I am waiting for a reply to this question from people in the geology department who are experts in vertabrate fossils. My guess is that there was not a single species ancestor. About the only other thing I know is that the early mammals were small and shrew-like. Also, I think they were nocturnal, able to fill a niche the cold-blooded reptiles could not because of low night-time temperatures. But, let's see what the experts say...I hope to get back to you again soon.

I don't have a solid answer to this question, but I think I can clarify some points.For starters, many people have been using differences in the genetic sequence to determine how related different animals are. If the animals are closely related (e.g. us and monkeys), there sequences of DNA are very similar. If they are distant relatives (e.g. monkeys and slime molds) the sequences are very dissimilar. Anyway, if you map this all out using a computer program, you get these lines called "branches" (just like branches of a tree). Thats where they get the "tree of life" which shows how related animals and plants and bacteria are. Anyway, these branchews have to meet somewhere, and the computer programs extrapolate out the lineages to form branch points. People often ask "what creature is at that branchpoint, that is the ancestor of all the mammals?" Well, that's an artificial construct. There is in theory an animal or animals that is the ancestor of all mammals, but I don't know what it is, and that branch point on a tree is an artificial construct. There are some very good paleontology sites (The american museum of natural history website is fantastic), and I would recommend going to those sites and contacting on of the curators of the fossil exhibits. They'll have the best idea of which animals are the likely ancestors to the mammals.

I hope this helps! By the way, the people at the National museum (div of paleontology) are very friendly and will answer your question quickly.

This is a great time to ask such a question because in the last couple of months papers came out describing a new, fully articulated mammal skeleton that is the best, earliest one found yet. Nature v. 398, 25 March, 1999 shows a proposed mammalian 'family tree' on page 283 and skeletal drawings on page 327. Science v 283, 26 March 1999 p. 1990 shows an artist's conception of it. It is a very cool specimen with reptile-like hindlimbs and mammalian type forelimbs. It was found abut 400km northeast of Beijing. They think it's 120 million yers old.

Have fun with the mammals, I'd be happy to answer any mammal questions you have.