Is the modern koala (Phascolarctos cinereus) a derived dwarf of a Pleistocene giant? Implications for testing megafauna extinction hypotheses
Price, GJ 2008, Quaternary Science Reviews, vol. 27, pp. 2516-2521.
This study tested the hypothesis that the modern Australian koala (Phascolarctos cinereus) is a dwarf descendent of the extinct giant koala (Ph. stirtoni), which underwent body-size reduction during the Late Pleistocene-Holocene period. The implication of this hypothesis is that Ph. stirtoni survived the megafaunal extinction event of the Late Pleistocene, with Ph. cinereus being the evolutionary ‘end-point’. Findings suggest that the giant koala should be considered a species distinct from the modern koala.
To test the koala dwarfing hypothesis, biogeographical records of fossil koalas and morphological character analyses were used. Initially, a comprehensive literature survey was conducted to identify dated contexts of Phascolarctos fossil remains. Following this, Ph. cinereus specimens were directly compared to the Ph. stirtoni holotype, including examination of dental nomenclature. Paleobiogeography results indicate the firm establishment of the Phascolarctos genus in the late Tertiary, with Ph. stirtoni clearly distinguishable by the Early Pleistocene. During the Middle Pleistocene, records show that Phascolarctos was widely distributed across southern and eastern Australia and both Ph. stirtoni and Ph. cinereus occurred within the same geographic area. Late Pleistocene fossils reveal the same results. Other than an obvious contrast in body size, morphological comparisons show significant differences in upper premolar nomenclature of Ph. stirtoni that fall outside the range of variation exhibited in Ph. cinereus.
For the dwarfing hypothesis to be validated, a number of criteria must be met: (1) taxa must not exist sympatrically; (2) Ph. cinereus and Ph. stirtoni should not overlap in time; and (3) taxa must be morphologically identical in all aspects other than size. Findings demonstrate that the taxa were not allopatric, possessed different morphological characters and further, they overlapped temporally throughout the Middle and Late Pleistocene. In addition, the co-occurrence of these species in both cool and warm phases during the last 350 thousand years indicates that physiological response to climactic conditions was not a factor influencing their body-size relationship. Accordingly, the data suggests that Ph. cinereus and Ph. stirtoni do not stem from the same, evolving lineage. It remains unclear, however, what factors allowed for the survival of Ph. cinereus, while also driving Ph. stirtoni to extinction.
Ultimately, this study disproves the hypothesis that the modern koala is simply a dwarfed descendent of the giant koala. Ph. stirtoni is among a range of other megafaunal vertebrates that underwent extinction at some point during the Late Pleistocene. Given this result, it is important that the dwarfing of other Australian megafauna (e.g. Macropus giganteus titan) also be questioned to validate the origins of extant taxa.
Summarised by Julian Radford-Smith
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