Scott Bradley R, Brainerd Elizabeth L, Wilga Cheryl A D
Department of Evolution, Ecology, and Behavior, University of Illinois Urbana-Champaign, 61801 IL, USA.
Department of Ecology, Evolution, and Organismal Biology, Brown University, 02912 RI, USA.
Integr Org Biol. 2022 Jul 25;4(1):obac024. doi: 10.1093/iob/obac024. eCollection 2022.
Long-axis rotation (LAR) of the jaws may be an important component of vertebrate feeding mechanisms, as it has been hypothesized to occur during prey capture or food processing across diverse vertebrate groups including mammals, ray-finned fishes, and sharks and rays. LAR can affect tooth orientation as well as muscle fiber direction and therefore muscle power during feeding. However, to date only a handful of studies have demonstrated this LAR in vivo. Here, we use XROMM to document LAR of the upper and lower jaws in white-spotted bamboo sharks, during suction feeding. As the lower jaw begins to depress for suction expansion, both the upper jaw (palatoquadrate) and lower jaw (Meckel's cartilage) evert, such that their toothed surfaces move laterally, and then they invert with jaw closing. Eversion has been shown to tense the dental ligament and erect the teeth in some sharks, but it is not clear how this tooth erection would contribute to suction feeding in bamboo sharks. Two recent XROMM studies have shown LAR of the lower jaws during mastication in mammals and stingrays and our study extends LAR to suction feeding and confirms its presence in shark species. Examples of LAR of the jaws are becoming increasingly widespread across vertebrates with unfused mandibular symphyses. Unfused lower jaws are the plesiomorphic condition for most vertebrate lineages and therefore LAR may be a common component of jaw mechanics unless the mandibular symphysis is fused.
颌骨的长轴旋转（LAR）可能是脊椎动物进食机制的一个重要组成部分，因为据推测，在包括哺乳动物、硬骨鱼以及鲨鱼和鳐鱼在内的各种脊椎动物群体捕食或处理食物的过程中都会发生这种现象。LAR会影响牙齿的方向以及肌肉纤维的方向，进而影响进食时的肌肉力量。然而，迄今为止，只有少数研究在活体中证实了这种LAR。在这里，我们使用XROMM技术记录白点竹鲨在吸食进食过程中上、下颌的LAR。当下颌开始下压以进行吸食扩张时，上颌（腭方软骨）和下颌（梅克尔软骨）都会外翻，使得它们的齿面横向移动，然后在颌骨闭合时再内翻。在一些鲨鱼中，外翻已被证明会拉紧牙周韧带并使牙齿竖起，但目前尚不清楚这种牙齿竖起对竹鲨的吸食进食有何作用。最近的两项XROMM研究表明，在哺乳动物和黄貂鱼咀嚼过程中下颌存在LAR，而我们的研究将LAR扩展到了吸食进食，并证实了其在鲨鱼物种中的存在。颌骨LAR的例子在具有未融合下颌联合的脊椎动物中越来越普遍。未融合的下颌是大多数脊椎动物谱系的原始状态，因此，除非下颌联合融合，否则LAR可能是颌骨力学的一个常见组成部分。
