The caudal cranium and occipito-cervical region, although usually overlooked, are informative about the paleobiology of fossil mammals, allowing inquiry into vision and hearing abilities, as well as head and neck postures. Particularly for Pachyrukhinae, some related features remain unexplored. In this contribution, 22 specimens of Paedotherium and Tremacyllus were analyzed in a mainly qualitative and comparative framework. Pachyrukhines are characterized by having large orbits and hearing cavities, moderate to short necks with generalized morphologies, and S-shape postures. These features allow rejecting some types of specialized digging habits, and support the preference of open or low-humidity environments. Paedotherium typicum is distinguished by the presence of laterally oriented eyes, marked vaulted cranium and predominant short extensor and stabilizing neck muscles, and cervico-occipital-hyoid configurations suitable for ventro-flexed resting posture. These features indicate accentuated frontation and panoramic-vision, upward head postures enhancing substrate perception, and the resistance of impacts during leaping-cursorial locomotion. Conversely, P. bonaerense, and to lesser degree Tremacyllus spp., show less frontation and probably adopted more horizontal head postures. More particularly, stronger ventral and lateral neck and head flexors and extrinsic arm musculature are reconstructed for P. bonaerense, compatible with generalist or scratch-digging habits. Its smaller auditory cavities and stronger ear musculature, compared to the contemporary P. typicum and especially Tremacyllus spp., would indicate larger ears and microhabitat segregation. The integrative analysis proposed here and in the accompanying contribution aims to shed light on convergences with extant models, paleobiology, niche partitioning, and external appearance of the latest rodent-like ungulates.

中文翻译：

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追踪 Paedotherium 和 Tremacyllus（Pachyrukhinae，Notoungulata）的古生物学，最新的 Sciuromorph 南美本土有蹄类动物——第二部分：轨道、听觉和枕颈区域

尾颅骨和枕颈区域虽然通常被忽视，但对化石哺乳动物的古生物学提供了丰富的信息，允许对视觉和听觉能力以及头部和颈部姿势进行调查。特别是对于 Pachyrukhinae，一些相关的特征仍未被探索。在这篇文章中，主要在定性和比较框架中分析了 22 个 Paedotherium 和 Tremacyllus 标本。Pachyrukhines 的特点是具有大的眼眶和听觉腔，具有广泛形态的中至短颈部，以及 S 形姿势。这些功能允许拒绝某些类型的专门挖掘习惯，并支持开放或低湿度环境的偏好。典型的Paedotherium typicum 的特点是存在横向定向的眼睛，明显的拱形颅骨和主要的短伸肌和稳定的颈部肌肉，以及适合腹屈休息姿势的颈枕-舌骨结构。这些特征表明突出的正面和全景视觉，向上的头部姿势增强了基板的感知，以及跳跃式光标运动过程中的冲击阻力。相反，P. bonaerense 和较小程度的 Tremacyllus spp. 表现出较少的正面，并且可能采用更多的水平头部姿势。更具体地说，为 P. bonaerense 重建了更强的腹侧和外侧颈部和头部屈肌以及外在手臂肌肉组织，与通才或抓挠习惯兼容。与当代的 P. typicum 尤其是 Tremacyllus spp. 相比，它的耳腔更小，耳朵肌肉组织更强壮，将表明更大的耳朵和微生境隔离。这里和随附的贡献中提出的综合分析旨在阐明与最新的啮齿动物类有蹄类动物的现存模型、古生物学、生态位分区和外观的融合。