Abstract：
Opossums (marsupials of the Didelphidae family) retain a generalized masticatory apparatus and tribosphenic molars, often used as models to understand the evolution of mastication in early therian mammals. Like all marsupials, their growth goes through a stage when pups complete their development while permanently attached to the mother\'s teats before weaning and starting feeding on their own. Yet, while the masticatory muscles of adults are known, as is the ontogeny of the cranium and mandible, the ontogenetic changes in the masticatory muscles remain unknown. Here we describe for the first time the changes in the masticatory muscles observed in lactating pups, and weaned juveniles, subadults, and adults in the White-eared opossum, Didelphis albiventris, through dissection of 25 specimens and quantification of relative muscle masses, lines of actions and mechanical advantages whenever possible. We also assessed the scaling patterns of muscle masses and mechanical advantages through ontogeny. The main changes, as expected, were found between suckling and weaned specimens, although some changes still occurred from juveniles to adults. The adult adductor musculature is similar to the other Didelphis species already known, with a dominant m. temporalis that originates on the lateral wall of the skull, up to the sagittal and nuchal crests, and fills the zygomatic arch when inserting into the lateral and medial surfaces of the coronoid process, respectively through the pars superficialis and pars profunda. The m. masseter is also subdivided in superficial and deep bundles which originate posteriorly in the maxilla and zygomatic arch, and insert into the angular process and masseteric fossa in the mandible. The m. pterygoideus medialis originates from the palatine, the pterygoid bone and the alisphenoid, and it inserts on the angular process medially. Suckling pups showed muscles with more restricted attachments, reduced muscle lines of action, and less diversity in the fiber orientation. The absence of the postorbital constriction also resulted in a distinct morphology of the m. temporalis pars profunda, through two bundles, one anterior and one posterior, which insert more inferiorly into the mandible. These major changes can be related to the onset of mastication and to size-related changes in growing weaned age classes. In general, all adductor muscles grew with positive allometry, and increased their fixation areas through, in part, the development of specific regions of the cranium and mandible. Their lines of action also increase and diversify along ontogeny. These changes can be related to the functional requirements for fixation during lactation, which shift to adduction and mastication movements after weaning.
摘要：
负鼠（Didelphidae家族的有袋动物）保留了广泛的咀嚼器和三七磨牙，通常用作模型来了解早期哺乳动物咀嚼的进化。像所有有袋动物一样，它们的生长经历了一个阶段，幼崽在断奶并开始自己进食之前，永久地附着在母亲的乳头上，完成它们的发育。然而，虽然成年人的咀嚼肌肉是众所周知的，颅骨和下颌骨的个体发育也是如此，咀嚼肌的个体发育变化仍然未知。在这里，我们首次描述了在泌乳幼犬中观察到的咀嚼肌的变化，和断奶的青少年，亚成人，白耳负鼠中的成年人，迪德菲斯·阿尔比文特里斯，通过解剖25个样本和量化相对肌肉质量，只要有可能，动作线和机械优势。我们还通过个体发育评估了肌肉质量的缩放模式和机械优势。主要变化,正如预期的那样，在哺乳和断奶标本之间发现，尽管从青少年到成年人仍然发生了一些变化。成年内收肌肌肉组织与已知的其他Didelphis物种相似，具有起源于头骨侧壁的显性颞叶，直到矢状和颈冠，并在插入冠状突的外侧和内侧表面时填充the弓，分别通过浅部和深部。m。咬肌也细分为浅层和深束，它们起源于上颌骨和the弓的后部，并插入下颌骨的角突和咬窝。中叶翼状肌起源于腭，翼骨和蝶骨,它在中间插入角度过程。幼崽表现出肌肉的附着更受限制，减少肌肉的动作线，纤维取向的多样性较小。不存在眶后收缩也导致了m的明显形态。颞叶深，通过两束，一个前一个后一个,更深入地插入下颌骨。这些主要变化可能与咀嚼的开始以及断奶年龄类别中与大小相关的变化有关。总的来说,所有内收肌都以正异速肌生长，并通过增加它们的固定区域，在某种程度上,颅骨和下颌骨特定区域的发育。他们的行动路线也随着个体发育而增加和多样化。这些变化可能与哺乳期固定的功能要求有关，断奶后转向内收和咀嚼运动。
