Anatomy of the nasal and auditory regions of the fossil lagomorph Palaeolagus haydeni: systematic and evolutionary implications

Palaeolagus, a late Eocene to early Miocene North American lagomorph genus, represented by numerous and well-preserved specimens, has been long considered a basal leporid, although it is currently understood as a stem lagomorph. Based on micro-computed tomography (μCT) data and 3D reconstructions, here we present the first description of intracranial structures of the nasal and auditory regions of a complete skull of Palaeolagus haydeni from the early Oligocene of Nebraska. Although Palaeolagus haydeni shows a puzzling mixture of extant leporid and ochotonid characters, it helps to polarize and re-evaluate already known lagomorph intracranial characters based on outgroup comparison with Rodentia and Scandentia. Common derived features of Palaeolagus haydeni and extant Lagomorpha are the dendritic maxilloturbinal and the excavated nasoturbinal that contacts the lamina semicircularis. Generally, Palaeolagus haydeni and Leporidae have several characters in common, some of which are certainly plesiomorphic (e.g., thin wall of bulla tympani and flat conic cochlea). Palaeolagus haydeni resembles Leporidae in having an interturbinal between the two frontoturbinals, and three ethmoturbinals plus one interturbinal between ethmoturbinal I and II. Now, this should also be regarded as a plesiomorphic grundplan pattern for Leporidae whereas ochotonids are derived from the lagomorph grundplan as concerns the number of frontoturbinals. Concerning the middle ear, Palaeolagus haydeni significantly contributes to the polarization of the anterior anchoring of the malleus in extant lagomorphs. Palaeolagus haydeni resembles the pattern observed in early ontogenetic stages of Ochotonidae, i.e., the attachment of the malleus to the ectotympanic via a short processus anterior. The patterns in adult ochotonids and leporids now can be regarded as two different and apomorphic character states. Autapomorphic characters of Palaeolagus haydeni are the reduced frontoturbinal 2 and the additional anterolaterally oriented process of the lamina semicircularis. Interestingly, among the investigated intracranial structures the loss of the secondary crus commune is the only apomorphic grundplan character of crown Lagomorpha.

古兔属（Palaeolagus）是始新世晚期至中新世早期的北美兔形类属群，其化石标本数量众多且保存完好，长期以来被认为是基础兔科类群，不过当前研究将其归入兔形类茎群。基于显微计算机断层扫描（micro-computed tomography，μCT）数据与三维重建技术，本文首次对内布拉斯加州渐新世早期地层出土的海登古兔（Palaeolagus haydeni）完整头骨的鼻腔与听觉区域颅内部结构进行描述。尽管海登古兔呈现出现生兔科与鼠兔科特征的混合状态，令人费解，但通过与啮齿目（Rodentia）和树鼩目（Scandentia）的外类群比较，该属有助于明确兔形类颅内部性状的极性，并重新评估已知的兔形类颅内性状。海登古兔与现生兔形类共有的衍征包括：树突状上颌鼻甲，以及与半规管板（lamina semicircularis）相连的凹陷式鼻腔鼻甲。总体而言，海登古兔与兔科存在多项共有性状，其中部分无疑属于祖征（例如鼓室泡薄壁与扁平锥形耳蜗）。海登古兔与兔科类似，在两个额鼻甲之间存在一枚鼻甲间片，且在第一与第二筛鼻甲之间具3个筛鼻甲及1枚鼻甲间片。这一特征应被视为兔科的基础构型祖征模式，而鼠兔科类群则在额鼻甲数量上偏离了兔形类的基础构型。关于中耳结构，海登古兔为明确现生兔形类锤骨的前附着极性提供了重要依据。海登古兔的锤骨通过短前突附着于外鼓骨，这一模式与鼠兔科的早期个体发育阶段一致。当前可将成年鼠兔科与兔科的该特征视为两种不同的衍征状态。海登古兔的自衍征包括缩小的第2额鼻甲，以及半规管板额外的前外侧指向突起。值得注意的是，在所研究的颅内部结构中，次级总脚（secondary crus commune）的缺失是冠群兔形类唯一的基础构型衍征。