudsoni and † T. pascuali (see Reig, 1952; Goin and Montalvo, 1988)—although the latter was described as “dubious” by Forasiepi et al. (2009). † Hesperocynus dolgopolae was also originally described as a species of † Thylatheridium (see † Hesperocynus above). † Thylatheridium has been considered by most authors to be most closely related to the Recent didelphid Monodelphis (Reig, 1952; Reig et al., 1987; Goin and Montalvo, 1988; Goin, 1991, 1995; Goin and Rey, 1997; Goin et al., 2000; Voss and Jansa, 2009). This hypothesis was formalized by Voss and Jansa (2009), who referred † Thylatheridium to the didelphid tribe Marmosini , together with Marmosa , Monodelphis , and Tlacuatzin . Beck and Taglioretti (2020) proposed restricting the name Marmosini to the Marmosa lineage only (or possibly to Marmosa + Tlacuatzin , if these two genera form a clade), and using Monodelphini to refer to the Monodelphis lineage (see also Goin, 1991, 1995; Goin and Rey, 1997; Goin et al., 2000), in which case Thylatheridium may also be a monodelphin. Indeed, it is possible that † Thylatheridium is actually nested within Monodelphis (Reig, 1958: 90; Voss and Jansa, 2009: 101).

By contrast, Simpson (1972) argued in favor of a closer relationship between † Thylatheridium and Lestodelphys (a member of the Recent didelphid tribe Thylamyini ), and some of the phylogenetic analyses of Reig et al. (1987: figs. 65, 67) placed † Thylatheridium in a clade with both Lestodelphys and Thylamys , to the exclusion of Monodelphis . Regardless, it seems certain that † Thylatheridium is a member of Didelphidae sensu Voss and Jansa (2009) .