Wallabies that were imported to Hawaii after the settlement of Australia can no longer interbreed with Australian Wallabies. North American fire ants can no longer interbreed with the populations of fire ants in South American from which they came. Interbreeding is fundamental to determining whether different populations of similar animals are of the same species. We have two examples of animals, in the course of a hundred years or so evolving new species or sub-species. The fossil record is therefore going to have similar gaps.
Fossilization requires very specific and very rare conditions.
For a carcass to be fossilized it must be quickly covered up by soil that is not too moist or too dry and not too acidic. Since fossilization requires the carcass to be covered up, only those animals that die in or are moved to areas of soil or sand deposition become fossilized. That means there's going to be a huge bias in the fossil record towards low-land or desert animals, where these conditions occur. Indeed it does. For example, periods of large speciation occurred at the same time the Mesozoic inland sea of North America either grew or shrank.