How do fossils assist in our current understanding of evolution?

We see a gradual change of life and a few sudden upheavals in the fossil record.

On it's own, one fossil says little. A group of fossils from one bed (e.g. the Burgess shale) says more because they offer a sense of the variation within populations. The overall distribution in time (as measured by the sequence of sedimentary layers) and space (which relies upon an understanding of past continental drift) says volumes.

The Paradoxides trilobites, found in fresh water on either side of the Atlantic, make no sense until it is noted that there are paired rock formation on either side of the Atlantic, showing the freshwater populations were once contiguous. The early dinosaurs, found in layers just over coal-deficient layers, increasing in diversity over time with novel forms appearing in progressively shallower (younger) layers paints a picture of changing populations. The absence of dinosaurs above an iridium (a highly dense element often found in meteorites) rich layer points to their demise in an impact event associated with a crater off the Yucatan peninsula.

Individual fossils tell us a little. Together they speak volumes of the natural history of the Earth.

Through comparative analysis we can observe converging and diverging structures. The wings of a bird and wings of an insect, for example, evolved independently yet have similar functions. We can also see how vestigal structures such as the appendix and human tailbone waned via natural selection.

We see changes and that the fossils get more and more complex. Creationists deny this evidence as well.