- WHAT ARE FOSSILS?
- HOW ARE FOSSILS FORMED?
- FOSSIL TYPES
- WHERE DO YOU FIND FOSSILS?
- HOW DO YOU FIND FOSSILS?
- FOSSIL FOOTPRINTS
- SOME PHOTOS OF MAMMAL LIKE REPTILES
WHAT ARE FOSSILS?
We generally know that fossils are bones of ancient creatures that have turned into rock and are buried in the ground. But a buried bone isn’t the same thing as a fossil. To become a fossil, the bone has to become rock. The organic parts of the bone, like blood cells, collagen (a protein), and fat, eventually break down. But the inorganic parts of the bone, or the parts made from minerals like calcium, have more staying power.
HOW ARE FOSSILS FORMED?
Fossils are usually formed when a plant or animal dies in a watery environment and is buried in mud and silt. Soft tissues quickly decompose leaving the hard bones or shells behind. After an organism’s soft tissues decay in the sediment, the hard parts — particularly the bones — are left behind.
As more silt, mud and other sediments get laid down on top of the bones they get buried deeper into the Earth. The layers above start to put pressure on those below as they pile up. Ground water seeps into the remains, carrying dissolved minerals like iron and calcium carbonate picked up from the surrounding sediment. When these dissolved minerals come into contact with the bones they either form crystals (precipitate) on the bones or slowly replace the individual cells and bone tissues. The minerals reinforce this bone, turning into a fossil.
This happens in one of two ways:
- Permineralisation: to understand this type of fossilisation you can think of the bone as a sponge that is being filled with glue– the sponge’s physical structure stays the same, but the pores and pockets within it fill up. Over time the sponge might break apart but the shape of the sponge is preserved by the glue. In the same way the bones might decay over time but the spaces in between the bone are filled with minerals that take the shape of the bone.
- Replacement: the dissolved minerals in the groundwater actually replace the original bone tissue to make a copy of it – but now the bones are made out of rock.
As this process continues, the bone becomes more and more rocklike. Large, thick bones, which have more room for mineral glue, usually make better-preserved fossils than small, flat bones. Over millions of years, more sediment builds over the top and the pressure from these added layers and the mineralisation from ground water hardens the bones into rock.
There are many different types of fossils…
- Body Fossils: these are bones, or other parts of the animal’s body that have been turned into rock
- Trace Fossils: footprints made by ancient walking animals or any other kind of creature slithering, swimming or moving across a surface
- Moulds and Casts: a mould forms in the mud or sand surrounding a dead animal. Over time this mud and sand is turned to rock but the dead animal rots away and is not turned into a fossil. A mould, impression of the animals is all that is left. Casts form when these moulds are filled up.
Other types of fossils are:
- Carbon Impressions of plants
- Wood turned into rock (Permineralisation usually)
- In some rare cases the actual tissues of an ancient animal are preserved. Examples of these are insects trapped in amber and mammoths preserved in ice.
WHERE DO YOU FIND FOSSILS?
If fossils form when ancient creatures are buried in the earth, how can we discover them on the surface? The answer lies in Plat Tectonics. The Earth’s lithosphere is made up of plates that don’t bend or buckle easily. Processes below these plates (in the Earth’s mantle) cause these plates to move slowly over time. This happens at about the same rate that your finger nails are growing at. Plates can collide and form mountains (like the Himalayas today). This allows ancient rocks, containing fossils, to be brought back up to the surface. Once these fossils are at the surface they are uncovered through the processes of erosion.
Most fossils in the Karoo are found in green grey mudrock but there are also other kinds of rocks that can be seen here. These reflect the different parts of the ancient rivers that were once here (about 250 million years ago).
- Mudrock is basically mud that has turned to stone. Mudrock settles in quite waters. Today we find it in ponds, lakes and overbanks (parts of the river banks that are broken and flooded during high rainfall). If an animal dies in these quiet settings there is little chance of it being carried downstream by the river. These muddy areas are good places for animals to be preserved as fossils.
