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Fossils, for those who don't know, is an identification event similar to rocks and minerals, herpetology, bugs, tree, etc. In doing this event, you will become (hopefully) experts on identifying various fossilized animals and plants, along with being able to determine the environment it lived in, its mode of life, how it formed, and other things. Let's start with the basics.

Fossil Formation

There are several types of fossils. These are:

  • External Molds: Imprints of the organism embedded in rocks.
  • Casts: These are formed when external molds are filled with sediment.
  • Internal molds: These occur when sediment fills the shell of a deceased organism, such as a bivalve or a gastropod. These remain after the organism's remains decompose to show the internal features of the organism
  • Permineralization/Petrification: These occur when minerals slowly replace the various organic tissues with minerals.
  • Carbonization/Coalification: These occur when over time all parts of the original organism except the carbon are removed from the fossil over time. The remaining carbon is the same carbon that the organism was made of.
  • Recrystallization: These occur when original minerals in the fossil over time revert into more stable minerals, such as an apatite shell recrystallizing into the more thermodynamically stable calcite.
  • Replacement: This occurs when the hard parts of the organism are replaced with minerals over time.
  • Trace fossils: Trace fossils are fossils that aren't exactly part of the organism. These include footprints, burrows, eggshells, and my personal favorite, coprolite (or fossilized excrement).They give insight into an organism's behavior.
  • Actual remains: These are much rarer than other fossil types. These are still intact parts of the organism. Actual remains can be seen preserved in ice, tar, or amber. A good example is mammoth hair. It is often frozen and still preserved.

Fossils almost always form in sedimentary rocks. In igneous and metamorphic rocks extreme heat and pressure needed to form them often destroys the fossilizing organisms, or warps it beyond recognition. When an organism dies, if the conditions are right, it becomes covered in sediments, which, after being subjected to pressure, become rock. This takes a very long time, and the actual organism decomposes by then. A soft organism (like a worm or jellyfish) does not get fossilized (usually) because it decomposes too fast. Only the hard parts (skeletons and teeth) remain long enough to keep the imprint in the rock while the rock is forming.

Fossil Environments

Fossils form (for the most part) in bodies of water, because that's where sedimentation occurs. Fossilization needs to occur in places where the dead organism won't be disturbed, so a place in the ocean devoid of wave activity is required. Most of these marine fossils do not form in the far depths of the sea known as the Abyssal Zone. This is because the sediment at the bottom of the Abyssal zone is generally dragged into the mantle of the Earth, as opposed to rising to the land.

Sedimentary Rocks

As said above, fossils usually form in water because that's where sedimentation occurs. Here are some of the common sedimentary rocks that fossils can be found in:

  • Sandstones/Siltstones: These rocks can usually be found in off-shore deposits or beaches. They commonly preserve water ripples, tracks, petrified wood, dinosaur bones and hard-shelled invertebrates.
  • Conglomerates: Fossilized bones and teeth, as well as amphibian and reptile fossils can be found in conglomerates.
  • Shale: Probably the most common fossil preserving rock, shales can contain fossils that are perfectly preserved. They can contain vertebrates, invertebrates, or plants.
  • Limestones: Also a very fossiliferous rock, they represent both shallow and deep tropical seas. Invertebrate fossils, as well as remains of armored fish and shark teeth can be found in limestones.
  • Coal/Coal Shales: Plants, fish, insects, marine invertebrates, and even dinosaur footprints can be found in coal deposits.

Modes of life

Different animals have different modes of life (these generally refer to oceanic dwellers, which makes up a bulk of the list). Here are some terms you need to know:

  • Pelagic: Free swimming, such as fish or scallops (scallops "swim" by flapping their shells. It's kinda cool).
  • Sessile. Rooted to the floor. Examples include crinoids (sea lilies) and sea anemones.
  • Benthic: Lives on the sea floor, e.g crabs, lobsters, crinoids.
  • Vagrant: Free swimming, same as pelagic.
  • Motile: The opposite of sessile; moves around. Examples include anything that is Pelagic/Vagrant, Benthic, or any other organism able to move around.

