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.

本研究基于显微计算机断层扫描（micro computed tomography）数据，重建并描述了北美布里杰阶始新世（Eocene）古有蹄类（archaic ungulate）纤细偶蹄兔（Hyopsodus lepidus）的骨迷路（bony labyrinth）形态。通过与始新世早期分化的偶蹄目（artiodactyl）类群双锥齿兽（Diacodexis）以及奇蹄目（perissodactyl）类群异马（Xenicohippus）的内耳结构进行对比，可进一步明晰真有蹄类（Euungulata）祖先内耳的形态特征。上述类群在形态上极为相近，仅在半规管（semicircular canal）的角度与轮廓上存在细微差异。三者均具有次级脚（secondary crus）结构，且外侧半规管（lateral semicircular canal）所在平面相对于后半规管（posterior semicircular canal）的位置偏低。这两个特征既被视为兽亚纲（Theria）的祖征，同样也可能是真有蹄类的共同祖征（symplesiomorphy）。纤细偶蹄兔与异马具有其他基干马总科（Equoidea）类群共有的特征，该结果支持此前文献中提出的二者亲缘关系较近的结论。本研究还对纤细偶蹄兔的耳蜗（cochlea）开展了功能学分析，以探讨其是否具备此前文献中提出的陆地回声定位（terrestrial echolocation）能力。纤细偶蹄兔的耳蜗形态并未显示出类似现生小蝙蝠亚目（microchiropteran bats）类群的复杂回声定位特化特征，但其估算的听觉频率范围（208 Hz至76.8 kHz）符合陆地回声定位的适配条件。