A project undertaken at Flinders University, and supervised by Gavin Prideaux
The rhinoceros-sized Diprotodon optatum was the largest marsupial that ever lived. It became extinct, along with 90% of Australia’s large marsupial species, 60,000–40,000 years ago. Diprotodon was one of the most widespread megafaunal marsupials, inhabiting regions with very different climates and vegetation, ranging from the desert to the tropics, and from the west coast to the east coast. Despite its iconic status, we know little about its basic biology, such as what it ate and how it used the landscape. As a resource to potentially shed light on Diprotodon, Lake Callabonna in northeastern South Australia is unparalleled. Renowned for its articulated skeletons of megafaunal species, countless individuals of Diprotodon mired in the mud and died around 50,000 years ago, while presumably en route to drink at mound springs in the centre of the lake. The aim of our study was to use strontium-, carbon- and oxygen-isotopic data to test a hypothesis that Diprotodon undertook seasonal migrations, an idea borne from an earlier isotopic study of an incisor tooth of an individual from Queensland. Lake Callabonna is situated on the southern margins of the Lake Eyre Basin, to the east of the geologically-diverse Flinders Ranges. This has produced 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 carbon-isotopic data, which informs on proportions of C3 and C4 plants in the diet, and oxygen-isotopic data, which reflects changes in relative humidity, allowed us to analyse movement patterns in Diprotodon at Lake Callabonna. Using a combination of mechanical sampling for carbon and oxygen, and laser ablation for strontium, we took 142 sequential, paired samples at 2-mm intervals from along the length of a Diprotodon incisor. The data reveal clear, cyclical trends in diet and relative humidity across what we interpret as a total of at least eight years of growth. Our individual ate a mixture of C3 and C4 vegetation, but this fluctuated seasonally, along with relative humidity, i.e., seasonally-varying effective rainfall. During drier seasons, it consumed a greater proportion of C4 vegetation, more than likely saltbush, which is the only widespread dicot plant that uses the C4 photosynthetic pathway and which has long been recognised in fossilised gut contents of some Diprotodon individuals from Lake Callabonna. Saltbush, by its nature, is nutritious but salty, and animals that eat it need to drink more, especially in warmer seasons.
This suggests that this individual maintained ready access to free water during those periods. Our Diprotodon was definitely a wanderer, with 60% of strontium values falling outside of the range for Lake Callabonna soils, which we established through analysis of modern soil samples collected from 17 sites from around Lake Callabonna and the Flinders Ranges. Strontium values fluctuated over time, which indicates marked movement across the landscape and episodic return to Lake Callabonna itself, but not in a way that aligns clearly with an annual, seasonal pattern. The evidence suggests that annual migration was not a strategy utilised by this individual, and that movements may have been determined more by locally fluctuating environmental conditions and resource availability. Continuing research will sample from more individuals of Diprotodon from Lake Callabonna to test for repeated patterns, as well as make comparisons with other species in the fossil assemblage. Expanding our number of modern soil samples from across the region will help us to tie down which water sources were episodically visited by Diprotodon and in which season. This study highlights the potential of such approaches to offer a window onto the life and times of extinct marsupials and to better understand the history of Australia’s unique biota.