Just some dead animal right? Well, kind of, but at the same time...not really.
What is a fossil... technically?
A fossil is a part of an organism that has been left behind to fossilise in the rock record. Given the high variety of organisms there have been and how many things they can do, this can entail quite a lot. Despite all of this (and despite how many fossils are out there), an organism becoming fossilised is an extraordinarily rare occurrence. Fossils can only be found in sedimentary rocks and the most common fossil out there, as well as the most well known type, is a body fossil. This normally includes all of the hard parts of the organism, such as parts of the skeleton or shells. There is a reason that the most common type of fossil is any hard part of animals, though, which is explained when we examine what a fossil actually is. That is because a fossil is, first and foremost, a rock.
The organic material of an animal cannot, under any circumstances, last for longer than approximately 30,000 years. Not a single molecule of the original animal is left behind after this point, even if it's DNA preserved in amber (sorry Jurassic Park fans). Instead, what is left behind is a sort of cast made from specific types of rocks of what was once there. So why are the hard parts of an animal specifically the things that most commonly fossilise? Well, let's have a look at how an organism fossilises to get a better understanding.
How to become a fossil
Step 1: Die
After that, perk up! Because all of your hard work is done!
Now it's up to chance.
The process in between an organism dying and being found is known as 'taphonomy'. It is within the process of taphonomy that fossilisation will occur. Once the remains of an organism have settled, it will be consumed by other organisms while it is exposed. This could be anything from scavengers to microbial organisms that contribute to general decay. Once (and if!) it has been buried in sediment, however, what remains of the animal is fairly protected. At this point, the minerals will gradually fill in the cavities of the organism on a cellular level, gradually creating a cast of the body. This process in called permineralisation and the precipitation of the minerals most readily fill within empty spaces or are filtered into other mineralised parts, such as bone or shell.
The reason that fossils of underwater organisms are so much more common is that water makes this process much easier, with more sediment being carried to bury the organism and a more 'floaty' medium for permineralisation to occur. The reason that it is mostly hard parts of an animal is that these are the parts that are most durable to weathering and decomposition, and are the most mineral-y part of the organism, so other minerals can more readily 'make friends' with it. The most abundant minerals in hard body parts are calcite, which all bones and most shells are made up of, and aragonite, which some shells are made up of. Aragonite-based shells are a rarer occurrence, but since aragonite breaks down easier than calcite, the fossilisation of aragonite gives a fossil a beautiful iridescence:
It is because of this dexterity of hard parts that soft tissue, plants, and microbial life doesn't fossilise quite so easily and the ideal conditions of water that we don't see as many land fossils. But they do exist!
If an organism is buried quickly enough, an incredible amount can be preserved, for example, 'mummified' dinosaurs:
The ideal occurrence (for paleontologists of course, not the animal!) is when an organism is buried via an obrution deposit. An obrution deposit is when an organism is subject to a rapid burial, sometimes while the organism is alive. The best example of this is when a Velociraptor and Protoceratops were buried alive mid fight by a collapsing sand dune.
When organisms become fossilised under conditions like this, amazing things can be preserved such as internal organs, skin impressions and even melanin cells, which can give us an idea of a dinosaur's colour.
After all of that over millions of years, you simply need to be brought to the surface and found by a nerd like me!
Types of fossils
Ok, so we now know what a fossil is, but did you know there are several types? Let's take a look at some of these fossils...
These are most often found after rapid burial in very soft, fine grained sediment. This is where the sediment compacts around the body of an organism so that it creates a cast impression of the body. This type of fossil is most often seen with shells such as ammonites, but can sometimes preserve other things on the organism's body such as a dinosaur's skin impressions.
Ichnites are trace fossils. This is a fossil that an organism has left behind that wasn't part of its original body, such as burrows, footprints and even coprolites (fossilised poo!). Trace fossils are an absolutely essential part of natural history study, since we can learn quite a lot about an animal's behaviour. Ichnology is such a broad and important subject in fact, that you often have specialists whose job it is to study them.
Fossils made of different things
Sometimes you can get fossils that don't look like they're made of your typical 'fossily' stuff. A good example is the aforementioned iridescent fossils. Another one is known as a pyritised fossil. This is essentially a fossil that has been infilled and fossilised in pyrite. You probably know of pyrite, but you'll probably know it better by its nickname, 'fool's gold'.
That's right, a fossil can be made out of what looks like gold! It looks fake, but these fossils are more common than you think. Pyrite is an anoxic iron sulphide, which means it only forms in environments with high iron content and little-to-no oxygen. Often, when a marine fossil sinks to the bottom of the ocean, it can sink down to sections very low in oxygen where the pyrite forms and so pyrite does the permineralisation instead. Just be careful if you ever find one of these babies in the wild, though, as many paleontologists collected these fossils and stored them away, only to come back weeks later and find a hole had melted through the drawer where the fossil once stood. This is because pyrite only forms when there is no oxygen, so once oxygen hits it, it starts to break down very quickly and one by-product of this chemical reaction is sulphuric acid! This is why it is safer to buy pyritised fossils as they have been treated with a sealant that stops this reaction from occurring.
So, there you have it! You now know exactly what a fossil is. Of course, there is a lot more detail I could go into about what a fossil is and all the various ways that fossils form, but the basics are here to act as a good foundation of knowledge.