Late Cretaceous (Maastrichtian)
Hell Creek Formation
Powder River County, MT, USA

23mm jaw section with 4 teeth.

None of the mammal groups we know today (rodents, rabbits, dogs, cats, bats, deer, cattle, horses, seals, whales, etc.) were present during the Mesozoic Era.   You need an expert familiar with groups of mammals most of us have never heard of to identify their teeth.  Because few paleontologists specialize in these groups, many mammal fossils from the Cretaceous go unidentified or misidentified.

Most mammals from the Hell Creek Formation are either marsupials or multituberculates but there are also a few species of eutherian mammals.  After the K/T extinction it would be the eutherians that would better weather the storm and greatly diversify during the Early Cenozoic.