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