The Pleistocene kangaroo Procoptodon goliah, the most extreme of the short-faced kangaroos, was the largest and most heavily built kangaroo known. It had an unusually short, flat face and forwardly-directed eyes, with a single large toe on each foot (reduced from the more normal count of four). Each hand had two long, clawed fingers that would have been used to bring leafy branches within reach.
The heavily built Procoptodon goliah was the most extreme of the sthenurines, or short-faced kangaroos. It had a very short, deep 'brachycephalic' skull and lower jaw, and eyes that were partly forward-facing (giving it a primate-like appearance). The lower jaws (dentaries) were massive and fused or ankylosed, and a 'chin' was developed. Both upper and lower incisors were small, and would have been used to nip vegetation. The last premolar in Procoptodon was complex and late-erupting. The ape-like molar teeth of Procoptodon were brachyodont (low-crowned) and tended to develop additional longitudinal enamel folds. Tim Flannery has compared them to the molars of the huge australopithecine Australopithecus boisei.
The forelimbs were unusually long and mobile in Procoptodon. The middle two fingers were elongated with long, recurved claws (forming a 'grappling hook' to grasp leafy branches). Procoptodon had a functionally monodactyl hind foot: the lateral metatarsals II, III and V were so reduced that their distal ends and phalanges, or toe bones, were lost, the well developed digit IV forming a 'single' toe. Correlated with this reduction was simplification of the digital tendons. Flexion of the ankle joint was restricted to fore and aft movement. As in other sthenurines, many bony elements of the foot and heel in Procoptodon were wider than in other kangaroos, in order to stabilize the single-toed foot. This is in part correlated with the loss of the fifth metatarsal, which helps to balance the foot in most kangaroos. These wider bones provided greater surface area for the insertion of the digital tendons, giving a powerful spring action to the hind foot. At the end of the toe was a hoof-like claw, possibly an adaptation to help give this massive kangaroo a greater measure of speed.
Note: The height of Procoptodon goliah has often been estimated at about 3 metres. This estimate may have come about because sthenurines, including Procoptodon, could reach above their heads using their long fore limbs and their tails as a prop (the only kangaroos able to do so). Given the size of Procoptodon, it conceivably could have reached branches about three metres off the ground. However, this is not the measurement used to indicate the actual height of the animal, and the three metre measure is an overestimate. In a normal upright standing position (not the above-mentioned possible feeding position or the raised 'threat' position of male kangaroos), Procoptodon goliah would have stood no more than about 2 metres in height, roughly comparable in height to a large Red Kangaroo but much more robust in build (about two and a half times as heavy). In addition, the ability of Procoptodonto stand in an extended upright position was limited by the construction of the spine, which in sthenurines was unable to be fully flexed.
Procoptodon goliah is most closely related to other species of Procoptodon(Procoptodon rapha and Procoptodon pusio). The relationships of sthenurine kangaroos are debated, although they may be closely related to the living Banded Hare-wallaby, Lagostrophus fasciatus.
Procoptodon goliah is known from a variety of habitats, mainly semiarid areas of New South Wales and South Australia. Many parts of its range were harsh environments, characterized by vast areas of treeless, wind-blown sand dunes. However, at Lake Menindee in western New South Wales, the climate was cooler and wetter than today, with lake-full conditions and a rich mammalian fauna. The surrounding area was a mosaic of sclerophyll forest, woodland, savannah and plains, yet windblown sand dunes also formed along the edge of the lake. Procoptodon goliah has also been found in cave deposits on the Nullarbor Plain, in an environment interpreted to have been a floristically diverse, dry open environment dominated by dry sclerophyll vegetation.
Procoptodon goliah is now known from all states except Tasmania (although there is one questionable record of Procoptodon from there). As yet, there are no records of Procoptodon from the Northern Territory. In Queensland, P. goliah is known from the Darling Downs in the southeast (the type locality). In New South Wales, P. goliah has been found in three areas, all west of the Dividing Range in the Murray-Darling drainage: from Lake Menindee in the west (the most abundant large kangaroo at the site), Tocumwal near the Murray River, and Bingara in the northeast. In South Australia, Procoptodon goliah is reported from the western Eyre Peninsula and from Naracoorte Caves. The most complete material comes from the recently discovered Nullarbor Plain cave deposits in Western Australia. All sites are interpreted as arid or semi-arid regions.
Feeding and diet
The complex teeth of Procoptodon goliah tell us that it was a browser rather than a grazer, able to handle the tough leaves and stems typical of arid and semi-arid environments. It would have used its long forelimbs to grasp branches and bring them close for feeding.
Life history cycle
Like all marsupials, Procoptodon would have had tiny, hairless young that developed to maturity in a pouch after birth. Living kangaroos have unique physiological adaptations (such as embryonic diapause) but it is not known if the extinct sthenurines also shared these adaptations.
The type specimen of Procoptodon goliah is a right maxillary fragment with molars M1-3/ in place. The first associated skeletal material came from Lake Menindee, New South Wales. Fossil material has also come from Lake Callabonna in South Australia and, in 2002, several other complete skeletons were found in caves on the Nullarbor Plain. The complete skeleton is now known, and hair and foot impressions have also been found. In areas where it is found with fossils of the Red Kangaroo, Macropus rufus, fossil of Procoptodon are usually more abundant, suggesting that it was a very successful species until its decline and eventual extinction about 15,000 years ago.
Sthenurinae is a subfamily within Macropodidae (the kangaroo family). There were two types of sthenurines: long-snouted (doliocephalic) and short-snouted (brachycephalic) forms. The doliocephalic Hadronomus puckridgei, from Alcoota Station in the Northern Territory, is currently considered the oldest sthenurine. The teeth of sthenurines differ between species, suggesting that the divergence between forms occurred well before the Pleistocene. This would mean that other features may have evolved in parallel. Procoptodon represents the extreme for a brachycephalic sthenurine, with long arms, forward facing eyes and an upright posture.
Based on similarities in the dentition, Tim Flannery (1983b) proposed that the closest living relative of sthenurines (short-faced kangaroos) was the tiny Banded Hare-wallaby (Lagostrophus fasciatus). Still extant, the Banded Hare-wallaby survives on two islands off the coast of Western Australia. This proposal, however, is debated.
Species of Procoptodon are all Pleistocene in age. Several species of Procoptodonhave been erected, although some of these are considered invalid. Valid species of Procoptodon in addition to P. goliah include Procoptodon pusio and P. rapha. Procoptodon goliah would have overlapped with Aboriginal people for as long as 30,000 years. In New South Wales, Aboriginal people still recount stories of a large, long-armed, aggressive kangaroo that would attack people. Its extinction may have been due to climate shifts during the Pleistocene.
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