ABSTRACT

During their long evolutionary history crocodylomorphs achieved a great diversity of body sizes, ecomorphotypes and inferred feeding ecologies. One unique group of crocodylomorphs are the thalattosuchians, which lived during the Jurassic and Cretaceous (ca. 191–125 Ma). They transitioned from shallow marine species, like teleosauroids, into fully pelagic forms with paddle shaped limbs and a vertically orientated tail fluke, the metriorhynchids. The osteological adaptations that allowed metriorhynchids to live in the water are generally well understood, but less is known about their neurosensory and endocranial systems, such as the brain, inner ears, sinuses and cranial nerves and how they relate to their aquatic lifestyle. Based on micro-computed tomography (μCT) data and three-dimensional models, we here describe the braincase and endocranial anatomy of a fully marine metriorhynchid, ‘Metriorhynchus’ cf. ‘M.’ brachyrhynchus (NHMUK PV OR 32617). We found several neuroanatomical features that likely helped this species function in its marine environment. These include a unique flexure in the brain endocast not seen in other thalattosuchians. Other features that have previously been seen in thalattosuchians include enlarged cerebral hemispheres, a hypertrophied venous sinus system, enlarged internal carotid arteries and foramina, and closed/absent lateral pharyngotympanic foramina. The specimen also possesses a pelagic metriorhynchid bony labyrinth morphology, with a compact and dorsoventrally short shape, thick semicircular canals, an enlarged vestibule and potentially a short cochlear duct. A review of character distribution confirms that some of these features evolved at the base of Thalattosuchia in semiaquatic species, long before metriorhynchids became pelagic, suggesting that endocranial anatomy helped allow metriorhynchoids colonize the ocean realm.

摘要

在其漫长的进化历史中，鳄鱼的体型大小、生态形态类型和推测的摄食生态都非常多样化。一种独特的鳄鱼形动物是 thalattosuchians，它们生活在侏罗纪和白垩纪（约 191-125 Ma）。它们从浅海海洋物种（如远程龙类）转变为具有桨状四肢和垂直方向的尾鳍的完全远洋形态，即河豚。允许 metriorhynchids 生活在水中的骨骼适应性通常被很好地理解，但对它们的神经感觉和内分泌系统（例如大脑、内耳、鼻窦和颅神经）以及它们与它们的水中生活方式的关系知之甚少。基于微计算机断层扫描（μCT）数据和三维模型，Metriorhynchus ' cf. ' M. ' 短吻鳄（NHMUK PV 或 32617）。我们发现了一些神经解剖学特征，可能有助于该物种在其海洋环境中发挥作用。这些包括在其他 thalattosuchians 中没有看到的大脑内模的独特弯曲。先前在拟鳄类动物中发现的其他特征包括大脑半球增大、静脉窦系统肥厚、颈内动脉和椎孔扩大，以及闭合/不存在的外侧咽鼓孔。该标本还具有远洋 metriorhynchid 骨迷路形态，具有紧凑的背腹短形状、厚的半规管、扩大的前庭和可能的短耳蜗管。对特征分布的回顾证实，其中一些特征在半水生物种的 Thalattosuchia 底部进化，早在 metriorhynchids 成为远洋物种之前，