For the next two weeks we are going to be working on our new science topic of
Rocks and Fossils
We are going to start by thinking... are dinosaurs real? What do you think? How could we know?
Write a sentence in your book explaining whether or not you think they could be real or not and why.
It is believed that dinosaur fossils have been found for centuries and these gave rise to some of the mythical creatures in ancient cultures. However, without documented evidence we can not know this for sure.
What we do know is that our current knowledge of dinosaurs and palaeontology(the study of fossils) started in the 1800s. So we really have only known about them for the last 200 years! We know about dinosaurs due to the discovery of fossils and fossilised skeletons.
There are some key concepts we need to know before moving on.
What is the difference between bones and fossils?
Bones are any piece of the hard whitish tissue that makes up the skeleton in animals including humans.
Fossils are more than
just ancient bones, which
is what many people think.
There are three types of fossils – body fossils, trace fossils and chemical fossils.
Chemical fossils contain carbon, which is proof that they must be formed from once living things. Examples of chemical
fossils include coal,
petroleum oil and
Body fossils are the remains of an animal or plant such as bones, shells or leaves.
There are three types of body fossils:
Mould and Cast Fossils
Mould fossils form when all the parts (including the bones) have decayed and all that is left is the mould of the animal.
Cast fossils form from mould fossils as the mould fossil is filled up with sediment – so it is not made up of the original matter of the animal or plant.
Replacement fossils form when water dissolves the original hard matter of the bones and replaces them with mineral matter – this is what we think of when we discuss dinosaur fossils. They still look like the original bones but are not made up of the same matter.
Whole Body Fossils
Whole body fossils form when the original body has been preserved – for example a woolly mammoth in ice or a mosquito in amber.
There are also fossils such as footprints, trackways and coprolites (fossilised pooh!)
There are many different ways that fossilisation occurs. However, you will focus on how fossils form in rocks (both body and trace fossils).
Fossilisation only takes place in sedimentary rocks as the heat from the lava that creates igneous rocks and changes the structure of metamorphic rocks would be too high for fossils to survive.
Step 1: An animal or creature dies and ends up in the sea (if not already in it!). It gets covered by a layer of rock.
Step 2: Over time more layers of rock cover it and by this time the only thing to remain of the animal would be its bones (except in the case of mould fossils where the bones would also be decayed).
Step 3: Over thousands of years the mould fossil might become a cast fossil with sediment entering the mould. In the case of replacement fossils, the original bone matter changes to mineral matter but this does not affect the shape of the bones.
Step 4: Over a long period of time the sea will recede in certain places.
Step 5: As erosion and weathering takes place, eventually the fossils become exposed.
In your books, I'd like you to write out and draw your own version of the 5 steps of fossilisation.