Fossils are the preserved remains or traces of prehistoric animals and plants found in sedimentary rocks. A fossil could be a bone, a shell or leaf but it could also be a footprint or burrow. In fact, any preserved evidence of the activity of prehistoric life on the planet could be considered a fossil. There is no clear definition of how old something has to be to be classified as a fossil but it's fair to say that anything older than 1 million years is a safe bet.

The fossils from the Jurassic Coast come from rocks that formed between 250 and 65 million years ago. This part of Earths history is known as the Mesozoic Era and is split into to three geological periods: the Triassic, Jurassic and Cretaceous. The Jurassic Coast is designated a World Heritage Site partly because the rocks form an almost complete record of the Mesozoic. The fossils trapped in those rocks are the record of life during that time, giving us glimpses at lost habitats, vanished ecosystems and the slow evolution of life on Earth. This is one of the most exciting stories from the Jurassic Coast and, with new discoveries made almost every year, it is one that is still developing.

How fossils are formed

Watch this short animation about how fossils are formed

Well, there is the short version or the long version. The long version could take up several books so we'll cover the short version here. What are the basic steps to make a fossil?

Step 1. Death - The easy part. Everything living dies and once it does it has a chance of becoming a fossil, depending on how the next few steps go. (a footprint or a burrow obviously doesn't involve death to be formed)

Step 2. transport - have the remains been moved around much? This could be done by scavenging animals, rivers, wind, ocean currents. Simple rule: the more transport the more of the remains could be destroyed.

Spet 3. Burial - the quicker the better. Once buried the remains of an animal or plant are protected from further damage. This is the vital step. If done straight after death even soft tissue can be preserved.

Step 4. alteration. This is when the sediment the remains are trapped in is compacted, crushing the fossil. It is also when bone or shell can be replaced by other minerals and when crystals grow in cavities. This process alters the remians, usually turning them into stone. This is essentially when the remains become a fossil.

If the remains or traces of plants and animals go through these stages they will become fossils. However there is a catch. The rock that the fossil is trapped in can't be buried too deeply, or be squashed and distorted by major Earth movements. If that happens the rock could be transformed by metamorphism or even melted making an igneous rock. Either way, any fossils would be destroyed.  

Slight differences in these stages of fossilisation can greatly affect what the fossil turns out like. It is why Triassic fossils from East Devon are rare and incomplete compared to Jurassic fossils from Lyme Regis that are beautifully preserved and relatively common. The Triassic rocks formed in deserts where the remains of animals would have been picked over by scavengers, possibly washed down rivers and blasted by the harsh desert winds. The rocks in Lyme Regis formed at the bottom of a stagnant and poisonous sea floor where the remains of animals were buried rapidly and were not disturbed by other creatures.

The last piece of this journey, and the most exciting for us, is the eventual discovery of amazing fossils on our coast. Giving us glimpses of the kind of life that existed here million of years ago.