[PaleoMammalogy • 2023] the Pliocene Marsupial Ambulator keanei gen. nov. (Marsupialia: Diprotodontidae) from inland Australia and its Locomotory Adaptations

 Ambulator keanei (Stirton, 1967)

in van Zoelen, Camens, Worthy et Prideaux. 2023.  

DOI: 10.1098/rsos.230211 

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Abstract

Diprotodontids were the largest marsupials to exist and an integral part of Australian terrestrial ecosystems until the last members of the group became extinct approximately 40 000 years ago. Despite the frequency with which diprotodontid remains are encountered, key aspects of their morphology, systematics, ecology and evolutionary history remain poorly understood. Here we describe new skeletal remains of the Pliocene taxon Zygomaturus keanei from northern South Australia. This is only the third partial skeleton of a late Cenozoic diprotodontid described in the last century, and the first displaying soft tissue structures associated with footpad impressions. Whereas it is difficult to distinguish Z. keanei and the type species of the genus, Z. trilobus, on dental grounds, the marked cranial and postcranial differences suggest that Z. keanei warrants genus-level distinction. Accordingly, we place it in the monotypic Ambulator gen. nov. We, also recognize the late Miocene Z. gilli as a nomen dubium. Features of the forelimb, manus and pes reveal that Ambulator keanei was more graviportal with greater adaptation to quadrupedal walking than earlier diprotodontids. These adaptations may have been driven by a need to travel longer distances to obtain resources as open habitats expanded in the late Pliocene of inland Australia.

Keywords: megafauna, Pliocene, Australia, fossil marsupials, Diprotodontidae, cranial and postcranial morphology

 

 Reassembled partial skeleton Ambulator keanei (SAMA P54742) with silhouette.

Systematic palaeontology

Class: Mammalia Linnaeus, 1758 

Superorder: Marsupialia Illiger, 1811 

Order: Diprotodontia Owen, 1877  

Superfamily: Diprotodontoidea Archer & Bartholomai, 1978 

Family: Diprotodontidae Gill, 1872 

Genus: Ambulator gen. nov.

 

Ambulator keanei (Stirton, 1967) comb. nov.

 Genus Etymology: ‘Ambulator’ is Latin for ‘walker’. This is in reference to the taxon's many postcranial adaptations that to plantigrade, graviportal walking. The gender of the genus is masculine.

 

 Species etymology: Named for Mr Vincent P. Keane, who provided heavy equipment for the excavations in the Tirari Desert in 1962 conducted by Ruben A. Stirton, Paul F. Lawson, Richard H. Tedford and Michael O. Woodburne. 

 Holotype: SAMA P13844, partial lower mandible with both ascending rami missing; right and left I1; left and right P3; left M1; right M4, partial maxilla containing left M2–M4. Collected by R. A. Stirton in 1962.

 Type locality: Keane Quarry, Lake Palankarinna, South Australia. Palankarinna Local Fauna, 3.9 Ma.

Conclusion: 

We describe the first postcranial material known for Ambulator keanei. This is only the fourth species of post-Miocene diprotodontid for which postcranial material has been described and includes the first three-dimensional description of the footpad morphology for the family. We found that Am. keanei expresses the earliest evidence for specialized graviportal locomotion within diprotodontids, suggesting that it may have been relatively vagile, which may correlate with late Pliocene grassland expansion. We also found evidence that the phalanges of graviportal diprotodontids were not used in weight bearing during locomotion. Systematic comparisons of established diprotodontid taxa indicate that inclusion of postcranial morphology in phylogenetic analyses is needed in order to resolve both species- and the higher-level taxonomy for the Diprotodontidae. Rare, well-preserved skeletal and soft tissue material found in the Main Body of the Tirari Formation indicates considerable potential for discovery of additional material. Future efforts should target collecting more vertebrate material from this formation. This may lead to not only a better understanding of marsupial diversity but a better understanding of the faunal change of Australia through time.

Jacob D. van Zoelen, Aaron B. Camens, Trevor H. Worthy and Gavin J. Prideaux. 2023. Description of the Pliocene Marsupial Ambulator keanei gen. nov. (Marsupialia: Diprotodontidae) from inland Australia and its Locomotory Adaptations. Royal Society Open Science. DOI: 10.1098/rsos.230211 

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