The inner ear morphology of the ‘condylarthran’ Hyopsodus lepidus

We describe the bony labyrinth morphology of the Eocene ‘archaic ungulate’ Hyopsoduslepidus (Bridgerian, North America) reconstructed from micro computed tomography scan data. Comparisons with the inner ear of the Eocene early diverging artiodactyl Diacodexis and perissodactyl Xenicohippus allow refining the picture of the ancestral inner ear morphology of Euungulates. These taxa are very close morphologically and mostly differ by slight differences in their semicircular canal angulations and profile. They all present a secondary crus and a low position of the plane of the lateral semicircular canal relative to the posterior semicircular canal. These two characters, considered as ancestral features for Theria, might be symplesiomorphies of Euungulata as well. Hyopsodus and Xenicohippus share characters also observed in other basal Equoidea, which would support the close relationship between these two taxa previously proposed in the literature. A functional study of the cochlea of Hyopsodus lepidus is also realised to discuss its putative ability of using terrestrial echolocation previously proposed in the literature. The morphology of the cochlea of Hyopsodus lepidus does not indicate a specialisation to sophisticated echolocation such as observed today in microchiropteran bats. However, its estimated audible range of frequencies (208 Hz to 76.8 KHz) would be compatible with terrestrial echolocation.本研究描述了基于显微计算机断层扫描（micro computed tomography）数据重建的北美布里杰阶始新世（Eocene）古有蹄类（archaic ungulate）瘦似蹄兽（Hyopsodus lepidus）的骨迷路（bony labyrinth）形态。通过与始新世早期分化的偶蹄类（artiodactyl）黛科迪西斯兽（Diacodexis）以及奇蹄类（perissodactyl）异马兽（Xenicohippus）的内耳结构进行比较，得以细化真有蹄类（Euungulata）祖先内耳形态的认知图景。这些类群在形态上极为相近，其差异主要体现于半规管（semicircular canal）的角度与轮廓的细微差别。三者均具备次级骨脚（secondary crus），且外侧半规管所在平面相较于后半规管位置更低。这两个特征曾被认为是兽亚纲（Theria）的祖征，同时也可能是真有蹄类的共享祖征（symplesiomorphies）。瘦似蹄兽与异马兽所共有的特征，也见于其他基干马超科（Equoidea）类群中，这一结果支持此前文献中提出的二者间存在紧密亲缘关系的观点。本研究还对瘦似蹄兽的耳蜗（cochlea）开展了功能学分析，以探讨其此前文献中提出的陆地回声定位（terrestrial echolocation）推定能力。瘦似蹄兽的耳蜗形态并未显示出特化出如现生小翼手亚目蝙蝠（microchiropteran bats）那般精巧的回声定位能力。不过，其估算得到的听觉频率范围（208赫兹至76.8千赫）与陆地回声定位的需求相契合。