The braincase and inner ear of ‘Metriorhynchus’ cf. ‘M.’ brachyrhynchus – implications for aquatic sensory adaptations in crocodylomorphs

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.在漫长的演化历程中，鳄形类（crocodylomorphs）演化出了高度多样的体型、生态形态类型以及推测的摄食生态格局。其中一支独特的演化支系为海鳄类（thalattosuchians），它们生存于侏罗纪至白垩纪（约1.91亿至1.25亿年前）。该类群从浅海栖息的类群（如地蜥鳄超科类群（teleosauroids））逐步演化为具备桨状四肢与垂直尾鳍的完全远洋性类群——中喙鳄科类群（metriorhynchids）。学界对中喙鳄科类群适应水生生活的骨骼解剖学特征已有较为充分的研究认知，但针对其神经感觉与颅内腔系统（涵盖脑、内耳、鼻窦与脑神经），以及这些结构与水生生活方式的关联，目前仍所知有限。本研究基于显微计算机断层扫描（micro-computed tomography, μCT）数据与三维模型，对一件完全海生的中喙鳄科类群标本——‘Metriorhynchus’ cf. ‘M.’ brachyrhynchus（NHMUK PV OR 32617）的脑颅与颅内腔解剖结构进行了详细描述。研究团队发现了多项可能助力该物种适应海洋环境的神经解剖学特征，其中包括一项在其他海鳄类中从未被观测到的独特脑颅内模弯曲结构。其余此前已在海鳄类中被报道的特征包括：膨大的大脑半球（cerebral hemispheres）、肥大的静脉窦系统（venous sinus system）、增大的颈内动脉（internal carotid arteries）与骨孔（foramina），以及闭合/缺失的咽鼓管外侧孔（lateral pharyngotympanic foramina）。该标本同时具备远洋中喙鳄科类群特有的骨迷路（bony labyrinth）形态特征：结构紧凑且背腹向短小、半规管（semicircular canals）粗壮、前庭（vestibule）膨大，以及可能存在短缩的耳蜗管（cochlear duct）。对相关性状分布的回顾分析证实，部分此类特征早在中喙鳄科类群演化为远洋类群之前，就已在半水生的海鳄类基干类群中演化形成，这提示颅内腔解剖结构的演变曾推动中喙鳄超科类群（metriorhynchoids）成功开拓海洋生境。