The anatomy of a crushing bite: The specialised cranial mechanics of a giant extinct kangaroo

Abstract

The Sthenurinae were a diverse subfamily of short-faced kangaroos that arose in the Miocene and diversified during the Pliocene and Pleistocene. Many species possessed skull morphologies that were relatively structurally reinforced with bone, suggesting that they were adapted to incorporate particularly resistant foods into their diets. However, the functional roles of many unique, robust features of the sthenurine cranium are not yet clearly defined. Here, the finite element method is applied to conduct a comprehensive analysis of unilateral biting along the cheek tooth battery of a well-represented sthenurine, Simosthenurus occidentalis. The results are compared with those of an extant species considered to be of most similar ecology and cranial proportions to this species, the koala (Phascolarctos cinereus). The simulations reveal that the cranium of S. occidentalis could produce and withstand comparatively high forces during unilateral biting. Its greatly expanded zygomatic arches potentially housed enlarged zygomaticomandibularis muscles, shown here to reduce the risk of dislocation of the temporomandibular joint during biting with the rear of a broad, extensive cheek tooth row. This may also be a function of the zygomaticomandibularis in the giant panda (Ailuropoda melanoleuca), another species known to exhibit an enlarged zygomatic arch and hypertrophy of this muscle. Furthermore, the expanded frontal plates of the S. occidentalis cranium form broad arches of bone with the braincase and deepened maxillae that each extend from the anterior tooth rows to their opposing jaw joints. These arches are demonstrated here to be a key feature in resisting high torsional forces during unilateral premolar biting on large, resistant food items. This supports the notion that S. occidentalis fed thick, lignified vegetation directly to the cheek teeth in a similar manner to that described for the giant panda when crushing mature bamboo culms.

Temporomandibular joint shape in anthropoid primates varies widely and is patterned by size and phylogeny

“…Statistical significance of the rPLS value was assessed using a resampling procedure with 9999 iterations. For each model, we also report the multivariate effect size (Adams & Collyer, 2016, 2019 and the proportion of the total variance in size and shape that is expressed by that singular warp for that block of data (i.e., the percent of the total variance for a single block represented by that block's first singular axis).…”

“…In regard to leverage patterns, one important aspect of biomechanics to note is that changes in the position of the bite point relative to the joint may explain the covariation we observe between relatively longer faces and glenoids with increased robusticity and/or joint relief (i.e., larger processes). Specifically, more anteriorly positioned bite points (i.e., as would occur when the face is elongated) should result in higher joint reaction forces (i.e., Hylander, 1977Hylander, , 2006Mitchell, 2019), which must then be adequately dissipated by either soft or hard tissue structures at the joint. Future analyses will explore covariation between TMJ, craniofacial, and dental shape as well as relationships between the glenoid fossa and measures of masticatory performance and efficiency.…”

Multiple modes of inference reveal less phylogenetic signal in marsupial basicranial shape compared with the rest of the cranium

“…This means that phylogenetic signal in shape variation may conceivably be related to variation in size or function, which could lead to homoplasy in shape [31,32,68] and might be more easily evaluated using metrics of size, locomotion, or diet alone. We explored this by asking how much shape variation was associated with overall cranial centroid size (see below) and two widely cited contributors to cranial shape: diet [47,49,51] and locomotion (reviewed in [69]). For this, we used phylogenetically informed generalized least squares (GLS) analysis as implemented in the mvMORPH package [70].…”

More Challenging Diets Sustain Feeding Performance: Applications Toward the Captive Rearing of Wildlife

“…Stress refers to the amount of force per unit area experienced at a specific location of an object during an action. In the context of this study, stress is defined as force per unit of bone, so an increase in bone mass at a specific region of the cranium will decrease the amount of stress experienced at that region for a given amount of force (see Mitchell 2019 ). We expected group differences in stress magnitudes to be most obvious in cranial regions that demonstrate high levels of strain when biting, namely the superior muzzle base and inferior aspect of the zygomatic arch ( Hylander and Johnson 1997 ; Franks et al.…”