Geologic Time

Earth's history is broken up several ways. The largest section is the supereon, the only one is the Precambrian. After this the next largest are eons There are four; the Hadean Eon (before 3800 mya (million years ago), the Archean Eon (3800-2500MYA), the Proterozoic Eon (2500-542MYA) and the Phanerozoic (540 mya to present). Not much is know about the Precambrian, because all of the life forms lacked hard shells or skeletons, making preservation very unlikely. There are, however, fossils called stromatolites that show indications of cyanobacteria. These are first found in the Archaean. It is possible that the first lifeforms and self replicating RNA strands emerged as early as the mid-Hadean. The Phanerozoic Eon is when shelled invertebrates began to emerge, and the fossil record expands.

Next, it's split into Eras. Eras are divided based on the dominant life forms at that time. The Paleozoic (meaning "ancient animals", from 540 mya to 243 mya) was dominated by marine invertebrates. Reptiles dominated the Mesozoic (middle animals) Era (from 243 mya to 65 mya), and mammals dominate the Cenozoic Era (65 mya to present, meaning "recent animals"). We are living in the Cenozoic Era now.

The next breakdown are periods. Each era is broken down into periods, except for the Archaean and Hadean Eons, which are only divided into eras. Periods are broken down into Epochs starting after the beginning of the Phanerozoic Eon. All epochs are then further divided into Ages, which can, though rarely are, divided into Chron. All divisions of time may be distinguished from each other by certain species that lived only in that period, called index fossils, this method is called biogeochronology. These divisions all have counterparts in chronostratigraphy, as Eon/Eonthem, Erathem/Era, System/Period, Series/Epoch, Stage/Age, and Chronozone/Chron. More on that later. First up, we have the periods of the Paleozoic (there are periods before this but they are rarely used):

  • Cambrian: (540 mya to 505 mya) The first period, this is when marine invertebrates start to emerge.
  • Ordovician: (505 mya to 438 mya) Primitive fish start to form. Index fossil is the trilobite genus Cryptolithus.
  • Silurian:(438mya to 408 mya) Early land animals began to emerge.
  • Devonian: ( 498 mya yo 360 mya) First forests and amphibians form. Index fossils include Mucrospirifer (brachiopod genus) and Phacops (trilobite genus).
  • Mississippian: (360 mya to 320 mya) Widespread shallow seas form.
  • Pennsylvanian: ( 320 mya to 286 mya) Coal bearing rocks form.
  • Permian: ( 286 mya to 245 mya) Earliest gymnosperms (cone bearing trees).

Next come the Mesozoic periods. It was during this time that dinosaurs dominated.

  • Triassic: ( 245 mya to 208 mya) First dinosaurs and earliest mammals.
  • Jurassic: ( 208 mya to 144 mya) Earliest birds.
  • Cretaceous:(144 mya to 65 mya) Flowering plants (angiosperms) develop.

Last are the periods of the Cenozoic. The periods in the Cenozoic differ from the other two eras by being broken down even further in epochs.

  • Tertiary: Apes begin to appear. It is broken down into epochs:
    • Paleocene (65 mya to 58 mya)
    • Eocene: (58 mya to 37 mya) further development of mammals.
    • Oligocene: (37 mya to 24 mya) Rise of true carnivores
    • Miocene: (24 mya to 5 mya) Grasses and grazing animals develop.
    • Pliocene: (5 mya to 2 mya) First modern animals
  • Quaternary: Humans appear and develop. This is the period we are still in today.
    • Pleistocene: (2 mya to 10,000 ya): First humans.
    • Holocene: (10,000 ya to present): The epoch in which we live today. The Holocene is further divided into the Boreal Age, followed by the Atlantic Stage.

And who knows? Maybe another type of animal will dominate us and another era will be born. You never know.

