Diprotodon, Owen, 1838

Price, Gilbert J., 2008, Taxonomy and palaeobiology of the largest-ever marsupial, Diprotodon Owen, 1838 (Diprotodontidae, Marsupialia), Zoological Journal of the Linnean Society 153 (2), pp. 369-397 : 386

LAKE CALLABONNA DIPROTODON

At least six large- and six small-form Lake Callabonna individuals were examined in this study, derived mostly from collections of the South Australian Museum. That result contrasts with Stirling & Zietz (1899) who suggested that the large form was significantly more common than the small form. The difference in perceived abundance of the large form over the small form may be a result of misidentification of size classes in the 1899 investigations, or alternatively, a greater number of specimens (dominated by large-form individuals) were available to Stirling & Zietz (1899). However, it is pertinent to note that there are several unprepared and unregistered specimens (including several large-form individuals) in the South Australian Museum collections that were not examined in this study.

The Lake Callabonna Diprotodon assemblage is relatively autochthonous in comparison with most other assemblages examined in this study. Tedford (1973) suggested that several small groups of Diprotodon crossed the lake at various times of low water levels, eventually becoming mired in the muddy substrate. Both mature large- and small-form individuals undoubtedly occur in the deposits ( Fig. 8 ). However, previous collecting procedures have mixed the samples and it cannot be determined if the social groups were body size (i.e. gender) segregated.

As with the Darling Downs, there are no significant or consistent dental morphological differences to warrant separation of more than one Diprotodon morphospecies in the Lake Callabonna assemblage. Coupled with the observations that a bimodal body size distribution exists in the assemblage, and the high probability that both size forms were temporally coeval, the data suggest that a single, sexually dimorphic Diprotodon morphospecies is represented in the Lake Callabonna assemblage. Although Stirling & Zietz (1899) suggested that there were at least two species represented at Lake Callabonna (based on the occurrence of two size classes), they did not discount the possibility that the differences in body size represented intraspecific, rather than interspecific variation within the Diprotodon assemblage.

Again, as with other mentioned assemblages, it is not possible unambiguously to assign gender to either Lake Callabonna Diprotodon size form. Interestingly, at Lake Callabonna, neonatal remains of an individual Diprotodon were recovered in association with an adult individual, and from the position where the pouch would have been located ( Tedford, 1973). Although some remains of the neonatal individual were accessioned into museum collections, they were separated from the putative mother ( Pledge, 1994). Thus, the question of gender within D. optatum remains unresolved and ‘the most wonderful discovery ever made in the world’, according to George Hurst, discoverer of those Lake Callabonna specimens (fide Tedford, 1973), was squandered.

Lake Callabonna Diprotodon dental morphometric COV values are higher relative to Bacchus Marsh and extant populations of grey kangaroos, but are more similar to those of the Darling Downs ( Tables 2–4). That may indicate that there was a different temporal sampling range at Lake Callabonna (presumably of longer duration) than those other assemblages. Again, that hypothesis is difficult to test, but it would support Tedford’s (1973) suggestion that the accumulation of individuals was quite slow, possibly occurring over several thousands of years, and would be consistent with sampling from more than one herd.

Figures

Figure 8. Dimensions of Diprotodon dentaries at position below M3 for the Darling Downs (N = 34), Lake Callabonna (N = 19) and Bacchus Marsh (N = 23) assemblages. Closed markers indicate large-form individuals, open markers indicate small-form individuals.