Data from: Morphology of the petrosal and stapes of Borealestes (Mammaliaformes, Docodonta) from the Middle Jurassic of Skye, Scotland

\We describe, in unprecedented detail, the petrosals and stapes of the docodont Borealestes from the Middle Jurassic of Scotland, using high resolution μCT and phase‐contrast synchrotron imaging. We describe the inner ear endocast and the vascularized interior structure of the petrosal, and provide the first endocranial view of a docodontan petrosal. Our study confirms some similarities in petrosal and stapedial morphology with the better known Haldanodon of the Late Jurassic of Portugal, including: (1) the degree of curvature of the cochlea; (2) multiple features related to the highly pneumatized paroccipital region; (3) the shape of lateral trough, the fossa of the M. tensor tympani, and the ridge on the promontorium; (4) the round shape of the fenestra vestibuli; and (5) overall morphology of the stapes. But Borealestes differs from Haldanodon in having a bony ridge that separates the tympanic opening of the prootic canal, the secondary facial foramen and the hiatus Fallopii, from the fenestra vestibuli. We identify two new vascular structures: the anterior and posterior trans‐cochlear sinuses, which traverse the pars cochlearis around the cochlear nerve (VIII). These trans‐cochlear sinuses have not been observed in previous docodont specimens, and could be an autapomorphy of Borealestes, or apomorphic for this clade. We also establish the anatomical relationship of the circum‐promontorium plexus to the inner endocast. The high quality of our scans has made these structures visible for the first time.

本研究借助高分辨显微CT（μCT）与相衬同步辐射成像技术，对苏格兰中侏罗世产出的柱齿兽类（Docodonta）北极兽属（Borealestes）的岩骨（petrosal）与镫骨（stapes）开展了前所未有的精细形态描述。本研究还详细描述了该物种的内耳内模（inner ear endocast）与岩骨的血管化内部结构，并首次呈现了柱齿兽类岩骨的颅腔内视图。本研究证实，北极兽与研究更为深入的葡萄牙晚侏罗世哈尔丹兽（Haldanodon）在岩骨与镫骨形态上存在多项相似特征，具体包括：(1) 耳蜗（cochlea）的弯曲程度；(2) 与高度气生副枕区相关的诸多解剖特征；(3) 外侧沟、鼓膜张肌（M. tensor tympani）窝以及鼓岬（promontorium）上的嵴的形态；(4) 前庭窗（fenestra vestibuli）的圆形轮廓；以及(5) 镫骨的整体形态。但北极兽与哈尔丹兽存在形态差异：前者具有一处骨性嵴，可将岩神经管鼓室口、第二面神经孔以及面神经管裂孔（hiatus Fallopii）与前庭窗分隔开来。本研究识别出两处全新的血管结构：环绕蜗神经（VIII）走行于耳蜗部（pars cochlearis）的前、后耳蜗跨窦（trans-cochlear sinuses）。此类耳蜗跨窦此前从未在柱齿兽类标本中被观测到，其既可能为北极兽的独有衍征（autapomorphy），也可能代表该演化支的衍征特征。此外，我们明确了岬周围丛（circum-promontorium plexus）与内耳内模的解剖学关联。得益于本次扫描的高质量成像，这些结构得以首次被清晰观测到。