Four micromammal localities from the upper Tremp Formation (Upper Paleocene, Lleida, Spain), closely below the llerdian stratotype, have delivered near 250 teeth from ten condylartha, insectívora, proteutheria and multituberculate species. The low diversity of these assemblages, together with the neighbour Campo (Huesca), and their intermediate biochronological situation between Cernaysian and Neustrian European mammal ages allow to define a new MP6b unit, situated in the Cernaysian age and correlated with lower Clarkforkian (Cf 1-2) north-American mammal unit, thus filling a gap in the European continental biochronology. They can be calibrated within NP9 nannoplancton biozone and between SBZ 3 (Clomalveolina primaeva) - SBZ 5 (Alveolina vredenburgi) shallow benthic zones and chrons C25n-C24r. The diversity curves fit in both continents: the MP6b-Cf 1-2 epochs represent a period of very low mammalian diversity, followed by increasingly diversified Neustrian European mammalian assemblages, here divided into MP7a and MP7b units. This epoch correlates well with the Clarkforkian-Wasatchian transition in North America (Cf 3-W 0-4), which correspond with the Late Paleocene Therma Maximum. According to the mammalian biochronology, the dCI3 anomalies detected in relation with European mammal sites must correspond to two successive episodes, one below the Palette MP7a site, and another above the Dormaal MP7b site. In any case, the mammalian migratory wave in both continents is not rapid and synchronous but gradual and diachronous
