European mesonychids are represented by few specimens found only in late Paleocene and early Eocene localities. Recent field works in Palette and La Borie (Ypresian of France) resulted in the discovery of new mesonychid specimens. Additionally, our review of the mesonychids housed by European scientific institutions allowed establishing the occurrence of Dissacus in Berru (Thanetian) and Sézanne-Broyes (Ypresian). Only the genera Pachyaena and Dissacus are recognized in Europe. Furthermore, two new Dissacus species can be defined from the Ypresian localities of Palette, Sézanne-Broyes and La Borie. We also identified from three localities several postcranial elements that could be referred to Dissacus. The preliminary study of this postcranial material suggests a cursorial locomotion. As a result of our study, we propose a scheme of evolution comprising three phases for these particular mammals in Europe. (1) The mesonychid Dissacus dispersed into Europe during Thanetian, probably from North America; it survived the PETM event and even possibly experienced a dwarfism during this global warming. (2) The very large mesonychid Pachyaena migrated into Europe shortly after the Paleocene–Eocene boundary, but it seems that it did not coexist with Dissacus: it was restricted to Northern Europe, while Dissacus was possibly present at that time only in southern areas. (3) However, Pachyaena rapidly disappeared from the European environments, while Dissacus dispersed subsequently into Northern Europe. These data support the existence of a faunal turnover in Northern Europe shortly after the PETM event: it has been notably proposed for explaining the disappearance from Europe of the carnivorous oxyaenids and hyaenodont sinopines, and their subsequent replacement by taxa from the southern areas. The persistence of Dissacus in Europe shows that size was not the main selective factor during this dramatic period. Dissacus has evolved endemically during Thanetian and Ypresian in Europe where it reached a very large size, especially in southern France and Spain. Due to its size, dental, and locomotor adaptations, Dissacus was probably the top mammal predator in Europe. It survived in Europe after its disappearance from North America, possibly due to its ecologic position and the lack of competitors. Grant Information This abstract is a contribution to the project BR/121/A3/PALEURAFRICA funded by the Belgian Science Policy Office.

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Paleontology