New material of Seggeurius amourensis (Paenungulata, Hyracoidea), including a partial skull with intact basicranium

Seggeurius is the basal-most hyracoid known to date, and one of the earliest known fossil afrotherians. This taxon has a dramatic importance in mammalian phylogeny, but until now it was known from dental remains only. Here we describe previously unpublished material of Seggeurius, including some mandibular and maxillary fragments, and a braincase. This study is the first to thoroughly describe and discuss the morphology of the ear region and osseous inner ear of a Paleogene hyracoid. It allows a reappraisal of some important characters that are often used in cladistic analyses of Paenungulata. In particular, it seems that in stem hyracoids the orbit is located caudally, the eustachian sac is absent or reduced, the course of the internal carotid artery is intrabullar and medial, and a deep epitympanic sinus, a basisphenoid contribution to the bulla, and the stapedial artery are present. The cochlear canal is rather flattened, and the secondary common crus was rapidly lost. A cladistic analysis of stem placental mammals highlights three derived traits shared by Seggeurius and stem sirenians (the tuber-shaped postglenoid process, the squared and massive mastoid apophysis of the petrosal, and the presence of a swelling on the septum metacochleare) and supports molecular analysis that hypothesizes a Hyracoidea-Sirenia sister-group relationship. This study demonstrates how useful the petrosal and labyrinthine characters of fossils can be for mammalian phylogenetic inference. SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP Citation for this article: Benoit, J., J.-Y. Crochet, M. Mahboubi, J.-J. Jaeger, M. Bensalah, M. Adaci, and R. Tabuce. 2015. New material of Seggeurius amourensis (Paenungulata, Hyracoidea), including a partial skull with intact basicranium. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2015.1034358.

塞盖兽（Seggeurius）是目前已知最基干的蹄兔目（Hyracoidea）类群，也是已知最早的非洲兽总目（Afrotheria）化石类群之一。该类群在哺乳动物系统发育研究中具有重要学术价值，但此前仅通过牙齿化石遗骸对其有所认知。本文首次记述了塞盖兽此前未发表的新材料，包括部分下颌骨、上颌骨碎片以及一件完整的脑颅。
本研究首次全面描述并讨论了古近纪蹄兔类的耳部区域及骨性内耳形态，为重新评估有蹄下纲（Paenungulata）支序分类分析（cladistic analysis）中常用的若干关键性状提供了依据。
研究结果显示，在蹄兔目干群类群中，眼眶位置偏后，咽鼓囊缺失或退化，颈内动脉（internal carotid artery）走行于鼓室内侧且位于鼓室内部；同时存在深大的鼓室上隐窝（epitympanic sinus）、基蝶骨（basisphenoid）参与构成鼓泡（bulla），以及镫骨动脉（stapedial artery）。其耳蜗管（cochlear canal）相对扁平，次级总脚（secondary common crus）快速退化消失。
针对胎盘类干群（stem placental mammals）的支序分析显示，塞盖兽与海牛目（Sirenia）干群共享3处衍征：结节状关节后突（tuber-shaped postglenoid process）、岩骨（petrosal）的方形粗大乳突突起（mastoid apophysis），以及后蜗隔（septum metacochleare）上的隆起结构；该结果支持分子生物学分析提出的蹄兔目与海牛目互为姊妹群关系（sister-group relationship）的假说。
本研究证实了化石类群的岩骨与迷路形态特征（labyrinthine characters）在哺乳动物系统发育推断（phylogenetic inference）中的重要应用价值。
补充数据——本文补充材料可免费通过以下链接获取：www.tandfonline.com/UJVP
本文引用信息：Benoit, J., J.-Y. Crochet, M. Mahboubi, J.-J. Jaeger, M. Bensalah, M. Adaci, and R. Tabuce. 2015. 塞盖兽阿莫伦种新材（Seggeurius amourensis，Paenungulata有蹄下纲，Hyracoidea蹄兔目）及其带有完整颅底的部分头骨。《脊椎动物古生物学杂志》（Journal of Vertebrate Paleontology）。DOI: 10.1080/02724634.2015.1034358.