Types of Fossils Original Fossils Original fossils formed when a part of the organism is preserved, with its chemical composition being about the same as it was when it was living. An original fossil could be a complete skeleton, bones, tooth/teeth or a shell. Bone is composed of minerals (such as calcium carbonate) that are resistant to decay and which scavengers find difficult to eat. The flexibility of living bone comes from proteins within. These proteins normally quickly decay after death, leaving behind the minerals as hard but brittle bones. Original fossils commonly include: ● sea creatures that have shells, such as molluscs like scallops, mussels and clams ● vertebrates because they had teeth and a bony skeleton Examples are the remains of dinosaurs and of Australian megafauna such as 3m tall giant kangaroos, sheet-sized echidnas, diprotodon, 3 metre long wombats and marsupial lions (a skull of which is shown on the next page - 45 000 years old, found in WA). Replacement Fossils A replacement fossil forms when a part of the organism is chemically changed into another mineral. This takes a long time to happen so most of these fossils date back to over 60 million years ago. Replacement fossils commonly form when the calcium carbonate found in shells and bony skeleton is replaced by another mineral such as silica, also called silicon dioxide. Silica is like sand. Sometimes the bone or shell even turns into opal, another form of silica. This means that the bone or shell is now a lump of solid silica or opal. This is shown on the next page. If the materials being replaced is wood, it’s called being petrified. Carbon Film Fossils Carbon film fossils (carbon trace fossils) occur when the dead body partially decays and leaves a thin black deposit of carbon. On the right you can see a carbon film fossil of an ancient fern. Lines are left in the shape of the organism that decayed and often show finer details. Indirect Fossils / Trace Fossils These are fossils that aren’t a part of the organism, but traces left by them like footprints, burrows, faeces, etc. A mould is an imprint left in rock showing the outside of an organism. It’s a negative image, so it has space, but no body. Molluscs often leave these moulds, since the shell is covered in sediment, turned it to rock, then the original shell is dissolved. This happens commonly deep in the ocean. Sometimes even internal moulds can happen e.g. a snail shell could fill up with mud, and turn into rock. A cast forms when an organism in rock decomposes and the space fills with soil that turns into rock. We can make these by pouring plaster into footprints - like in forensics. Questions 1. Complete the table below Type of Description Example Fossil Original Replacement Carbon Film Indirect/trace 2. How old are most replacement fossils? ________________________________ 3. If wood is replaced, it is said to be ________________ wood 4. How could an internal mould be created? ________________________________ ________________________________ 5. True or False: Forensics uses some principles that are applied in palaentology.