Bearing up well? Understanding the past, present and future of Australia's koalas
Black, KH, Price, GJ, Archer, M & Hand, SJ 2014, Gondwana Research, vol. 25, pp. 1186-1201.
Evidence from fossil records of the family Phascolarctidae indicates that these specialist folivores, of which the modern koala Phascolarctos cinereus is the last surviving member, are particularly sensitive to climate change. Knowledge about the nature and rate of change in previous palaeocommunities of phascolarctids together with data relating to the responses of modern koala populations to historic climatic extremes indicate that the future survival of the species is under threat.
Due to the paucity of koala fossil material, much of which is poorly preserved, researchers have debated that there may have been eight to ten genera and from 13 to 22 species. This has made the task of accurately assessing the palaeodiversity and palaeoecology of phascolarctids challenging. Moreover, this problem has substantially hindered researchers in their attempts to form an accurate picture of the evolution of the koala’s lineage and its responses to previous environmental changes. There is one fossil record that dates the modern phascolarctid genus from the late Miocene or Pliocene of South Australia, while assemblages show that the modern species appeared about 350,000 years ago. While the fossil record of koalas is extended, the pre-Pleisteocene record is scant and geographically restricted with many species being studied from a single deposit. Overall, there are only 163 historical specimens from 58 deposits. This paucity of fossil assemblages has resulted in three major areas of systematic conflict among researchers: whether to include the koala-like Koobor within the Phascolarctidae; what the species boundaries are in koala lineages; and what speciation and morphological differentiations there are within the modern genus Phascolarctos.
Presently, three extinct species of Phascolarctos are recognised, and these koalas were within the same size range as the modern species (4.1-13.5 kg). Most of the recorded fossil koalas, however, had dentitions 15-65% larger than the modern koala. It has been suggested that one of these fossils from the early to middle Miocene, Nimiokoala greystanesi, which shows the most complex dentitions of all koala species and a different cranio-mandibular morphology to the modern koala, had a diet that consisted not only of eucalyptus leaves but also seeds and fruits. Further, it has been argued that the earliest probable morphological evidence for koalas primarily favouring eucalypts dates from the late Miocene–Pliocene species, which display dentitions precisely matching those of Phascolarctos cinereus, highly suggestive that they also shared a similar diet.
Research in this area needs to continue to achieve a deeper understanding of the diversity of phascolarctids and the complex driving forces behind their speciation and extinction rates. This means investigating further the life histories, macroevolutionary processes and environmental factors of this unique biological entity. Importantly, studying the history of the koala provides us with a greater understanding of its future, and in particular the threats it is likely to be vulnerable to. These include disease, reduced genetic diversity and a changing climate, and given the range contractions and localised extinctions of koalas that have occurred in the past, the authors argue that conservation measures for the species should be prioritised.
Summarised by Rosemary Shaw
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