Revision of a pretribosphenic mammal Arguimus from the Early Cretaceous of Mongolia

Arguimus khosbajari is redescribed, based on five additional specimens from the topotypic Early Cretaceous (Aptian– Albian) Höövör locality in Mongolia. The teeth preserved in the holotype of A. khosbajari are interpreted as p4–5, m1–3. The original identification of the teeth preserved in the holotype and single specimen of Arguitherium cromptoni from Höövör as p4–5, m1 is confirmed and this specimen is considered conspecific with A. khosbajari. Thus Arguitherium cromptoni Dashzeveg, 1994 and Arguitheriidae Dashzeveg, 1994 are junior subjective synonyms of Arguimus khosbajari Dashzeveg, 1979 and Arguimuridae Dashzeveg, 1994 respectively (syn. nov.). Arguimus is a stem−lineage zatherian characterized by the lower postcanine formula p1–5, m1–4, a premolariform p5, a “partially molariform” m1 having a widely open trigonid basin, trigonid cusps less angulated than in m2–4, a low and small paraconid, and a small but distinct metaconid, a single cusped talonid with an incipient talonid basin on m1–4, a distinct labial mandibular foramen, and total lack of the Meckel’s groove. A similar “partially molariform” m1 was apparently characteristic also for the stem−lineage zatherian Nanolestes from the Late Jurassic of Portugal, based on reinterpretation of the isolated tooth Gui Mam 1005, considered previously to be a deciduous premolar, and for Chunnelodon from the Early Cretaceous of England, described originally as an indeterminate dryolestoid, but referred here to as a stem−lineage zatherian.