Joint mobility is a key factor in determining the functional capacity of tetrapod limbs, and is important in palaeobiological reconstructions of extinct animals. Recent advances have been made in quantifying osteological joint mobility using virtual computational methods; however, these approaches generally focus on the proximal limb joints and have seldom been applied to fossil mammals. Palorchestes azael is an enigmatic, extinct ~1000 kg marsupial with no close living relatives, whose functional ecology within Australian Pleistocene environments is poorly understood. Most intriguing is its flattened elbow morphology, which has long been assumed to indicate very low mobility at this important joint. Here, we tested elbow mobility via virtual range of motion (ROM) mapping and helical axis analysis, to quantitatively explore the limits of Palorchestes' elbow movement and compare this with their living and extinct relatives, as well as extant mammals that may represent functional analogues. We find that Palorchestes had the lowest elbow mobility among mammals sampled, even when afforded joint translations in addition to rotational degrees of freedom. This indicates that Palorchestes was limited to crouched forelimb postures, something highly unusual for mammals of this size. Coupled flexion and abduction created a skewed primary axis of movement at the elbow, suggesting an abducted forelimb posture and humeral rotation gait that is not found among marsupials and unlike that seen in any large mammals alive today. This work introduces new quantitative methods and demonstrates the utility of comparative ROM mapping approaches, highlighting that Palorchestes' forelimb function was unlike its contemporaneous relatives and appears to lack clear functional analogues among living mammals.

The Palorchestidae are a family of marsupial megafauna occurring across the eastern Australian continent from the late Oligocene through to their extinction in the Late Pleistocene. The group is known for their odd ‘tapir-like’ crania and distinctive clawed forelimbs, but their appendicular anatomy has never been formally described. We provide the first descriptions of the appendicular skeleton and body mass estimates for three palorchestid species, presenting newly-identified, and in some cases associated, material of mid-Miocene Propalorchestes, Plio-Pleistocene Palorchestes parvus and Pleistocene Palorchestes azael alongside detailed comparisons with extant and fossil vombatiform marsupials. We propose postcranial diagnostic characters at the family, genus and species level. Specialisation in the palorchestid appendicular skeleton evidently occurred much later than in the cranium and instead correlates with increasing body size within the lineage. We conclude that palorchestid forelimbs were highly specialised for the manipulation of their environment in the acquisition of browse, and that they may have adopted bipedal postures to feed. Our results indicate palorchestids were bigger than previously thought, with the largest species likely weighing over 1000 kg. Additionally, we show that P. azael exhibits some of the most unusual forelimb morphology of any mammal, with a uniquely fixed humeroulnar joint unlike any of their marsupial kin, living or extinct.

Pentapedal locomotion is the use of the tail as a fifth leg during the slow gait of kangaroos. Although previous studies have informally noted that some smaller species of macropodines do not engage in pentapedal locomotion, a systematic comparative analysis of tail use during slow gait across a wide range of species in this group has not been done. Analysis of relative movement of the pelvis, tail, and joint angles of the lower limbs during slow gait, using 2D landmark techniques on video recordings, was carried out on 16 species of Macropodinae. We also compared the relative lengthening of the tibia using crural index (CI) to test whether hindlimb morphology was associated with pentapedal locomotion. Pentapedal locomotion was characterised by three features: the presence of the ‘tail repositioning phase’, the constant height of the pelvis and the stationary placement of the distal tail on the ground during the hindlimb swing phase. The mean CI of pentapedal species was significantly greater than that of non-pentapedal species (1.71 versus 1.36; P < 0.001). This lends support to the hypothesis that the use of pentapedal locomotion is associated with the relative lengthening of the hindlimb, which in turn is associated with body size and habitat preference within the Macropodinae.