Index Fossils

Index fossils are fossils of organisms that lived only in one period. They developed near the beginning of the period, and became extinct before the end. Note that this refers to genera or species, not entire classes or families. Index fossils are extremely useful for dating rock. They cant be used to tell absolute age (we need carbon-14 testing for that), but we use them for relative dating. By comparing two rock outcrops with the same index fossil, we can conclude that they are roughly the same age, (give or take several million years, which, in the span of Earth's 4.6 billion year, is not much).

For example, Genus Mucrospirifer can be an index fossil for the Devonian Period because they only existed during that period. Therefore, if you find a rock with a Mucrospirifer in it, you can guess that the rock is from the Devonian Period.

There's not much else I can write now without the list of event description. I'm gonna have to wait, it looks like. I hope this helps right now, and if you know something that I don't, or forgot to add, feel free to put it in. Good luck with fossils!

Relative Dating

Relative dating orders events in chronological order. It tells you which events came first, but it does not tell you the exact date of which it occurred. There are different methods that are used for relative dating. They are the principle of superposition, the principle of original horizontality, the principle of cross-cutting relationships, and the principle of inclusions.

  • Principle of Superposition: If you have undisturbed layers of sedimentary rocks, than the layers will be younger as they near the top. The oldest layers are on the bottom and the tallest layers are on the top.
  • Principle of Original Horizontality: Rocks are originally layered horizontally. If you have layers that are higher on one side than on the other, it is due to the tilting of rocks caused by a geological event.
  • Principle of Cross-Cutting Relationships: This principle states that a fracture or cut in a rock caused by another rock (igneous intrusion) is always younger than the rock it cuts.
  • Principle of Inclusions: Fragments of one rock in another rock must be older than the rock it is contained in.

Absolute Dating

Absolute dating is similar to relative dating in that they both order events in chronological order. However, unlike relative dating, absolute dating can determine the ages of rocks. There are several methods that are used in absolute dating, including radiometric dating, half-life, and carbon dating.

  • Half-life: The half-life of an isotope is how much time it takes for half the atoms in that isotope to decay. After that many years, half the atoms in the isotope will decay. After that many years again, half of that half (one quarter of the whole or two half-lives) will decay. After that many years again, half of the half of that half (one eighth of the whole or three half-lives) will decay. It will go on until the isotope decays to its daughter product.
  • Radiometric Dating: As time goes on, the amount of parent material in a rock decreases as the amount of daughter product in the rock increases. Geologists can determine the age of rocks by measuring the amount of parent and daughter material in the rock and knowing the half-life of the parent rock.

Choosing a Guide

There are 3 main fossil guides used for this event, They are Simon and Schuster's Guide to Fossils, National Audubon Guide To North American Fossils, and Eyewitness Handbook: Fossils (also called Smithsonian Guide to Fossils). Here is my humble option on the books. Simon & Schuster's Guide: Good info but it's organized awkwardly and is VERY hit and miss on the fossils (more miss then hit). National Audobon: Very good info,has everything you need wen it comes to ID, buts its EXTREMELY bulky, and there's a lot of flipping back and forth. Smithsonian: Good straight forward info (something the other two don't have, but it doesn't have all the samples. If you where buying one I would recommend buying in this order.

1) Audubon: It has almost all invertebrates on the list, that automatically puts it first, thought I think it is i bit bulky for these purposes.

2) Smithsonian: I recommend this book because it's really straight forward, not very bulky, but the only thing wrong is that it doesn't have all the specimen's on the fossils list. "The fossils rules recommended you use it and so do I," says GGuy5.

3) Simon and Schuster's: It's ok... it doesn't have many of the samples, but what is does have is great, but its the only guide of the three here that has information on dinosaurs.


[1] This has vast quantities on information on several taxa.

[2] The standard resource for all SciOly events :p

[3] This has a large amount of information on taxonomy of each specimen.

[4] This has a lot of useless stuff, but some good information, such as the Latin word roots common to dinosaurs.