can you name a few mammals during that period?
Cimolodont, multituberculates, metatherians and eutherians
Late Cretaceous
In the northern hemisphere, cimolodont, multituberculates, metatherians and eutherians were the dominant mammals, 
how many years ago was the thescelosaurus in existence? More than 66 million years ago which period was that? Late Cretaceous period describe this creature's physical characteristics. Its size has been estimated in the 2.5–4.0 m range for length, a weight of 200–300 kilograms and it may have been sexually dimorphic, with one sex larger than the other in which region were their fossils discovered? Niobrara County, Wyoming, USA who discovered them? John Bell Hatcher and William H. Utterback were these fossils recovered and preserved somewhere in an institution? Smithsonian Institution's National Museum of Natural History were there any body organs of this creature recovered? Remnants of a four-chambered heart and an aorta but these conclusions have been disputed can you name a few other species from that period you mentioned earlier? Hadrosaurs, ankylosaurs and ceratopsians can you name a few birds which existed during that period? Vegavis, Yungavolucris and Avisaurus
wiki:13066056
Late Cretaceous [SEP] Mammals
Several old mammal groups began to disappear, with the last eutriconodonts occurring in the Campanian of North America. In the northern hemisphere, cimolodont, multituberculates, metatherians and eutherians were the dominant mammals, with the former two groups being the most common mammals in North America. In the southern hemisphere there was instead a more complex fauna of dryolestoids, gondwanatheres and other multituberculates and basal eutherians; monotremes were presumably present, as was the last of the haramiyidans, "Avashishta". Mammals, though generally small, ranged into a variety of ecological niches, from carnivores (Deltatheroida), to mollusc-eater (Stagodontidae), to herbivores (multituberculates, "Schowalteria", Zhelestidae and Mesungulatidae) to highly atypical cursorial forms (Zalambdalestidae, Brandoniidae).