Soft tissues of early mammals would preserve under the following conditions:
Amber in Russia
Frozen soil in the Arctic
Ice sheets in Greenland
The three possibilities shares a characteristic, which is the impossibility of microbes to act upon the organic compounds soft tissues are made of as they would under normal conditions. However, couple of years ago, paleontologist discovered a small mammal with preserved fur and internal organs (soft tissues), unearthed from 125-million-year-old rocks in central Spain, which could open the possibility for "sandstone in the Badlands" to be a correct answer too, but the fact which discard this presumption is that Badlands oldest sandstone are 65 to 75 myo (Cretaceous Period), not fitting in the period in which the small mammal was found in Spain.
