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Systematics and Evolution of the Sthenurine Kangaroos


The subfamily Sthenurinae (Macropodoidea, Diprotodontia) is an extinct group of robust kangaroos. The earliest sthenurine appears in the late Miocene of central Australia, but the group is most common in the Pleistocene faunas of southern and eastern Australia. Since the Sthenurinae was last reviewed over three decades ago, species diversity has more than doubled. Many species are now also represented by series of well-preserved specimens, including complete crania and skeletons. New insights generated by these discoveries provided the major impetus for this review of sthenurine systematics, functional morphology, paleoecology, biochronology and zoogeography.

Sthenurinae is diagnosed on the basis of nine craniodental synapomorphies, making it the best-defined kangaroo subfamily. Two new genera and four new species are recognized here, bringing the total to six genera and 26 species. A new tribe (Simosthenurini) is raised to include the three short-faced genera: Archaeosimos gen. nov., Simosthenurus and Procoptodon. A cladistic analysis of interrelationships within the Sthenurinae has prompted the dismissal of concerns previously expressed about the distinction of Simosthenurus and Sthenurus. However, Simosthenurus (sensu lato) is paraphyletic, because Procoptodon is derived from within it. Several molar crests hypertrophied in the more derived Procoptodon species are believed to be a result of phylogenetic character reversal, a phenomenon that may be an important but previously neglected mechanism of evolutionary change within the Macropodoidea. Compelling evidence suggests a paraphyletic origin for the Sthenurinae from within the late Oligocene to middle Miocene subfamily Bulungamayinae. The middle Miocene bulungamayine Wanburoo is the sister taxon of the Sthenurinae, which along with the late Miocene occurrences of Hadronomas in central Australia and Archaeosimos in southern Australia, supports a middle Miocene origin for the subfamily. No support is found for the hypothesis that Troposodon and Lagostrophus are sthenurines. These two genera have a closer affinity with the macropodines and are probably most closely allied to each other and Protemnodon. Craniodental similarities shared with the sthenurines are likely to reflect dietary convergence.

A fundamental step in the phylogenesis of the sthenurine lineage was the attainment of relatively large body size. The plesiomorphic Hadronomas exemplifies the sthenurine bauplan: it is the earliest macropodoid in the fossil record larger than a small wallaby, and several craniodental attributes indicate that it was capable of generating larger occlusal forces during the apprehension and mastication of food than any other Miocene macropodoids. The evolution of sthenurines was most likely a direct faunal response to the decline in mesic conditions from the middle Miocene. All sthenurines apart from Hadronomas are united by nine craniodental traits that represent a crucial circumvention of the constraints that probably restricted Hadronomas to only a moderately fibrous diet.

The masticatory muscles became more anteriorly oriented, the orbits became anterolaterally projected, the mandibular symphysis was rigidified, grinding became the primary function of the premolars, the cheek tooth row became laterally curved, and molar complexity increased. These adaptations improved the capacity of the craniodental system for breaking down tough vegetation and they paved the way for the cranial foreshortening characteristic of the Simosthenurini, which originated in the late Miocene.

No less than 14 of the 18 simosthenurin species recognized occur in the middle or late Pleistocene cave assemblages of southern and eastern Australia. Simosthenurins filled many of the browser niches available throughout the temperate woodlands and open forests of the Pleistocene, and the lineage may have originated in transitional areas between the retracting wetter forests and expanding sclerophyll habitats of the southeast during the late Miocene. Some Pleistocene species, such as Simosthenurus maddocki, were apparently highly selective feeders, while others were generalists. The evolution of Procoptodon is likely to have been prompted by the marked expansion of dry sclerophyll vegetation and open conditions following the intensification of aridity in the latter half of the Pliocene. P. goliah, the largest and most robust of all macropodoids, was the only simosthenurin widely distributed throughout the continental interior and was apparently adapted to the consumption of a tougher diet than any other sthenurine. In contrast to Procoptodon, three of the six Pleistocene species of Sthenurus had already evolved by the early late Pliocene. This lineage maintained a stronghold in the interior of the continent during the Pleistocene and is characterized by a more gracile build, longer face, broader incisors and higher-crowned molars than the Simosthenurini. Sthenurus species may have subsisted on the small-leafed, xeromorphic shrubs and low, dusty forbs still common throughout inland Australia today. Although sthenurine diversity and abundance reached its zenith during the middle and early late Pleistocene, all species were extinct by the end of the epoch. On the basis of data currently available, the demise of this remarkable radiation of browsing mammals appears most likely to have resulted from attrition over an extended duration due primarily to human hunting pressure.

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