Traces of organisms preserved in strata are fossils. Fossils are Earth's record media that tell us two things: paleoenvironment and age.
Basic Knowledge
Fossils are used from two perspectives.
① Facies fossils: Fossils that indicate the environment at that time. The more limited the living conditions, the more valuable they are.
・Coral → warm, shallow sea／・Clam → estuary where fresh and seawater mix／・Short-neck clam → shallow sea／・Scallop → cold sea／・Beech → temperate zone.
② Index fossils: Fossils used to identify an age. Species that were widely distributed and flourished for a short period are suitable.
・Paleozoic → trilobites and fusulinids／・Mesozoic → ammonites and dinosaurs／・Cenozoic → Vicarya and Naumann elephants.
Fossilization requires suitable conditions: after an organism dies, it must be buried in sediment quickly and isolated from oxygen.
Facies fossil（A fossil that indicates the environment at that time. It is better when living conditions were limited）
Index fossil（A fossil used to identify an age. Wide distribution and a short duration are required）
Trilobite（A typical Paleozoic index fossil. An arthropod）
Ammonite（A typical Mesozoic index fossil. A cephalopod）
Vicarya（A Cenozoic gastropod. Also a facies fossil of estuarine environments）
Trace fossil（A fossil preserving evidence of organism activity, such as footprints and burrows）
Further Study (Background and Significance)
The study of fossils is called paleontology and is closely connected with Darwin's theory of evolution. During Earth's approximately 4.6-billion-year history, organisms diversified explosively during the Cambrian Period (about 540 million years ago). This is called the Cambrian explosion.
At the end of the Mesozoic Era (about 66 million years ago), a large asteroid struck Earth, causing the mass extinction (K–Pg boundary event) of organisms including dinosaurs and ammonites. Iridium (an element of meteoritic origin) is concentrated in strata around the world, providing physical evidence for the impact theory.
The extinction of dinosaurs created an opportunity for mammals to flourish and eventually led to the emergence of humans. Fossils are truly “witnesses to evolution and environmental change.”
- Facies fossil = environment / index fossil = age
- Coral (warm sea)・clam (brackish water)・beech (temperate zone)
- Paleozoic = trilobites・fusulinids
- Mesozoic = ammonites・dinosaurs
- Cenozoic = Vicarya・Naumann elephants・mammoths
- Cambrian explosion = about 540 million years ago
- K–Pg boundary = about 66 million years ago・asteroid impact・dinosaur extinction
Points to Note (Easily Confused)
① Do not confuse the roles of facies fossils (environment) and index fossils (age). ② Ammonites are from the Mesozoic Era, while trilobites are from the Paleozoic Era. ③ Naumann elephants lived during the Cenozoic (Pleistocene). ④ The conditions for index fossils are “wide and short” (wide distribution and a short period of existence).
Practice
- What paleoenvironment can be inferred from coral fossils?
- In which era did ammonites flourish?
- What is the leading theory for the cause of dinosaur extinction?