Résumé
We report new complete craniomandibular and dental remains of the mesotheriine notoungulate Microtypotheriumchoquecotense, from the Choquecota-Hakallinca type locality, Totora Formation (middle Miocene, Bolivia). Microtypotheriumchoquecotense is the most abundant taxon in this locality, with ca. 57% of the mammalian fossil recovered (15 out of 26).We estimate for an adult individual (Museo Nacional de Historia Natural, La Paz, Bolivia, MNHN-Bol-V 13223), a body massof 21.69±3.79 or 21.26±4.88 kg, based on cranial or mandibular measurements, respectively. The studied fossils widelyimprove our knowledge of the morphology of the so-far elusive M. choquecotense, as well as other early-divergingmesotheriines with which it shares some characteristics (e.g., Altitypotherium spp.), such as I1 oval in occlusal view and withsmooth enamel, median lobe of M1-2 aligned lingually with the protoloph and metaloph, well-differentiated septum atthe incisive foramen, and parallel upper diastemata. Autapomorphic features include an oval i1 (which differentiates it from the trapezoidal shape as seen in Altitypotherium/Rusconitherium) and P4 with a marked enamel fold on the lingual edge.The presence of the latter trait is particularly striking as it has been either considered for differentiating late-divergingmesotheriines (Pseudotypotherium spp. and Mesotherium; also present in Caraguatypotherium) from earlier-diverging generaor for distinguishing species within Eutypotherium. In short, this character seems to occur much earlier in the evolutionaryhistory of mesotheriines than previously thought and it might document intraspecific variation as well, as a polymorphicfeature, within Microtypotherium and Eutypotherium.