Do most of the known Mesozoic mammals exhibit evidence of their living in trees, on the ground...or is it like with Archie, and the evidence could be interpretted either way?
Most known Mesozoic mammals are only known from isolated teeth, at most jaw fragments... the ones where the postcranial skeleton is known are about 50:50, AFAIK (unless I've underestimated the terrestrial multituberculates from the Late Cretaceous of Mongolia).
(_Volatacotherium_'s a definate tree-dweller, I know; _Repomanus_(sp) seems too bulky to be very good at climbing)
*Volaticotherium*.*Repenomamus* -- a very clumsily formed name; -mam- as in Mammalia, + -us -- had, AFAIK, hands and feet that suggest a terrestrial lifestyle, as did its smaller relative *Gobiconodon*.