Fossil Record and Radiometric Dating

Fossil Record

• Everything known about extinct organisms originates from the fossil record
• Fossils represent traces of living organisms embedded in the Earth's crust
• Fossils commonly are petrified remains of an organism's tissues
• Tissue becomes petrified when organic matter gets replaced with minerals
• Fossil finds are usually bones, but can be soft tissue
• Fossils can also be impressions of organisms like footprints
• Fossilization hardly ever transpires, scientists estimate only 0.001% of living organisms will fossilize
• For fossilization, organisms must be covered by sediment without disturbance in anaerobic conditions, and water must filter through carrying minerals to replace remaining tissue
• Following this, fossils must be discovered after exposure via erosion

Radiometric Dating

• Radiometric dating uses the half-life of radioisotopes to date rocks
• Radioisotopes prove unstable, undergoing decay into stable daughter elements as neutrons/protons break down into high-energy particles
• The half-life of a radioisotope refers to the time needed for half the radioisotopes in a sample to decay into the daughter element
• By gauging a sample's ratio of parent radioisotope to its daughter element alongside the radioisotope's half-life, scientists can ascertain a rock's age
• Uranium-lead dating of zircon crystals proves reliable because zircon crystals in igneous/volcanic rocks exclude lead, thus, lead found in zircon crystals derives from uranium radioisotopes

History of Life On Earth

• The Big Bang brought about the present universe ~13.8 billion years ago (bya)
• The age of the earth was calculated via measurements of the expansion rate coupled with extrapolating backwards, as well radiometric U-Pb dating of meteorites that originated when the solar system first formed
• Earth/solar system formed in the Milky Way galaxy ~4.54 bya
• The geologic time scale splits Earth's history into four eons

Four Eons

• Hadean ("age of Hades") from 4.5-4.0 bya
• Archean ("the first age") from 4.0-2.5 bya
• Proterozoic ("first animals") from 2.5 bya to 541 million years ago (mya)
• Phanerozoic ("visible life") from 541 mya to the present
• First three eons get collectively referred to as the Precambrian
• Precambrian precedes the Cambrian period of the Phanerozoic eon
• During the Cambrian era, recognizable animal lineages emerged

Hadean and Archaean Eons

• Hadean Eon - Earth forms as molten matter for hundreds of millions of years
• Archaean Eon - Land formations take shape ~4 bya; life begins ~3.7 bya
• Primitive cellular life gave rise to bacteria and archaea ~3.5 bya

Proterozoic Eon

• An endosymbiotic event between archaea and proteobacteria caused the rise of eukaryotes ~1.8 bya
• A second endosymbiotic event between eukaryotes and cyanobacteria brought about photosynthetic eukaryotes ~1.5 bya
• Multicellular eukaryotic life surfaced ~1.5 bya
• First animals surfaced ~600-550 mya

Phanerozoic Eon

• Over the following 541 million years, animals, fungi, and plants would diversify and generate present-day organisms
• The Cambrian explosion gave rise to most of the recognizable animal phyla ~541 mya

Phanerozoic Subdivisions

• The Phanerozoic contains an extensive fossil record
• Geologists divide this eon into three eras based on fossils that define respective rock layers
• Paleozoic Era - Meaning ancient life
• Mesozoic Era - Meaning middle life
• Cenozoic Era - Meaning new life
• Each era has subdivisions into periods, further subdivided into epochs

Paleozoic Era

• The Paleozoic Era began with the Cambrian explosion (in the first 25 million years).
• Fish, insects, tetrapods, and reptiles later appeared.
• Dominated by fish.
• A remarkable mass extinction at the end of the Permian Period created emptiness in evolutionary niches, paving the way for the rise of dinosaurs

Mesozoic Era

• Dinosaurs, mammals, and bird species emerged.
• Reptiles dominated the Mesozoic.
• A mass extinction at the end of the Cretaceous Period left an open door for mammals to diversify and flourish

Cenozoic Era

• Mammals underwent significant diversification and became the predominant species.
• Most major groups of mammals came into existence, including primates

Mass Extinctions

• Five mass extinctions happened during the Phanerozoic Eon
• The two most significant mass extinctions are:
• End-Permian (the great dying) - 95% of marine species perished along with 70% of land species
• End-Cretaceous - 75% of animal species perished, including all non-avian dinosaurs

Explore fossil formation, the information that they provide and the process of radiometric dating. Learn how the fossil record gives insights into extinct organisms and how radiometric dating uses radioisotopes to estimate the age of rocks.