The internal cranial morphology of the terrestrial sloth Glossotherium robustum is described here based on a neurocranium from the late Pleistocene of the Pampean region of Buenos Aires, northeastern Argentina. The first published data on the morphology of the brain cavity of this species date back to the latest nineteenth century. The novel techniques of CT scanning and digital reconstructions enable non-destructive access to the internal cranial features of both extinct and extant vertebrates, and thus improve our knowledge of anatomical features that had previously remained obscure. Therefore, we performed CT scans on the posterior half of a skull of G. robustum and created digital models of the endocasts and internal structures. The results reveal the morphology of the brain cavity itself, as well as the paranasal sinuses and the trajectory of several cranial nerves and blood vessels. These features have been compared with the two extant folivoran genera, the two-toed sloth Choloepus and the three-toed sloth Bradypus. For many characteristics, especially those related to the paranasal pneumaticity and the brain cavity, a closer similarity between Glossotherium and Choloepus is observed, in accordance with the most widely accepted phylogenetic scenarios. However, other features are only shared by the two extant genera, but are probably related to allometric effects and the convergence that affected the two modern lineages. This study, which represents the first exhaustive analysis of digital endocasts of a fossil sloth, reveals the importance of the application of new methodologies, such as CT scans, for elucidating the evolutionary history of this peculiar mammalian clade.

中文翻译：

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阿根廷晚更新世已灭绝的树懒 Glossotherium strongum（Xenarthra，Mylodontidae）的数字颅内模型：描述和与现存树懒的比较

陆生树懒 Glossotheriumrobustum 的内部颅骨形态在此基于阿根廷东北部布宜诺斯艾利斯潘佩斯地区晚更新世的神经颅骨进行描述。最早发表的关于该物种脑腔形态的数据可以追溯到 19 世纪。CT 扫描和数字重建的新技术能够无损地访问已灭绝和现存脊椎动物的内部颅骨特征，从而提高我们对以前仍不清楚的解剖特征的了解。因此，我们对 G.robustum 头骨的后半部分进行了 CT 扫描，并创建了内铸件和内部结构的数字模型。结果揭示了脑腔本身的形态，以及副鼻窦和几个颅神经和血管的轨迹。已经将这些特征与两个现存的食叶动物属，两趾树懒 Choloepus 和三趾树懒 Bradypus 进行了比较。对于许多特征，尤其是与鼻旁充气和脑腔相关的特征，根据最广泛接受的系统发育情景，观察到 Glossotherium 和 Choloepus 之间更接近的相似性。然而，其他特征仅由两个现存属共享，但可能与影响两个现代谱系的异速生长效应和趋同有关。这项研究代表了对化石树懒数字内铸的首次详尽分析，揭示了应用新方法的重要性，例如 CT 扫描、