A project undertaken at Flinders University, and supervised by Gavin Prideaux
Many large placental mammals on other continents undertake annual migrations to exploit geographically-patchy, seasonally-variable resources. In Australia, kangaroos may travel long distances in search of food and water, but no marsupials migrate. Why?
Perhaps there is something special about marsupial herbivores or Australian climate patterns that preclude migration, or perhaps those marsupials that did migrate became extinct during the megafaunal extinction interval 60,000–40,000 years ago. Recent isotopic evidence obtained from the incisor tooth of an individual of the rhinoceros-sized marsupial, Diprotodon optatum, has suggested that it undertook a 200-km seasonal migration through southeastern Queensland 300,000 years ago (Price et al. 2017, Proceedings of the Royal Society B).
This, alongside the inferred herd-living behaviour of Diprotodon (Price 2008, Zoological Journal of the Linnean Society), has been used to evoke the image of an “Australian Serengeti” (Price 2017, The Conversation). But Diprotodon was very widely distributed before it become extinct 40,000 years ago, inhabiting regions with very different climates and vegetation to the subtropics. This study will examine whether annual migration was an attribute of Diprotodon in the arid interior of Australia, with a focus on the region encompassing the famous megafauna graveyard of Lake Callabonna, northeastern South Australia.
Lake Callabonna is renowned for its articulated skeletons of megafaunal species. Countless individuals mired in the mud and died during the late Pleistocene (Pledge 1994, Records of the South Australian Museum). Their remarkable preservation offers an exceptional opportunity for generating insights into the biology of iconic Australian giants.
A wealth of material exists in the South Australian Museum and at Flinders University for isotopic studies, yet until now, none has been carried out. Moreover, Lake Callabonna is situated on the southern margins of the Lake Eyre Basin and to the east of the geologically-diverse Flinders Ranges. This results in a region with marked soil heterogeneity, each isotopically distinctive in strontium. Plants growing on these different soils incorporate strontium isotopes into their tissues in proportion to their soil concentration. In turn, so do the herbivores that consume these plants.
This means that by sampling tissues from these herbivores, most especially the sequentially-deposited tooth enamel of the ever-growing incisors characteristic of wombats and Diprotodon, it is possible to track where individuals roamed across one year to multiple years. Combining this strontium dataset with oxygen- and carbon-isotopic data for modern kangaroos and wombats would establish regional isotope heterogeneity that will allow us to assess movement patterns in Lake Callabonna megafauna.
Results of this study will improve our understanding of marsupial evolutionary ecology and help test ideas about what drove the late Pleistocene extinctions and why some species may have been more susceptible than others. Oxygen and strontium isotope data may establish whether permanent water sources were visited seasonally by megafaunal species. Carbon isotopes may help shed light on the kinds of plants extinct herbivores relied upon and whether this varied between seasons.