A new species of hegetotheriid notoungulate, Hemihegetotherium trilobus, is described from the middle Miocene (Laventan SALMA) Quebrada Honda Fauna of southern Bolivia. It differs from all previously described hegetotheriines in possessing a distinctly trilobed m3, a character state previously recorded only in pachyrukhine hegetotheriids. It further differs from other species of Hemihegetotherium in having a proportionately larger p2 and in overall size (being smaller than H. achataleptum and larger than H. torresi). A phylogenetic analysis of hegetotheriid genera coded for 26 characters supports referral of this species to Hemihegetotherium, but does not support the monophyly of Hegetotheriinae; the monophyly of Pachyrukhinae (including Prosotherium and Propachyrucos) is strongly supported. The resulting tree suggests that the lack of tibiofibular fusion in Oligocene pachyrukhines is a secondarily derived trait, but additional postcranial data are needed to provide a stronger test of this hypothesis.
Hemihegetotherium trilobus is the most abundant ungulate at Quebrada Honda and is the only species of hegetotheriid present. This contrasts with most other South American paleofaunas in which pachyrukhines are more common than hegetotheriines (e.g., Arroyo Chasicó) and/or other small notoungulates are more common than hegetotheriids (e.g., Chucal, Nazareno). Although the absence of pachyrukhines at Quebrada Honda may be due to a lack of suitable habitat, it is also possible that pachyrukhines had not dispersed north to southern Bolivia by the middle Miocene. Further investigations of Quebrada Honda may help discriminate between these two alternatives and should also help clarify the biogeographic histories of other Miocene taxa.