Beck, Louys, Brewer, Archer, Black & Tedford, 2020
Reconstruction by Peter Schouten.
We describe the partial cranium and skeleton of a new diprotodontian marsupial from the late Oligocene (~26–25 Ma) Namba Formation of South Australia. This is one of the oldest Australian marsupial fossils known from an associated skeleton and it reveals previously unsuspected morphological diversity within Vombatiformes, the clade that includes wombats (Vombatidae), koalas (Phascolarctidae) and several extinct families. Several aspects of the skull and teeth of the new taxon, which we refer to a new family, are intermediate between members of the fossil family Wynyardiidae and wombats. Its postcranial skeleton exhibits features associated with scratch-digging, but it is unlikely to have been a true burrower. Body mass estimates based on postcranial dimensions range between 143 and 171 kg, suggesting that it was ~5 times larger than living wombats. Phylogenetic analysis based on 79 craniodental and 20 postcranial characters places the new taxon as sister to vombatids, with which it forms the superfamily Vombatoidea as defined here. It suggests that the highly derived vombatids evolved from wynyardiid-like ancestors, and that scratch-digging adaptations evolved in vombatoids prior to the appearance of the ever-growing (hypselodont) molars that are a characteristic feature of all post-Miocene vombatids. Ancestral state reconstructions on our preferred phylogeny suggest that bunolophodont molars are plesiomorphic for vombatiforms, with full lophodonty (characteristic of diprotodontoids) evolving from a selenodont morphology that was retained by phascolarctids and ilariids, and wynyardiids and vombatoids retaining an intermediate selenolophodont condition. There appear to have been at least six independent acquisitions of very large (>100 kg) body size within Vombatiformes, several having already occurred by the late Oligocene.
Order Diprotodontia Owen, 1866 New Definition
Suborder Vombatiformes Woodburne, 1984 New Definition
Infraorder Vombatomorphia Aplin and Archer, 1987 New Definition
Superfamily Vombatoidea Kirsch, 1968 New Definition
Included taxa: Mukupirna nambensis new species
Mukupirna nambensis gen. et. sp. nov.
Differential diagnosis: differs from known members of Wynyardiidae in possessing a P3 that lacks a posterolingual cusp (=”hypocone”), less well-developed selenodonty, a less well-developed masseteric process, palatal vacuities entirely enclosed by the palatines, a proportionately longer deltopectoral crest and broader distal end of the humerus (Epicondylar index = 0.4422), a proportionately longer olecranon of the ulna (Index of Fossorial Ability = 0.4223), and a much larger body size (estimated body mass based on postcranial measurements = 143–171 kg); differs from vombatids in lacking bilobate molars (molars are only slightly bilobate in Nimbavombatus, Rhizophascolonus and Warendja, but strongly bilobate in other vombatids); differs from all vombatids except Nimbavombatus in retaining three upper incisors and the upper canine; differs from Nimbavombatus in larger size, more bicuspid P3, and palatal vacuities entirely enclosed by the palatines; differs from vombatids known from postcranial remains in lacking a laterally extensive deltopectoral crest of the humerus; differs from hypselodont vombatids in having closed premolar and molar roots; differs from known members of Thylacoleonidae in retaining only a single upper premolar (P3), with this tooth not as elongate or bladelike, lacking a marked reduction in molar size posteriorly, having a proportionately longer deltopectoral crest and broader distal end of the humerus, and having a proportionately longer olecranon of the ulna; differs from known members of Phascolarctidae in lacking strongly selenodont molars, having a less well-developed masseteric process, a proportionately longer deltopectoral crest and broader distal end of the humerus, and a proportionately longer olecranon of the ulna; differs from known members of Ilariidae in lacking posterobuccal and lingual cusps on P3, in lacking strongly selenodont molars, and in lacking a well-developed masseteric process; differs from known members of Diprotodontidae and Palorchestidae in lacking a molariform P3, molars not strongly bilophodont, in lacking a well-developed masseteric process, and in retaining palatal vacuities. Mukupirna nambensis cannot be compared directly with Marada arcanum (the only known representative of the vombatiform family Maradidae), because Mu. nambensis is only known from the cranium and upper dentition whereas Ma. arcanum is known only from the lower dentition, and it is possible that they represent the same taxon or are closely related (see the supplementary information).
Holotype: AMNH FM 102646 (previously, QMAM 16824), a badly crushed cranium (preserved length = 197 mm; dorsal surface not preserved) with left and right P3-M4, and associated partial postcranial skeleton comprising vertebrae, ribs, left and right scapulae, left humerus, left ulna, left radius, left and right femora, left tibia, left fibula, and parts of the autopodia. The adult dentition is fully erupted, except possibly for M4, which does not appear to be in line with the occlusal surfaces of M1-3 (although this may be the result of post-mortem displacement); the molars are only lightly worn. In the postcranium, most fracturing has occurred at the epiphyseal plates. Collectively this suggests that this individual was probably a late subadult or young adult.
Etymology: The generic name is from the words muku (“bones”) and pirna (“big”) in the Dieri (Diyari) language traditionally spoken in the area around Lake Eyre and refers to the large size of the animal. The species name nambensis is after the Namba Formation in which the only known specimen was found.
Type Locality and Age: Lake Pinpa Site C, Namba Formation, Lake Frome area, South Australia. The Namba Formation has been correlated with the Etadunna Formation, which has been estimated to be 26-24 Ma old (i.e. latest Oligocene) on the basis of isotopic, foraminiferal, magnetostratigraphic and radiometric (Rb-Sr dating of illite) data. More recently, the Etadunna Formation has been proposed to be 26.1-23.6 Ma old based on a best-fit age-model of magnetostratigraphic data. The Pinpa Local Fauna is the oldest of the three distinct faunal units recovered from stratigraphic levels in the Namba Formation, and has been correlated with the oldest faunal zone (Zone A) of the Etadunna Formation, which has been dated as 25.3-24.9 Ma old (chrons 7An and 7Ar) based on magnetostratigraphy25. In summary, available evidence suggests a probable age of between approximately 26 and 25 MYA for the Pinpa Local Fauna.
Robin M. D. Beck, Julien Louys, Philippa Brewer, Michael Archer, Karen H. Black and Richard H. Tedford. 2020. A New Family of Diprotodontian Marsupials from the latest Oligocene of Australia and the Evolution of Wombats, Koalas, and their Relatives (Vombatiformes). Scientific Reports. 10, 9741. DOI: 10.1038/s41598-020-66425-8