I suspect you have an arrangement like this:
Mammals
Reptiles
Amphibians
Fish

How did you come up with this order?
Perhaps you based it on information you already knew before you started this course. But even without previous education in evolution you would have realised that there are intermediate forms linking different groups. For example, amphibians have gills (when young) like a fish but legs (when adults) like a reptile. Perhaps you realised that mammals are very different from all the rest and certainly most different from fish so you put mammals at the top and fish at the bottom of the list.

The fossil record supports this ordering. Mammals are the most recent group of vertebrate animals to appear (evolve) and fish are the most ancient. Very old rock sediments contain fossils of fish but no amphibians, reptiles or mammals. More recent sediments have the remains of amphibians along with those of fish. Sediments even more recent contain fossil reptiles (along with amphibians and fish). The most recently formed sediments have fossils of mammals in them, along with the other earlier animal groups in this list.

We can also trace a few lineages, evolutionary family trees, in the fossil record. This is a very exciting bit of evidence for evolution by Natural Selection but usually the lineages have gaps in them. Indeed, most fossil lineages are incomplete. There are "holes" in the fossil record so only a few lineages can be traced well.

Why? Why isn't the fossil record complete and precise in details about each species' lineage?
Once you've given that some thought, go to the next page for the reasons.