What Is A Fossil?
Callan Bentley, Karen Layou, Russ Kohrs, Shelley Jaye, Matt Affolter, and Brian Ricketts and Charlene Estrada
Fossils
Fossils are evidence of ancient life. Fossils are found in sedimentary rocks. Here are ways in which they are preserved (We will go into depth about these in the next chapter):
- Shells and coral – the shells of animals can be preserved in the rock. This is common in limestone formed at coral reefs.
- Bones – the bones of animals are hard enough to be preserved in soft sedimentary rocks such as mudstone (shale), sandstone, and limestone.
- Petrified wood by mineral replacement – some materials, such as the carbon in wood, can be replaced by other minerals such as silica (SiO2). The original structure can still be seen after the replacement process.
- Impression – some materials can leave behind just the impression. Leaves leave behind a carbon film, while bony fish can leave the entire fish impression behind.
- Constructed mounds left behind by the organism. – some animals create structures, like an ant hill, that can be preserved.
- Molds or casts – are cavities that can be filled, forming the original shape of the missing material.
- Amber (tree sap) – organisms and material can get trapped in sab or tar and be preserved.
Figure Above: How are fossils preserved? They are preserved as shells and hard parts, bones, through replacement, as a cast or mold, as a thin carbon film, as impressions, within amber, and through constructed features left by the organism.
Most creatures are never preserved because preservation usually requires hard parts and rapid burial after death. If a creature remains on the surface, it will physically or chemically decompose or can be eaten by predators. Preservation is easier beneath the sea than on land because objects tend to get buried more rapidly, and it’s a low oxygen environment.
Figure Above: How fossils vary with age. Mammals and grasses are from the Cenozoic (65 Myr to the present). Dinosaurs and flowering plants are from the Mesozoic (251 to 65 Myr). Crinoids, coral, clams, some fish, plants, insects, and amphibians are from the Paleozoic (542 to 251 Myr). Simple creatures and fossils such as bacteria and stromatolites are from the Precambrian (life started around 3.5 billion years ago through 542 Myr)
- Cenozoic: (Recent Life) The age of mammals because they became the dominant life form.
- Mesozoic: (Middle Life) The age of dinosaurs because they dominated during this time. The end of the Mesozoic marks the extinction of the dinosaurs.
- Paleozoic: (Ancient Life) The age of fish. Hard-shelled invertebrates developed along with vertebrates. During this time, plants, insects, amphibians, and reptiles came into existence. At the end is the Great dying.
- Precambrian: The largest of the periods. Ninety percent of earth’s history is here. Only simple life forms such as bacteria, algae, and stromatolites exist.
What factors determine whether a species survives or becomes extinct? Almost all species that ever existed are now extinct. Below is a list of why species become extinct.
- Environmental Niche: Animals and plants have certain environmental settings (niches). Organisms can sometimes take advantage of new changes to niches, but others are very picky. If that niche becomes threatened due to changes in the organism population or climate, the creature will become extinct.
- Climate and Habitat: Changes in climate or environmental disease can wipe out animals. Earth history is marked by extreme changes in climate and species loss.
- Reproductive Strategy: How a plant or animal reproduces can be advantageous– or not depending on how the environment changes. For example, some creatures, like Pandas today, have trouble reproducing naturally.
- Adaptability: The more adaptable a species is, the more likely it will survive changes. Some species are not adaptable to climate changes because they are specifically evolved for certain climates (i.e., consider polar bears in the face of climate change).
- Competition: Competition among species for sparse resources can lead to life or death.
- Predators and Prey: Becoming a new food source for some other creature is never good!
Figure Above: The Geologic Time Scale. See below for detail on the three main eras.
Mesozoic – Cenozoic: This boundary was based on the mass extinction of the dinosaurs at the end of the Mesozoic. The boundary is called the KT boundary because it separates the Cretaceous (K) and Tertiary periods.
Paleozoic – Mesozoic: This boundary was based on a mass extinction called the Great Dying, which occurred at the end of the Paleozoic. 1/3 of all the marine life died.
Precambrian – Paleozoic: This boundary was based on the widespread appearance of hard-shelled organisms at the beginning of the Cambrian period.
