Roots of the major Recent mammalian clades (monotremes, marsupials, placentals) can be traced back to the Mesozoic, when their earliest representatives lived alongside more basal mammal groups that are now extinct. One very important such group were the multituberculates, non-therian mammals with a peculiar dentition that are commonly called the ‘rodents of the Mesozoic’, despite the fact that they also survived into the early Cenozoic. Multituberculates frequently dominated the late Mesozoic mammalian assemblages.This is also the case of the latest Cretaceous (~75 to 66 millions of years ago) continental vertebrate faunas that inhabited what is today Transylvania, an area that can be reconstructed as a subtropical island at that time. This island (the ‘Hațeg Island’) was the home of a peculiar endemic fauna of dwarfed dinosaurs, huge pterosaurs, relictual turtles and crocodyliforms, accompanied by a group of multituberculates called Kogaionidae. Intriguingly, the kogaionids were strictly endemic to the Hațeg Island in the endCretaceous, also representing the only mammals in this region, whereas in western Europe rare contemporaneous mammals were ancient relatives of the placentals or marsupials. Until now, 29 different fossil localities in Transylvania yielded kogaionid remains, including incomplete skulls and partial skeletons. Review of these 29 kogaionid occurrences shows that the kogaionids were both common and widespread in the area, often being represented by several sympatric taxa of different sizes; our review also suggests that at least some kogaionids favored well-drained floodplain areas as their habitat and may had been fossorial, which contributed to their excellent and preferential preservation in certain beds. This work was supported by a grant of the Ministry of Research, Innovation and Digitization,CNCS/CCCDI –UEFISCDI,projectnumberPN-III-P4-ID-PCE-2020-2570, within PNCDI III.