A few weeks ago, I was demonstrating on a geology field course in Wales. It was fantastic for several reasons - I had a great time, it was out on the coast of Wales mostly on the coast, and more importantly, it reminded me of my love for geology. It's been a while since I've done some geology, about 6 years to be exact, and I really enjoyed looking at rock types, folds, faults, and all those other fun things.
One of the other things I remembered on the trip was the occasional animosity that is seen between geologists and palaeontologists. I remember in my undergrad geology classes how geologists would rant about how they hated palaeontology and fossils, and vice versa for palaeontologists regarding rocks and geology. Then in Wales, one of the lead staff members would make a face each time we found a fossil or discussed them. While I will admit to preferring fossils over rocks (obviously, I'm a palaeontologist...), there is one important thing to remember: palaeontology and geology NEED each other. And palaeontology especially would not exist without geology.
To start with the obvious side, fossils are basically rocks made of bones, shells, soft tissue, etc. of long dead animals or plants. The process of fossilisation means that the organic material is literally replaced over time with rock and mineral. This is done primarily by highly-mineralised pore water in the ground and sediment. As an animal dies and is buried by sediment, highly mineralised water flows through the sediments and around the dead body, and over time, the minerals in the water will replace the tissues of the body, primarily the hard tissue. Soft tissue is generally decayed and broken down, while the hard tissues of bone and teeth remain replaced as rock. Being able to tell what type of rock now makes up the fossils can be important in figuring out the environment that the fossil was deposited in. Even more important is that the rocks surrounding the fossils can contain clues about how the animal died and the environment in which it lived. For example ripples in sandstone tell us there was a current, while fine laminated sediment like mudstone tells us that the animal died in a quiet low-energy environment like a lake or lagoon where fine sediments settled over a long period of time to the bottom. The presence of specific minerals can tell us things as well. Pyrite is formed in low-oxygen environments, so pyritised fossils mean a low-oxygen environment like the bottom of a deep ocean. Having a background in geology can definitely help in understanding palaeontology, specifically in order to understand the environment that the plant or animal was living in.
On the other hand, palaeontology and fossils can tell us specific things about the environment and geology of an area. There are fossils called Index Fossils, which are fossils that lived for a very little amount of time, and a specific time. They are obviously identifiable, and the fact that they are only found at very specific times means that when they are found, we know exactly what time period those fossils come from. Additionally, fossils can be helpful in determining geological structures like bedding planes. Sedimentary rocks are deposited in layers called beds. These layers are deposited as horizontal beds as sediments fall from lakes or rivers, for example. Through geological processes, these beds can be folded, faulted and overturned, making it difficult to tell which way is up and what has happened geologically speaking. This is where fossils, and in particular trace fossils can come in to help clear up some of the problems. If an animal is walking around on the bottom of the ocean or lake, the footprints or traces will be on a single bedding plane. If preserved and fossilised, these traces can tell us which way was "up" on the bed when it was deposited. This can help us understand what geological processes have happened in the past in terms of folding and faulting.