Low elbow mobility indicates unique forelimb posture and function in a giant extinct marsupial

Abstract

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.

A multimethod approach to the differentiation of enthesis bone microstructure based on soft tissue type

“…There are many instances where researchers have made inferences about the presence of soft tissue anatomy in fossils, either based on exceptional preservation (Bell et al, 2014; Bennett, 2002; Brown et al, 2017) or interpretation of bone morphology (Burch, 2014; Carrano & Hutchinson, 2002; Drysdale et al, 2018; Witzmann & Schoch, 2013). Not only this, but the ability to determine soft tissue type can lead to further refinement of functional models (Manafzadeh, 2020; Richards et al, 2021). Knowledge of the presence of aponeuroses and articular cartilage can constrain range of motion studies.…”

Paleobiological reconstructions of articular function require all six degrees of freedom

“…The no‐translation condition resulted in 197,173 hip joint configurations and 50,653 hinge joint configurations tested per joint. Under the single‐translation approach, we allowed translation of the rotational pivot along the axis representing joint distraction−compression to the minimum, mid‐range, and maximum values measured from cadavers to simulate pooling the results of joint space sensitivity analyses (e.g., those of Demuth et al, 2020; Nyakatura et al, 2015; and Richards et al, 2021). The single‐translation condition evaluated three translations per rotational pose, resulting in 591,519 hip joint configurations and 151,959 hinge joint configurations tested per joint.…”

“…The all‐translation condition resulted in 5,323,671 hip joint configurations and 1,367,631 hinge joint configurations tested per joint. To our knowledge, no virtual ROM study to date has systematically allowed combinations of motions in all three rotational and all three translational degrees of freedom (but see a partial six degree of freedom sensitivity analysis by Richards et al, 2021).…”

“…Traditionally, paleobiologists began these reconstructions by physically manipulating pairs of fossil bones to estimate the rotational range of motion (ROM) of individual joints (e.g., Bramwell & Whitfield, 1974; Carpenter, 2002; Carpenter & Wilson, 2008; Fröbisch, 2006; Jenkins, 1971; Padian, 1983; Paul & Christiansen, 2000; Senter & Robins, 2005; Senter & Sullivan, 2019). Although virtual ROM analyses have largely superseded physical ones (e.g., Demuth et al, 2020; Mallison, 2010a,b; Molnar et al, 2021; Nyakatura et al, 2015, 2019; Pierce et al, 2012; Richards et al, 2021), their primary objective has remained the same: to identify impossible joint poses and to exclude potential behaviors that require them.…”

“…If viable joint poses were to be omitted from ROM estimates, they would also erroneously be excluded from reconstructions before functional variables such as bone strain, energy expenditure, muscle moment arms, or balance (e.g., Bishop et al, 2021; Carrano et al, 1998; Molnar et al, 2021; Nyakatura et al, 2019) are even considered. This possibility is especially concerning because ROM‐based exclusion of limb configurations has played a large role in shaping not only our understanding of the stance and gait of individual taxa (e.g., Gatesy et al, 2009; Lai et al, 2018; Mallison, 2010a,b; Richards et al, 2021) but also that of major transformations in locomotor evolution (such as the origins of terrestriality, bipedality, and flight; e.g., Demuth et al, 2020; Molnar et al, 2021; Nyakatura et al, 2019; Pierce et al, 2012).…”