The new earliest diverging Mesotheriinae (Mammalia, Notoungulata) from the Early Miocene (Burdigalian) of the Puna region, Catamarca, Argentina

Mesotheriidae (Notoungulata) is an extinct clade of herbivorous mammals that was widely distributed in South America from the early Oligocene to the Early–Middle Pleistocene. Within this clade, only Mesotheriinae is recognized as a monophyletic group, encompassing Early Miocene–Pleistocene taxa. The earliest mesotheriine records date back to the Early Miocene and are from Río Negro Province (Argentina), but most complete specimens are from Mendoza Province (Argentina) and northern Chile. During the Middle to Late Miocene, mesotheriines diversified and became abundant across Bolivia, Argentina and Chile. However, their distribution decreased to only Argentina from the Pliocene until their extinction in the Pleistocene. Recent findings from north-western Argentina indicate a more complex history for this lineage, with morphological affinities with Bolivian taxa. Here, we present a new mesotheriine from the Puna of Catamarca Province, which represents the northernmost record in Argentina and the first fossil record from the Potrero Grande Formation (fossil levels dated to ∼18 Ma; Burdigalian, Early Miocene). The specimen MHAS 161 consists of an almost complete palate with P2–M3 and has unique features such as rooted premolars, developed P2, enamel fossettes and a lingual groove in the third lobe of M1–3. After comparative and phylogenetic analyses (descriptive, metrics and morphometric geometrics), we propose that MHAS 161 represents a new taxon (<i>Ichhutherium wayra</i> gen. et sp. nov.) characterized by a combination of ancestral and derived dental and palatal traits, with an estimated body mass of 7.16–15.34 kg. The phylogenetic analysis identifies <i>I. wayra</i> gen. et sp. nov. as the earliest diverging Mesotheriinae, indicating the first radiation of the lineage. This study reopens debates on Mesotheriidae ontogenetic stages and trait development. Future explorations and findings in outcrops of the early Neogene of the Argentine Puna could be crucial for a deeper understanding of South American native ungulates and other vertebrates. https://zoobank.org/urn:lsid:zoobank.org:pub:6DFDFBFE-ABA1-4FB1-9778-FEBF29A1AF8E

中柱兽科（Mesotheriidae，南方有蹄目Notoungulata）是一类已灭绝的植食性哺乳类演化支，在渐新世早期至早-中更新世期间广泛分布于南美洲。该演化支中仅中柱兽亚科（Mesotheriinae）被认定为单系群，涵盖了中新世早期至更新世的所有分类单元。最早的中柱兽亚科化石记录可追溯至中新世早期，发现于阿根廷内格罗河省（Río Negro Province），而迄今最完整的标本则出自阿根廷门多萨省（Mendoza Province）与智利北部。中新世中晚期，中柱兽亚科发生辐射分化，在玻利维亚、阿根廷及智利境内大量繁盛。但自上新世至更新世灭绝前，其分布范围逐渐缩减至仅存于阿根廷境内。近期在阿根廷西北部的化石发现表明，该支系的演化历史更为复杂，其形态特征与玻利维亚的同类群存在亲缘关联。
本文报道了一件产自阿根廷卡塔马卡省普纳（Puna）地区的中柱兽亚科新类群，该标本不仅是阿根廷境内已知最北端的中柱兽亚科化石记录，同时也是波托雷罗格兰德组（Potrero Grande Formation）的首件化石报道——该地层的化石层年代约为18 Ma，对应早中新世布尔迪加尔阶（Burdigalian）。标本MHAS 161为一件近乎完整的带有P2–M3齿列的腭骨，具备多项独有特征：具根的前臼齿、发育完善的P2、釉质窝（enamel fossettes）以及M1–3第三叶的舌侧沟（lingual groove）。经比较解剖学、度量学及几何形态测量学等系统发育与比较分析后，我们认为MHAS 161代表一个全新的分类单元——*Ichhutherium wayra* gen. et sp. nov.（新属新种），该类群兼具祖先型与衍生型的牙齿及腭骨特征，估算体重为7.16–15.34 kg。
系统发育分析显示，*Ichhutherium wayra* gen. et sp. nov.是中柱兽亚科中最早分化的类群，代表该支系的首次辐射演化事件。本研究重新引发了关于中柱兽科个体发育阶段与特征演化的学术讨论。未来在阿根廷普纳地区早新近纪露头中的勘探与新发现，将对深入理解南美本土有蹄类及其他脊椎动物的演化具有关键意义。https://zoobank.org/urn:lsid:zoobank.org:pub:6DFDFBFE-ABA1-4FB1-9778-FEBF29A1AF8E