The Head and Neck Anatomy of Sea Turtles (Cryptodira: Chelonioidea) and Skull Shape in Testudines

Background
Sea turtles (Chelonoidea) are a charismatic group of marine reptiles that occupy a range of important ecological roles. However, the diversity and evolution of their feeding anatomy remain incompletely known.
Methodology/Principal Findings
Using computed tomography and classical comparative anatomy we describe the cranial anatomy in two sea turtles, the loggerhead (Caretta caretta) and Kemp’s ridley (Lepidochelys kempii), for a better understanding of sea turtle functional anatomy and morphological variation. In both taxa the temporal region of the skull is enclosed by bone and the jaw joint structure and muscle arrangement indicate that palinal jaw movement is possible. The tongue is relatively small, and the hyoid apparatus is not as conspicuous as in some freshwater aquatic turtles. We find several similarities between the muscles of C. caretta and L. kempii, but comparison with other turtles suggests only one of these characters may be derived: connection of the m. adductor mandibulae internus into the Pars intramandibularis via the Zwischensehne. The large fleshy origin of the m. adductor mandibulae externus Pars superficialis from the jugal seems to be a characteristic feature of sea turtles.
Conclusions/Significance
In C. caretta and L. kempii the ability to suction feed does not seem to be as well developed as that found in some freshwater aquatic turtles. Instead both have skulls suited to forceful biting. This is consistent with the observation that both taxa tend to feed on relatively slow moving but sometimes armoured prey. The broad fleshy origin of the m. adductor mandibulae externus Pars superficialis may be linked to thecheek region being almost fully enclosed in bone but the relationship is complex.

研究背景
海龟总科（Chelonoidea）是一类极具辨识度的海洋爬行动物类群，承担着多种重要的生态功能。然而，目前学界对其摄食解剖结构的多样性与演化机制仍未完全阐明。

研究方法与主要结果
本研究借助计算机断层扫描（Computed Tomography）与经典比较解剖学方法，对两种海龟——蠵龟（*Caretta caretta*）和肯氏丽龟（*Lepidochelys kempii*）——的颅骨解剖结构进行详细描述，以期深入理解海龟的功能解剖学特征与形态变异规律。两类海龟的颅骨颞区均为骨骼所包裹，颌关节结构与肌肉排布表明其具备后退式颌运动能力。其舌头相对短小，舌骨器也不如部分淡水水生龟类那般醒目。本研究发现蠵龟与肯氏丽龟的肌肉结构存在多处相似，但与其他龟类对比后发现，仅其中一处特征可能为衍征：内收下颌肌（musculus adductor mandibulae internus）通过中间腱（Zwischensehne）连接至下颌支部（Pars intramandibularis）。浅部外收下颌肌（musculus adductor mandibulae externus Pars superficialis）起自颧骨的宽大肉质起点，似乎是海龟类群的共有特征。

研究结论与意义
蠵龟与肯氏丽龟的吸食摄食能力似乎不如部分淡水水生龟类发达，二者的颅骨结构均适配强力咬噬行为。这与两类海龟多以行动相对迟缓、部分个体带有硬甲的猎物为食的观测结果相符。浅部外收下颌肌宽大肉质的起源可能与颅骨颊区几乎完全被骨骼包裹存在关联，但二者的具体关联机制仍较为复杂。