A New Contender For Oldest Known Mammal
linxx points to this Discover article, writing: "A 195 million year old mammal the size of a paper clip is thought to run around the earth along with the dinosaurs." Not everyone is convinced yet that this is a true mammal, either -- but like other aspects of science, it's an ongoing attempt to find a line of best fit.
The CBC Radio One science program Quirks and Quarks has an MP3 audio interview with Dr. Zhe-Xi Luo (the leader of the team that described this animal) in their archived show listing for May 26, 2001.