Department of Geology & Geophysics, Yale University, New Haven, CT, USA.
Peabody Museum of Natural History, Yale University, New Haven, CT, USA.
Nature. 2019 Feb;566(7745):528-532. doi: 10.1038/s41586-019-0940-x. Epub 2019 Feb 13.
Over the past two centuries, mammalian chewing and related anatomical features have been among the most discussed of all vertebrate evolutionary innovations. Chief among these features are two characters: the dentary-only mandible, and the tribosphenic molar with its triangulated upper cusps and lower talonid basin. The flexible mandibular joint and the unfused symphysis of ancestral mammals-in combination with transformations of the adductor musculature and palate-are thought to have permitted greater mobility of each lower jaw, or hemimandible. Following the appearance of precise dental occlusion near the origin of the mammalian crown, therians evolved a tribosphenic molar with a craggy topography that is presumed to have been used to catch, cut and crush food. Here we describe the ancestral tribosphenic therian chewing stroke, as conserved in the short-tailed opossum Monodelphis domestica: it is a simple symmetrical sequence of lower tooth-row eversion and inversion during jaw opening and closing, respectively, enacted by hemimandibular long-axis rotation. This sequence is coupled with an eversion-inversion rotational grinding stroke. We infer that the ancestral therian chewing stroke relied heavily on long-axis rotation, including symmetrical eversion and inversion (inherited from the first mammaliaforms) as well as a mortar-and-pestle rotational grinding stroke that was inherited from stem therians along with the tribosphenic molar. The yaw-dominated masticatory cycle of primates, ungulates and other bunodont therians is derived; it is necessitated by a secondarily fused jaw symphysis, and permitted by the reduction of high, interlocking cusps. The development of an efficient masticatory system-culminating in the tribosphenic apparatus-allowed early mammals to begin the process of digestion by shearing and crushing food into small boli instead of swallowing larger pieces in the reptilian manner, which necessitates a long, slow and wholly chemical breakdown. The vast diversity of mammalian teeth has emerged from the basic tribosphenic groundplan.
在过去的两个世纪中，哺乳动物的咀嚼和相关的解剖特征一直是所有脊椎动物进化创新中讨论最多的特征之一。其中最重要的两个特征是：仅下颌骨的齿骨和具有三角形上尖牙和下跟尖窝的三尖齿。灵活的下颌关节和祖先哺乳动物未融合的联合-与咀嚼肌和 palate 的转变相结合-被认为允许每个下颌骨或半下颌骨更大的活动性。在哺乳动物冠的起源附近出现精确的牙齿咬合后，有胎盘类动物进化出具有崎岖地形的三尖齿，据推测，这种齿用于捕捉、切割和粉碎食物。在这里，我们描述了短尾负鼠 Monodelphis domestica 中保守的祖先三尖齿类咀嚼运动：它是一个简单的对称序列，分别在下颌打开和关闭期间，下颌牙齿行的反转和内翻，由半下颌长轴旋转执行。这个序列与反转-反转旋转研磨运动相结合。我们推断，祖先的三尖齿类咀嚼运动严重依赖于长轴旋转，包括从第一哺乳动物形式继承的对称反转和内翻，以及从 stem 有胎盘类动物继承的臼齿和三尖齿的臼齿和臼齿旋转研磨运动。灵长类动物、有蹄类动物和其他 bunodont 有胎盘类动物的主导性咀嚼周期是衍生的；它是由下颌联合的二次融合引起的，并通过降低高、互锁尖牙来允许。有效的咀嚼系统的发展-最终形成三尖齿装置-使早期哺乳动物能够通过剪切和将食物压碎成小团块开始消化过程，而不是像爬行动物那样以更大的块吞咽食物，这需要长时间、缓慢和完全的化学分解。哺乳动物牙齿的巨大多样性源于基本的三尖齿基础平面图。
