STHENURUS (MACROPODIDAE, MARSUPIALIA) FROM THE PLEISTOCENE OF LAKE CALLABONNA, SOUTH-AUSTRALIA

This study of the skeletal remains of three species of the extinct kan garoo Sthenurus (Sthenurinae: Macropodidae) from Lake Callabonna, nort hern South Australia, details the comparative osteology of these taxa and their functional, anatomical, and phylogenetic implications. Geolo gical study of the locality assigns these fossils to the base of the Q uaternary sequence in laminated clay and fine sands that are part of a unit correlated with the Millyera Formation of the Lake Frome area im mediately south of Lake Callabonna. These deposits accumulated in a la ke of variable salinity, several times the size of the present Callabo nna playa. The plant remains associated with the Callabonna Fauna sugg est a more arborescent flora than that near the present-day salina but one containing taxa that still exist in the surrounding region. These facts indicate a seasonal climate with fluctuating water table but a regionally more effective rainfall than at present. Direct C-14 dating of wood from the Sthenurus-bearing deposits establishes an age beyond the limit of the radiocarbon method and regional geological correlati ons suggest a medial Pleistocene age within the span 0.2-0.7 Ma as mos t likely for the sthenurine kangaroos and associated large marsupials and ratite birds that constitute the Callabonna Fauna. Like the other large-bodied vertebrates at Lake Callabonna, the Sthenurus species wer e mired in the clays while attempting to cross the floor of the lake d uring tow-water or dry times. This mode of accumulation has yielded an unprecedented opportunity for comparative study of the remains of ind ividual animals. Three closely allied sthenurine species coexisted at Lake Callabonna: a new giant taxon, Sthenurus stirlingi, a somewhat sm aller S. tindalei, and the considerably smaller S. andersoni. Strong s exual dimorphism is shown by the larger taxa, resulting in size overla p between individuals, but morphological criteria identify the sexes a nd taxa involved. Comparative osteology of these Sthenurus species wit h Macropus giganteus emphasizes how different they are from the living gray kangaroo, especially in their short, deep skulls, long front fee t with very reduced lateral digits, and the virtually monodactyl hind feet. These distinctions, and many others, limit functional analysis f or lack of a closely comparable living model. We have nevertheless tri ed to gather the evidence into a coherent picture of Sthenurus as a li ving animal. The cheek teeth of these Sthenurus species fit the browsi ng grade of Sanson's (1978) model. The slender forelimbs are better fi tted for feeding than locomotor function. These limbs could be raised above the head, and the hands, with their long phalanges and claws, co uld have grasped high vegetation. The vertebral column is more rigid t han in living kangaroos and flexion is limited, but considerable exten sion of the anterior part of the body could have been retained as an i mportant function in reaching for high browse. The pelvis is modified for flexion and adduction of the thigh, which would support a bulky an imal while standing or elevating the body. The hind limb is more massi ve than in Macropus, but the elements are of similar proportion and th ere appears to be greater emphasis on the elastic properties of the te ndons and ligaments to augment muscular action across joints, especial ly the knee and plantar parts of the pes. We conclude that species of Sthenurus, like other sthenurines, were bulky, browsing kangaroos that sacrificed quadrupedal or pentapedal movement for greater dependence on bipedal saltation and extension of the body and forelimbs for highe r browsing. This diverse subfamily was a prominent element in Pleistoc ene faunas in Australia. Several taxa of comparable size were often fo und together which suggests partitioning of the browsing feeding mode- a broad niche that seems to contain few adherents among living large k angaroos.