Where have fossils been found?
The two oldest skeletons were found in Lake Kanunka, South Australia and Fisherman's Cliff, New South Wales and were believed to come from late Pliocene. The most complete skeleton of quite a large Diprotodon was found in Tambar Springs in NSW, but is now located at the Australian Museum, Coonabarabran Visitor Centre in central New South Wales. What is odd about this Diprotodon specifically is that it had a small, square hole in one rib that was created when the bone was still fresh. This is why some scientists believe that these species lived among humans during the Pleistocene Epoch (2.6 million to 11,700 years ago). Every Fossil is typically found in sedimentary rocks because it was a sediment, but over time it has hardened around the rock. The fossil could be in a cast (when the rock forms around the fossil preserving it) but they are usually only imprints or moulds, not the whole fossil. Very rarely is there a full form fossil, meaning it contains the whole body of the animal or a whole animal part.
The Fossil Process
- The animal dies (usually around water) and sinks to the bottom of the lake.
- The animal rots leaving only the bones.
- Mud covers the bones.
- After millions of years the mud piles up and turns into rock. By this time the animal has been turned into mineral matter, meaning that it is officially a fossil.
- Weathering and erosion allow the rock to be seen again but it does not last for long until it deteriorates.
- If found the fossil will generally be put in a museum
Why are most fossils incomplete?
Most fossils are found incomplete or only imprints because a sediment was once around the rock. Over time the sediment hardened up and formed an imprint or sometimes even preserved part of the actual bone. Usually only hard parts on the animal (like the skull which is often separated from the rest of the body after death) are preserved. The teeth are very common to be found because they are very sturdy and last a long time.
How do scientists extrapolate information about an extinct creature based on available data?
Scientists gather information about an extinct animal from fossils. Fossils are the main factor to finding out anything about an extinct species. This could be the whole cast or just a mould. Both of these things posses vital information about the specie. Scientists could put the bones together if the whole structure is available. They could also test the fossil for any form of d.n.a and they could also gather weather it was a herbivore, omnivore or carnivore. They find this out from the skull structure, the part of the species that is usually preserved.
How do we know the age of fossilised material?
Determining the age of a fossil can be quite difficult but there are several ways around it. We categorize these methods in two categories, absolute dating methods and relative dating methods. The absolute dating method supplies us with an exact answer in years. Most of the time these methods use several types of minerals and organic matter and study the radioactive elements of them. The relative dating method isn't as accurate and can only tell us weather one fossil is older or younger than the other.
Absolute dating methods
The Fission track method - This method works as long as the fossil is 5000 - 100,000,000 years old. This method tests the volcanic minerals and the teeth. Most of the time in these rocks a form of Uranium is present, often in the form of Uranium-238. The fission process is created by the uranium atoms splitting, and then fission tracks are created from the damage of the rock, caused by high speed atom movement. The tiny marks on the rock or mineral are the fission tracks and the uranium content increase over time. The age of the rock and fossil is determined by how dense the fission tracks are and how much uranium it contains.
Potassium-argon dating - This method lasts from 200,000 - 4,000,000,000 years and it studies the volcanic rocks and minerals. A new set of ash and rock forms during an eruption. It releases a substance called argon-40 and radioactive potassium is also present in the rock. This method measures the age by the amount of argon-40 and radioactive potassium present in the the rock.
Argon-argon dating - This method works as far as 200,000 - 4,000,000,000 years also and it studies volcanic rocks and minerals. It is a newer method of the potassium-argon dating and it is also more accurate. It measures the amount of a more accurate and stable argon-39, as well as argon-40. This method is also efficient because it can extract both argon-39 and argon-40 from only one sample, where the older method requires two samples and it doesn't give a more accurate result
Radio-carbon dating - This method can date up to about 60,000 years old and dates the amount of c14 in the fossil. This method can only work with an actual organism of the animal. This is because bones don't contain this substance. Every living sentiment contains some form of carbon, generally c12 when alive. This contains 6 neutrons and 6 protons but sometimes cosmetic rays from high up hit the nitrogen, creating carbon with 6 protons and 8 neutrons. This carbon is now called c14. This substance acts very similar to c12 but it is unstable, meaning that it decays over time. After approximately 5,730 years on average, half of the c14 would have decayed into nitrogen. This is called its half-life. After 1 half-life it will have about half of the c14 left, after another half-life it will have about a quarter of c14 left, and after another half-life it will have about an eighth. During this process the c12 never changes. We can determine the age by looking at how much c14 the fossil has compared to how much c12 there is.
Relative Dating Methods
Chemical analysis - This method can range from 0 - 4,000,000,000 years and it dates the bone and fossilised bone. There is two techniques that a chemical analysis can be done. The first technique measures the surrounding uranium and fluorine that were absorbed from the ground surrounding the fossil. Over time more of these chemicals are absorbed in the fossil and there are to many variable factors to gather an absolute date. The second method measures the amount of nitrogen within the bone as it gradually reduces over time. This method isn't absolute because plenty of things could influence the deformation of the nitrogen such as the surrounding temperature, moisture, soil chemicals and bacteria
Stratigraphy - This method can range from 0 - 4,000,000,000 years and dates the fossils and the other substances found in the layers of sediment or sedimentary rock. Most fossils are found in a layer of sedimentary rock, the youngest on the top and the oldest on the bottom. If these layers aren't too damaged a rough age can be gathered giving a relative date.
Biostratigraphy - This method can range from 0 - 2,000,000,000 years and dates similar fossils from different sites. This method is useful for comparing fossils as there is usually a significant difference. Detailed chronological sequences can be discovered by scientists by studying various rock characteristics.
Paleomagnetic stratigraphy - This method can range from 0 - 80,000,000 years and dates the magnetic orientation in the fossil's layers. Over time the earth's magnetic field changes direction and iron oxide is magnetic, so it moves with it. This process is called a reversal since it keeps changing from north to south. Scientists can discover a relative date by studying what direction the iron oxide was facing when the rock formed around it.