Vombatomorphia Aplin and Archer, 1987

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 : 230-231

Vombatomorphia Aplin and Archer, 1987

CONTENTS: †Diprotodontoidea, † Ilariidae , † Muramura , † Namilamadeta , Phascolarctidae , and Vombatidae .

STEM AGE: 32.4 Mya (95% HPD: 29.1–36.4 Mya).

CROWN AGE: 30.5 Mya (95% HPD: 27.3–34.1 Mya).

UNAMBIGUOUS CRANIODENTAL SYNAPOMORPHIES: Frontal and squamosal in contact on lateral aspect of braincase (char. 26: 0→1; ci = 0.071); squamosal prevents parietal-mastoid contact (char. 32: 0→1; ci = 0.083); subarcuate fossa is a shallow depression (char. 73: 0→1; ci = 0.500); facial nerve exits the middle ear via a stylomastoid foramen formed by the ectotympanic and pars canalicularis of the petrosal (char. 79: 0→5; ci = 0.625); zygomatic epitympanic sinus shallow and largely open laterally (char. 86: 1→2; ci = 0.333); and condylar process transversely elongate and medially extensive (char. 101: 0→1; ci = 0.333).

COMMENTS: Beck et al. (2020: table 1 ) defined Vombatomorphia as the most inclusive clade including Vombatus ursinus but not Phascolarctos cinereus ; thus defined, Vombatomorphia comprises all vombatiforms except Phascolarctidae (note that † Thylacoleonidae is considered here Diprotodontia incertae sedis, rather than a member of Vombatiformes; see above). Among the unambiguous craniodental synapomorphies that diagnose this clade is the very shallow subarcuate (or floccular) fossa. The volume of the subarcuate fossa has been suggested to be associated with agility in mammals, with a larger volume indicating greater agility ( Olson, 1944; Gannon et al., 1988; Spoor and Leakey, 1996; Jeffery and Spoor, 2006). This hypothesis has not been supported by recent studies ( Rodgers, 2011; Ferreira-Cardoso et al., 2017), but there is evidence that, in rodents at least, the petrosal lobules (and the subarcuate fossae that house them) are larger in arboreal forms and smaller in fossorial forms ( Arnaudo et al., 2020; Bertrand et al., 2021). Vombatomorphia includes several very large (estimated body mass> 100 kg) fossil taxa ( Beck et al., 2020), some of which show graviportal adaptations ( Camens, 2008; Camens and Wells, 2009) and presumably had low agility. However, † Muramura also has a shallow subarcuate fossa, yet was considerably smaller (estimated body mass ~ 18 kg; Beck et al., 2020: supplementary information), with a relatively gracile postcranial skeleton ( Pledge, 2003), and was presumably much more agile. Pledge (1987: 399) concluded that † Muramura williamsi was not fossorial based on its pedal morphology, and postcranial indices for this taxon do not clearly support fossoriality ( Beck et al., 2020: table 2 ). However, in a subsequent paper, Pledge (2003: 554) noted similarities beween the feet of † Muramura and fossorially adapted vombatids, suggesting that the second † Muramura species known, † M. pinpensis , may have been better adapted to burrowing based on its shorter lower limbs.

Definitive vombatomorphians are known from late Oligocene sites in Australia ( Archer et al., 1999; Long et al., 2002; Archer and Hand, 2006; Black et al., 2012b), with their initial diversification estimated here as having taken place during the latest Eocene or Oligocene.