Abstract
The macropodine kangaroo, Wallabia kitcheneri, was first described in 1989 from a Pleistocene deposit within Mammoth Cave, southwestern Australia, on the basis of a few partial dentaries and maxilla fragments. Here, we recognize W. kitcheneri within the Pleistocene assemblages of the Thylacoleo Caves, south-central Australia, where it is represented by several cranial specimens and two near-complete skeletons, a probable male and female. We reallocate this species to the hitherto monotypic genus Congruus. Congruus kitcheneri differs from all other macropodid species by having a highly unusual pocket within the wall of the nasal cavity. It is distinguished from C. congruus by having a longer, narrower rostrum, a taller occiput and a deeper jugal. Congruus is closest to Protemnodon in overall cranial morphology but is smaller and less robust. In most postcranial attributes, Congruus also resembles Protemnodon, including general limb robustness and the atypical ratio of 14 thoracic to five lumbar vertebrae. It is distinguished by the high mobility of its glenohumeral joints, the development of muscle attachment sites for strong adduction and mobility of the forelimb, and large, robust manual and pedal digits with strongly recurved distal phalanges. These adaptations resemble those of tree-kangaroos more than ground-dwelling macropodines. We interpret this to imply that C. kitcheneri was semiarboreal, with a propensity to climb and move slowly through trees. This is the first evidence for the secondary adoption of a climbing habit within crown macropodines.
Highlights
Macropodids descended from arboreal possum-like ancestors during the Palaeogene before becoming the main ground-dwelling mammalian herbivores of the Australian continent over the past 20 million years (Myr) [1]
Tree-kangaroos can hop bipedally as well as move their hindlimbs alternately [7]. Their arboreal adeptness is reflected in a range of skeletal modifications manifested within the larger, extinct species of the genus Bohra Flannery & Szalay, 1982 [8], which are known from the Pliocene and Pleistocene [1,9,10,11,12]
Lower dentition and postcranial skeleton of C. kitcheneri are treated as generic traits until they are described for another species of Congruus
Summary
Macropodids (kangaroos and relatives) descended from arboreal possum-like ancestors during the Palaeogene before becoming the main ground-dwelling mammalian herbivores of the Australian continent over the past 20 million years (Myr) [1]. Eleven of the 12 extant macropodine genera are characterized by species that are principally ground dwelling They employ a bipedal hopping mode of locomotion when moving at speed, and a ‘pentapedal’ mode, involving the use of the tail as a ‘fifth limb’ when moving slowly [5]. Tree-kangaroos can hop bipedally as well as move their hindlimbs alternately [7] Their arboreal adeptness is reflected in a range of skeletal modifications manifested within the larger, extinct species of the genus Bohra Flannery & Szalay, 1982 [8], which are known from the Pliocene and Pleistocene [1,9,10,11,12]. The occurrence of two species of Bohra within the Pleistocene Thylacoleo Caves deposits of south-central Australia, which have been inferred to have accumulated under a moderate to relatively low rainfall regime [13,14,15], reveals that the modern association between tree-kangaroos and wetter forest habitats is an artefact of differential extinction in the late Quaternary
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