http://pandasthumb.org/archives/2013/10 ... .html#more
"Mammals are a group of species related by their evolutionary history".
A creationist - I forget where - recently suggested in a thread that I participated in that it might be difficult to be sure that an extinct animal found fossilised actually was a mammal. That may be true if the find is just a small fragment. But if the fossil is near-complete the task may be easier.
If I recall correctly, I think I referred the person to part 2 of the recent BBC Two Attenborough mini-series.
It appears that not all mammals were 'created' on the same single day in Genesis. Which is probably another Bible argument (but not a scientific argument) against evolution beyond 'kind' boundaries eg from land mammals to air-breathing sea creatures such as whales.