AC
A fossil record, by its very nature, will never be complete. However, even in Equus fossil tree, enough transition can be seen to fill in the blanks.

Say you have two populations of Wooly Critters, happily living in cold climes. Climate changes for Wooly Critter population A, and starts to warm up, while the climate for Wooly Critter population B stays constant.

For Wooly Critter population A, those with thinner coats are of wool will find the climate far more tolerable, and will have to expend less of their energy dissipating heat. Initally this may only give them a small advantage over the more wooly counterparts, but in each generation, the alleles for lighter coats of wool mean that eventually the heavier wooly coats will disappear.

Now go back to Wooly Critter population B, which had no such selection pressure. After many generations in reproductive isolation, it is quite possible that it will no longer interbreed with the descendants of population A (either due to accumulated behavorial differences or outright genetic blocks to producing viable hybrids).

This is evolution. It has been observed in modern populations, and explains the fossil record rather better than Creationism, unless your an advocate of Last Thursdayism.

--- AaronC