DOCTORAL DISSERTATION RESEARCH: Symphyseal Fusion in Ungulates and Anthropoids: A Case of Functional Convergence?

在过去的三十年里，人类学家一直是研究哺乳动物咀嚼器官最活跃的研究人员之一。除了大量的比较形态学研究外，体质人类学家对体内咀嚼功能的研究也做出了重要贡献。因此，我们对哺乳动物颅骨形态的了解大多是基于灵长类动物模型。这些研究揭示了一些关于灵长类动物颅面形态进化的有趣模式。特别是，研究表明，颚肌功能在灵长类动物咀嚼形式的进化中发挥了重要作用。在类人猿灵长类动物（即猴子、猿和人类）的早期发育过程中，下颌联合（下颌骨两半之间的关节）融合或完全骨化。然而，像大多数哺乳动物一样，最早的真正的灵长类动物（称为链鼻动物或原灵长类动物）保留了高度移动、不融合的联合的原始状态。有趣的是，最近的研究表明，类人猿和链球菌在咀嚼过程中，从平衡侧（即非咀嚼侧）肌肉中吸收横向力的时间和大小不同，下颌内收肌的吸收模式不同。特别令人感兴趣的是平衡侧深咬肌的活动，它的力有很大的横向分量。与链球菌相比，在类人猿中，这块肌肉相对于其他颚内收肌的招募时间较晚，而且来自这块肌肉的力的大小增加了。这种深咬肌的活动模式导致下颌骨的两半被拉开，导致联合关节受到严重的压力和拉伤。因此，有证据表明，在类人猿中，颚肌放电模式、联合骨负荷和联合骨融合的进化是相关的。本研究概述的山羊、羊驼和马的研究将有助于从体内角度研究哺乳动物咀嚼器官的文献的增长。由于我们对有蹄类动物，特别是羊驼和马的咀嚼功能知之甚少，因此本文收集的肌电图、应变和运动学数据将扩展我们对另外两个哺乳动物目的颅形和功能的理解。更重要的是，由于羊驼和马也有融合的联合骨，这项研究将揭示非灵长类动物联合骨融合的进化。为此，这些数据将有助于确定灵长类动物和有蹄类动物咀嚼形式进化的趋同趋势。具体而言，本研究将阐明尽管面部形态不同，但这些动物的联合关节是否存在类似的功能限制。除了上述科学贡献外，这项研究的更广泛影响还包括少数民族和女性共同负责人的参与和研究生培训。由于本研究中收集的许多数据将与未来的研究相关，因此本项目的资助将是推进该Co-PI的科学事业不可或缺的一部分。

In the last three decades anthropologists have been among the most active researchers on the mammalian masticatory apparatus. In addition to numerous comparative morphological studies, physical anthropologists have contributed significantly to the growing literature on in vivo masticatory function. As a result, much of what is known about the mammalian cranial form is based on primate models. These studies have revealed a number of interesting patterns regarding the evolution of primate craniofacial form. In particular, research suggests that jaw-muscle function has played a significant role in the evolution of primate masticatory form. During the early development of anthropoid primates (i.e., monkeys, apes and humans), the mandibular symphysis (the joint between the two halves of the mandible) fuses or becomes completely ossified. However, like most mammals, the earliest true primates (called strepsirrhines or prosimians) retain the primitive condition of a highly mobile, unfused symphysis. Interestingly, recent work shows that jaw adductor recruitment patterns during mastication differ between anthropoids and strepsirrhines with respect to the timing and magnitude of recruitment of transversely oriented force from the balancing-side (i.e., non-chewing side) muscles. Of particular interest is the activity of the balancing-side deep masseter muscle, which has a large transverse component to its force. Compared to strepsirrhines, in anthropoids this muscle is recruited late relative to the other jaw adductors and the magnitude of force from this muscle is increased. This pattern of deep masseter activity causes the two halves of the mandible to be pulled apart resulting in significant stress and strain in the symphysis. Thus, there is evidence that jaw muscle firing patterns, symphyseal loading and the evolution of symphyseal fusion are linked in anthropoids. The research on goats, alpacas and horses outlined in this study will contribute to the growing literature on the mammalian masticatory apparatus from an in vivo perspective. As little is known about masticatory function in ungulates, particularly in alpacas and horses, the electromyographic, strain and kinematic data collected here will expand our understanding of cranial form and function in two additional mammalian orders. More importantly, as alpacas and horses also have fused symphyses, this study will shed light on the evolution of symphyseal fusion in non-primates. To this end, these data will aid in identifying convergent trends in the evolution of masticatory form in primates and ungulates. Specifically, this research will clarify whether there are similar functional constraints on the symphysis in these animals despite differences in facial morphology. In addition to the scientific contributions outlined above, the broader impacts of this research include the participation and graduate training of a minority and female Co-PI. As much of the data collected in this study will be pertinent to future research, funding for this project will be integral in advancing the scientific career of this Co-PI.