- Sandstone is a sedimentary rock formed from cemented sand-sized grains. The cement that binds these grains can vary from clay minerals to calcite, silica or iron oxides. In rivers today we find sand in channels and parts of the river banks where there is flowing water. The chances of dead animals being preserved as fossils within sandstones are not very high because the high energy flowing river breaks the bones apart or transports it to places of lower energy downstream. Sometimes we find broken up fossil bones at the bottom of sandstone beds.
- Dolerite is a fine- to medium-grained, dark red to black intrusive igneous rock. It is extremely hard and tough to break apart. In the Karoo the dolerites formed about 180 million years ago when the ancient supercontinent called Gondwana broke apart. A hot spot under the supercontinent heated up the continent and melted the rocks to form magma. Sometimes this melted rock reached the surface and flowed out as larva (like we see in Hawaii today). Some of the magma stayed trapped inside the earth, somewhere between the surface and the hot spot. The heat from this magma baked the ancient river rocks (mudrocks and sandstones) and any fossils that were buried with them. Because of this you will not find any fossils in or close to the dolerite rocks.
HOW DO YOU FIND FOSSILS?
These are examples of how difficult it is to find fossils in different rock formations.
- Be careful when looking for fossils.
- Fossils are exceptionally rare remains of ancient creatures that are part of our collective heritage as South Africans (and people of the world).
- It’s important not to damage or remove a fossil. A fossil can only be removed from the ground by a qualified person who has a permit issued by the South Africa Heritage Research Agency.
- To search for fossils you need the land owner’s permission.
- Although you can’t take the fossils home with you, it is still lots of fun to search for them! If you find a fossil you should take photos, mark where you found it and contact a local museum.
- This will help palaeontologists (scientists who study fossils) at lot! They can’t cover all the areas where fossils are found alone and rely on citizen scientists to help. Often you will be able to help the museum scientist excavate the fossil that you found. If it turns out to be a really unique fossil it will probably be named after you as the one who discovered it! The fossil will be taken to a museum where it can be seen by many visitors and protected for future generations.
- Fossils are not found in all of the Karoo rocks and there are pockets where fossils are more likely to be found. It helps to go fossil hunting with someone who is trained to read the rocks and can help you see the clues that separate fossils from rocks.
- Reading the rocks means that you are able to look at the ground and see changes in colour, texture and shape that can tell you if a rock could be a fossil bone. Bones have very distinctive shapes so that is usually the easiest clue that what you are holding is a fossil.
- When you walk, think in bone shapes and not how the animal would have looked like before it died.
- About 255 million years ago, mammal ancestors (previously called mammal like reptiles) walked in the Karoo basin. These creatures were much older than the dinosaurs and although they looked a lot like bigger versions of today’s reptiles (crocodiles, snakes and lizards) they are actually more closely related to modern mammals. Many of these early mammal ancestors went extinct during the biggest mass extinction event ever! The Permian Mass Extinction.
- Much like modern animals congregating around a watering hole today, they left footprints in the ancient mud.
- Most of the time these footprints are washed away by the rivers and destroyed. Sometimes, under exceptional circumstances, the footprints can be preserved for us to discovered and enjoy.
- In 1968 Nick van Gass, a farmer who lived just outside of Fraserberg, discovered a beautifully preserved ancient muddy watering hole after part of his land was washed away during a flash-flood.
- It looked like the rocks were laid down yesterday because all the ripples, mud-cracks and small streams were preserved in such detail! These rocks preserved about 300 vertebrate footprints, invertebrate trackways and other trace fossils that showed a thriving ancient ecosystem. These trace fossils can tell us much more about the behaviour and interactions between these ancient creatures than fossil bones ever could.
- The site became known as the Gansfontein Palaeosurface and through the efforts of scientists like Coenie de Beer, Roger Smith and others it is protected as a Geological Heritage Site. Scientists continue to unlock the secrets of this ancient palaeosurface today but it’s also open to the public.
- You can now make an appointment with Marthinus Kruger in Fraserburg to visit the Gansfontein paleosurface and be transported back in time on a breath-taking stroll amongst the footprints of these magnificent ancient creatures.
Above information was edited by Dr Claire Browning, Karoo Curator, Iziko Museum.
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To see a VIRTUAL 3D EXPERIENCE of FOSSILS of the PERMIAN PERIOD, follow this link: