A project undertaken at the School of Veterinary and Life Sciences, Murdoch University, and supervised by Dr Natalie Warburton
An ability to climb trees is known for modern tree-kangaroos (Dendrolagus) and their extinct ancestors (Bohra) (Prideaux & Warburton 2008, 2009). Study of the skeletons of several new Pleistocene (2.6–0.01 Myr ago) kangaroo species, including the enigmatic genus Congruus and “giant” rat-kangaroo Propleopus, has led to surprising observations of adaptations consistent with tree-climbing. Further, hind-limb bones from a 3.5-Myr-old site in New Guinea tentatively referred to the forest-wallaby genus Dorcopsis (Plane 1967) also preserve a number of anatomical features that bespeak an ability to climb trees. The main aim of this study is to verify whether these species were indeed built for climbing, and to synthesise our results with other lines of evidence to shed light on the hitherto-unanticipated ecological roles that these species may have filled in Australian ecosystems.
Project Outcomes
The most detailed study (taxonomic, comparative and functional) analysis of the extinct kangaroo genus Congruus has been a major focus to date. The biology of the species in this poorly-understood genus has previously been entirely obscure. The work we have completed describes and revises the skeletal material referrable to Congruus has at least 20 individuals specimens from the Nullarbor Plain Thylacoleo Caves (WA) deposits, in addition to the specimens from Mammoth Cave (South West WA). The manuscript presenting taxonomic evaluation of all of these specimens, together with the first complete description of all the available postcranial elements of the most complete and informative skeletons (more than 100 individual bones) has been a very significant undertaking. Interpretations of its adaptations for tree-climbing, its significant sexual dimorphism, and its distribution through the fossil record have all added substantially to our understanding of the previous diversity of the kangaroo family. What is especially interesting about these large climbers is that they were primarily inhabitants of the drier parts of southern and eastern Australia, rather than forest dwellers analogous to orang-utan or bears.
By making comparisons through dissection and examination of a wide range of tree-climbing mammals, we have been able characterise a suite of morphological characteristics that are associated with the adaptation of kangaroos to tree climbing. From this information, we have been able to identify additional species of previously undescribed extinct kangaroos that display exceptional adaptations to tree-climbing, as well as being able to track the evolution of these characteristics through different lineages. Our synthetic analysis of tree-kangaroo evolutionary history has revealed that the origin of tree-kangaroos was tied to forest expansion 5–3.5 million years ago, but after Australia began to dry out, some species became denizens of a range of habitats, including open woodland. Anatomical differences between 4–3 million-year-old species and those that were around most recently (or remain extant) reveal successive stages in the transition from partial to full arboreality.
We have built a large dataset of modern kangaroo skeletal dimensions and regressed these against body mass. This shows that measures of the cranium, dentary, femur and calcaneus (heel bone) are tightly correlated with body size. This has allowed us to estimate adult body masses for species of Bohra, Congruus and Baringa. Remarkably, these range from 15 up to 70 kg.
Dental microwear analysis of the teeth of modern and fossil tree-kangaroos (Dendrolagus and Bohra) has revealed that, although they were clearly browsers (leaf and fruit consumers), as expected, their microwear patterns are quite distinct from those of ground-dwelling browsers. We think that this reflects the absence of dust and grit from food higher in the canopy versus closer to the ground. We are still analysis dental microwear data for the species of Congruus and Baringa. Given that these animals were distinctly larger than most tree-kangaroos, we are keen to see if or how their diets differed.
Ongoing work
The success of the work categorising tree-climbing attributes in kangaroos has led to the identification of more previously unknown species that appear to have similar attributes, than expected. The collection of new specimens from Lake Callabonna during the last two years by members of the Flinders University Palaeontology group have yielded outstanding new examples of fossil kangaroos of the genus Baringa. Most unexpectedly, these beautiful specimens have led to the discovery of tree-climbing adaptations in parts of the skeleton that were previously unknown, and as such have become an example of another lineage of what were thought to have been terrestrial wallabies that are likely to have actually been tree-climbers. Descriptions of these specimens as a new species of extinct wallaby with the most remarkable anatomy of the hands we have ever seen are well underway and will be of great significance to the understanding of the diversity of extinct marsupial fauna. We are very excited to be able to be continue this collaborative work into the future based on the ever-growing collection of fossil material.
- Continued identification, description and analysis of tree-climbing kangaroo forms from diverse Australian-Pacific fossil assemblages.
- Taxonomic description and functional analysis of a new species of Baringa with extreme and unique adaptations of the hands for tree-climbing.
- Ongoing examination and comparison of fossil material from Australian collections to
identify more specimens of Propleopus. Specimens of Propleopus has been more challenging to identify, and without access to other museum collections (see below) work on this genus has been slow to date but will be continued as travel permits.
ACADEMIC OUTPUTS
Warburton and Prideaux (In prep.) The skeleton of Congruus kitcheneri, a semiarboreal kangaroo from the Pleistocene of southern Australia.
Prideaux et al., (In prep) Systematics and evolution of tree-kangaroos (Macropodidae, Marsupialia).
Prideaux et al., (In prep) A review of the late Cenozoic kangaroo genus Congruus (Marsupialia: Macropodidae).
Warburton et al., Description of new species of Baringa based on outstanding skeletal material from Lake Callabonna, South Australia.
The APSF funding was instrumental in our gaining funding for the Australian Research Council project DP190103636 “Illuminating the evolutionary history of Australia’s most iconic animals”. This aims to pinpoint the nature and timing of key steps in macropod evolution and to test how these link with major climatic and biotic changes. It will help resolve fundamental questions such as how and why ancestral macropods evolved bipedal hopping, what the drivers of their initial descent from the trees were, and why, much later, three kangaroo lineages returned to the trees.
Conference presentations including: Conference of Australasian Vertebrate Evolution Palaeontology and Systematics (CAVEPS) in Queenstown, New Zealand (2017)