ACTA PALAEONTOLOGICA ROMANIAE, V. 3 (2001), p. 89-95 PALEOBIOGEOGRAPHIC IMPLICATIONS OF THE FOSSIL MAMMALS FROM THE MAASTRICHTIAN OF THE HAŢEG BASIN, ROMANIA Zoltán CSIKI & Dan GRIGORESCU * Abstract: Recently discovered mammal teeth from the HaĠeg Basin reveal the presence of a diverse assemblage of multituberculates, belonging to the Kogaionidae. At least three different taxa might have been present. This unique mammal assemblage provides useful data for interpreting the evolution of the European mammalian faunas around the Cetaceous/Tertiary Boundary. A proposed tentative evolutionary scenario suggests that kogaionids represented a vicariant, endemic European multituberculate clade that was restricted to and achieved a high diversity on the HaĠeg landmass during the Maastrichtian. At the beginning the Paleocene, kogaionids were able to spread westward to cratonic Europe, but did not reached the former dominance or diversity in those mammal faunas. Middle Paleocene immigration of North American taxa led to increased competition, and finally to the local extinction of the kogaionids in western Europe prior to the end of the Paleocene. Before that, a stock of derived kogaionids spread toward east and repopulated, together with diverse eutherians, the Transsylvanian landmass. The presence of putative eastern North American kogaionids during the latest Campanian is considered as a result of accidental waif dispersal from Europe. Further paleontological, phylogenetical and paleogeographical data are needed to test the proposed working hypothesis on kogaionid evolution and paleobiogeography. Keywords: multituberculates, HaĠeg Basin, Maastrichtian, kogaionid paleobiogeography * Faculty of Geology and Geophysics, University of Bucharest; 1 Bălcescu Blvd., 70111 Bucharest INTRODUCTION Mesozoic mammals are among the prized specimens of any paleontological collections; their importance in sheding light on the first two-thirds of the hstory of mammalian origin, evolution and early diversification is beyond doubts. However, although relatively large samples of Mesozoic (mostly Cretaceous) mammals are known for more than a century (Lillegraven et al., 1979), these were mostly recovered from continental deposits of North America and Asia. Only recently were significant Mesozoic mammal discoveries made in other continental landmasses, showing that mammalian evolution was far more complex than previously thought (e. g. Luo et al., 2001). In spite of recent advances, Cretaceous, and especially Late Cretaceous mammals and mammalian evolution of Europe is still largely unknown, due to the especially scarce fossil record. The first European Late Cretaceous mammal was discovered only as late as in 1966 (Ledoux et al., 1966) in the Champ- Garimond site of Southern France, and for almost 20 years this isolated tooth represented the only record of the group in Europe for that particular time period. Multituberculates are among the best known Mesozoic mammals. Although the origin of the group is still unclear, they have a relatively complete fossil record spanning the Late Jurassic to Eocene interval; their lower and higher level systematics and evolutionary history is also relatively well understood (Clemens & Kielan-Jaworowska, 1979; Kielan- Jaworowska & Hurum, 2001). Although several purported Gondwanan multituberculates were reported (e.g. Krause et al., 1993), most of these remains are now referred to the Gondwanan endemic Gondwanatheria classified within the Australosphenida (Luo et al., 2001); a few isolated remains suggest however, that multituberculates, though exceedingly rare, were indeed present on some of the southern landmasses (northern Africa, Madagascar, South America; Sigogneau- Russell, 1991; Kielan-Jaworowska & Hurum, 2001) during the Cretaceous. Regardless of these isolated southern occurences, multituberculates were a primarily Laurasian group of mammals; the first certain members of the group are known from the Late Jurassic of Europe and North America (Kielan- Jaworowska & Hurum, 2001) and they diversified in the Early Cretaceous of Europe and, after being introduced there from ?Europe, in Asia. All these early multituberculates can be included into the basal paraphyletic group of the Plagiaulacida. The first derived multituberculates of the suborder Cimolodonta, members of the Paracimexomys group, are known from and are restricted to the late Early Cretaceous - Late Cretaceous (Aptian-