Thylacomyidae Bensley, 1903

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 : 213-215

Thylacomyidae Bensley, 1903

CONTENTS: Macrotis (fig. 38).

STEM AGE: 16.5 Mya (95% HPD: 12.4–20.6 Mya).

CROWN AGE: N/A.

UNAMBIGUOUS CRANIODENTAL AUTAPOMORPHIES: Two or more discrete lacrimal foramina usually present (char. 10: 1→0; ci = 0.063); at least one lacrimal foramen located within suture between lacrimal and maxilla (char. 11: 0→1; ci = 0.286); pterygoids in midline contact ventral to presphenoid (char. 46: 0→3; ci = 0.111); large, erect paroccipital process directed ventrally (char. 93: 1→2; ci = 0.100); P2 and P3 subequal in height (char. 119: 2→1; ci = 0.118); M1 pseudopreparacrista absent (char. 139: 1→0; ci = 0.333); upper molar posterolingual cusp is the metacone (char. 143: 1→2; ci = 0.400); lower first molar (m1) paracristid and paraconid both indistinct or absent (char. 159: 0→1; ci = 0.400); neomorphic cuspid(s) in hypoflexid region present (char. 171: 0→1; ci = 0.167); and lower third molar hypoconid lingual to salient protoconid (char. 173: 0→1; ci = 0.045).

COMMENTS: Thylacomyidae , represented here by its sole Recent genus Macrotis , is recovered in our molecular (figs. 27–29) and totalevidence (figs. 32, 33) analyses as the next perameloid family to diverge after Chaeropodidae . This topology is also seen in most other recent molecular and total-evidence analyses in which representatives of all extant peramelemorphian families are included ( Westerman et al., 2012; Kear et al., 2016; Travouillon and Phillips, 2018: fig. 1B, D; but see Travouillon and Phillips, 2018: figs. 1C, E, 2; Travouillon et al., 2019, 2021).

Within Peramelemorphia , Macrotis is craniodentally distinctive, as demonstrated by the long list of apomorphies identified here; among the most notable of these are the presence of two lacrimal foramina (all other peramelemorphians except some specimens of Perameles have a single lacrimal foramen), midline contact between the right and left pterygoids ventral to the presephenoid (also seen in Isoodon ), the large and erect paroccipital process (also seen in some Isoodon specimens), the absence of a pseudopreparacrista on M1 (also seen in some Isoodon and Microperoryctes specimens), recruitment of the metacone as the posterolingual cusp of the upper molars (rather than, as in all other metatherians with a posterolingual cusp, the metaconule; Bensley, 1903; Archer and Kirsch, 1977), and the absence of a distinct paracristid and paraconid on m1 (as also seen in a few diprotodontians, namely Burramys , Acrobates , and some specimens of Distoechurus ).

† Ischnodon australis from the Pliocene Tirari Formation at Lake Palankarinna in South Australia, which is known from a single partial right dentary preserving p1–2 and m1–2 ( Stirton, 1955; Archer and Kirsch, 1977; Tedford et al., 1992; Travouillon et al., 2017), has been argued to be a thylacomyid, although it is markedly more plesiomorphic than Macrotis in retaining a distinct paraconid and paracristid on m1 and in having overall lower-crowned (less hyposodont) molars ( Stirton, 1955; Archer and Kirsch, 1977; Travouillon et al., 2017). However, whereas most phylogenetic analyses have placed † Ischnodon as sister to Macrotis ( Travouillon et al., 2014 a, 2015 b, 2017: [fig. 9B], 2021; Chamberlain et al., 2015) supporting thylacomyid affinities for the former taxon, a few have not ( Travouillon et al., 2017: fig. 9A).

As already discussed (see Perameloidea above), † Bulbadon warburtonae, known from a single partial mandible, from the late Oligocene Ditjimanka Local Fauna (Faunal Zone B) of the Etadunna Formation, has been described as the oldest known thylacomyid ( Travouillon et al., 2021), but it did not form a clade with other thylacomyids in the dated total-evidence analyses of Travouillon et al. (2021). † Liyamayi dayi from the middle Miocene of Riversleigh World Heritage Area has also been described as a thylacomyid ( Travouillon et al., 2014a). However, † L. dayi is currently known from only two teeth (identified as an M2 and m1) that show striking differences from the homologous teeth of Macrotis in, for example, the presence of a distinct metaconule (rather than the metacone) at the posterolingual corner of M2, and a very prominent paraconid and paracristid on m1 ( Travouillon et al., 2014a). A number of published phylogenetic analyses have failed to support thylacomyid affinities for † Liyamayi ( Travouillon et al., 2014 a, 2015b; Chamberlain et al., 2015), but most recently the dated total-evidence analyses of Travouillon et al. (2021) placed this taxon sister to Macrotis + † Ischnodon , suggesting that it is indeed an early thylacomyid.