Paleoneurology, auditory region, and associated soft tissue inference in the late Oligocene notoungulates Mendozahippus fierensis and Gualta cuyana (Toxodontia) from central-western Argentina

Noninvasive computed tomography (CT) scanning techniques performed on the high-quality preserved holotypes of Mendozahippus fierensis (grouped within the paraphyletic ‘Notohippidae’) and Gualta cuyana (Leontiniidae) allowed a thorough description of both endocasts, not only associated with the brain but also with closely related endocranial spaces, which proved to be highly diagnostic among Notoungulata and relevant from a paleobiological perspective. The specimens come from Quebrada Fiera (Mendoza, Argentina), an upper Oligocene locality extensively studied, especially during the last decade. Regarding brain endocast proportions and neocortical complexity, M. fierensis shows morphological affinities to coeval ‘notohippids,’ whereas G. cuyana resembles Leontinia gaudryi. An acute olfaction is inferred from the large olfactory bulbs and piriform lobes. Conversely, evidence concerning the development (and implications) of some necortical areas (i.e., somatosensory cortex and auditory cortex) were inconclusive. The encephalization quotients are around those previously reported for ‘notohippids’ and Toxodontidae Nesodontinae, and slightly lower than in Typotheria (except for Mesotheriidae). Other endocranial traits previously unexplored (such as the transverse diploic communication between the temporal sinuses in M. fierensis and the large temporal sinuses inferred for G. cuyana) show that there is relevant variability within Toxodontia that deserves further research. Extending similar approaches to early diverging taxa will certainly contribute to a better understanding of the morphological evolution of the brain, special sense organs, and associated blood circulation pattern in toxodontians and notoungulates in general.

本研究采用无创计算机断层扫描（computed tomography, CT）技术，对保存完好的费耶尔门多萨兽（Mendozahippus fierensis，归属于并系群「后弓兽科（Notohippidae）」）及瓜尔塔库亚纳兽（Gualta cuyana，隶属于莱昂蒂尼科（Leontiniidae））的正模标本开展扫描，得以对两者的颅内模（endocasts）展开详尽描述——这些颅内模不仅关联脑组织，还涵盖紧密相关的颅内腔隙，经证实其在南方有蹄目（Notoungulata）中具有极高的分类诊断价值，且古生物学意义重大。这些标本产自阿根廷门多萨省的费耶拉峡谷（Quebrada Fiera），该渐新世晚期地层自近十年以来已得到广泛研究。就颅内模的比例及新皮层（neocortical）复杂度而言，费耶尔门多萨兽与同期的「后弓兽类」存在形态亲缘关系，而瓜尔塔库亚纳兽则与高德里莱昂蒂尼兽（Leontinia gaudryi）形态相似。从硕大的嗅球（olfactory bulbs）及梨状叶（piriform lobes）可推断，该类群拥有敏锐的嗅觉。反之，关于部分新皮层区域（即躯体感觉皮层（somatosensory cortex）与听觉皮层（auditory cortex））的发育情况及其相关意义的证据仍不明确。脑化指数（encephalization quotients）大致与此前报道的「后弓兽类」及弯齿兽科（Toxodontidae）内的新齿兽亚科（Nesodontinae）相当，略低于异节目（Typotheria）的类群（除中兽科（Mesotheriidae）外）。此前未被探索的其他颅内腔隙特征（例如费耶尔门多萨兽的颞窦（temporal sinuses）之间横向板障（diploic）连通结构，以及瓜尔塔库亚纳兽所推断出的宽大颞窦）表明，弯齿兽类（Toxodontia）内部存在值得进一步研究的显著形态变异。将类似研究方法拓展至早期分化的类群，无疑将有助于我们更深入理解弯齿兽类乃至整体南方有蹄类的脑组织、特殊感觉器官及相关血液循环模式的形态演化历程。