Most Complete Pre-Deseaden Litoptern Postcranium (Gran Hondonada, Middle–Late Eocene, Chubut Province, Argentina)

Litopterna is an order of South American Native Ungulates, known for their cursorial adaptations.However, these features are only well documented in specimens from the late Oligocene (Deseadan Age) and Neogene.Here, we report a partial skeleton of a small litoptern from Gran Hondonada (middle-late Eocene).It consists of elements from both hind limbs of the same individual.Preliminary cladistic analyses (113 dental, cranial, and postcranial characters scored across 50 taxa) recovered the specimen (MLP-PV 67-II-27-375) as a branch of Lopholipterna that diverged before Macraucheniidae and Proterotheriidae, a position supported by two dental and four postcranial synapomorphies.It shares the following plesiomorphic traits with Protolipternidae: absence of the patellar groove on the tibia, a crest-like peroneal process, and the presence of a plantar process on the ectocuneiform bone.Like other lopholipterns, its tibia shows an anterior facet for the astragalus, a reduced medial malleolus, and more complex facets in the tarsal sinus.Various regression models were applied to associate MLP-PV 67-II-27-375 with a known taxon based on tooth size.An indeterminate Adianthidae was the only compatible match.However, MLP-PV 67-II-27-375 could also belong to a taxon not represented by dental remains.Regardless of its taxonomic identity, MLP-PV 67-II-27-375 suggests that many postcranial specializations characteristic of Macraucheniidae and Proterotheriidae were present in their shared ancestors since at least the late Eocene.Anisolambdidae and Adianthidae were recovered as a polyphyletic group of branches that diverged early.Future discoveries, especially of postcranial material, would help to better understand the evolution of Litopterna.