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

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.

短面袋鼠亚科（Sthenurinae）是一类多样性丰富的短面袋鼠类群，起源于中新世，并在上新世与更新世期间发生物种分化。多数物种拥有经骨骼强化的颅骨结构形态，暗示其已演化出适应以极具韧性的食物为食的食性。然而，短面袋鼠亚科颅骨诸多独特且粗壮的结构的功能角色目前仍未明确。本研究采用有限元法（finite element method），对具代表性的短面袋鼠物种西氏拟短面袋鼠（Simosthenurus occidentalis）的颊齿列单侧咬合过程开展全面分析。研究结果将与生态与颅骨比例均与该物种最为相近的现生同类——考拉（Phascolarctos cinereus）的对应分析结果进行对比。模拟结果显示，西氏拟短面袋鼠的颅骨在单侧咬合过程中可产生并承受相对较高的咬合力。其极度扩张的颧弓或容纳了肥大的颧下颌肌，本研究证实该结构可降低颞下颌关节在宽大颊齿列后部进行咬合时发生脱位的风险。这一功能可能同样存在于大熊猫（Ailuropoda melanoleuca）的颧下颌肌中——该物种同样具有扩大的颧弓与该肌肉肥大的特征。此外，西氏拟短面袋鼠颅骨的扩张额板与脑颅形成宽阔的骨弓结构，且加深的上颌骨均从前部齿列延伸至对侧的下颌关节。本研究证实，这类骨弓是在对大型韧性食物进行单侧前臼齿咬合时，抵抗高扭转力的关键结构。这一结果支持了西氏拟短面袋鼠会将厚实的木质化植被送至颊齿处进食的观点，其进食方式与大熊猫碾碎成熟竹秆的行为模式相似。