KEY CONCEPT Specific environmental conditions are necessary in order for fossils to form. Tollund man 2200 yrs. Ago Found in a peat bog in Denmark. Acidity preserved skin/hair.

Fossils can form in several ways. Permineralization occurs when minerals carried by water are deposited around a hard structure. (silica, carbonates, iron) Creates rock like fossil. ex. Dinosaur bones (Velociraptor)

A natural cast forms when flowing water removes all of the original tissue, leaving an impression. (Mold) Minerals can then fill the mold, recreating the shape. Crinoid (marine animal) Mollusk

Trace fossils record the activity or behavior of an organism. ex. Nests, burrows, imprints of leaves, footprints

Amber-preserved fossils are organisms that become trapped in tree resin that hardens after the tree is buried.

Preserved remains form when an entire organism becomes encased in material such as ice, volcanic ash, or peat bogs, tar pits. (La Brea tar pits – Los Angeles)

Specific conditions are needed for fossilization Specific conditions are needed for fossilization. Need to be immediately buried in sediments, sand, mud or tar. No oxygen. Usually found in sedimentary rocks, in bogs, wetlands, river mouths, lakebeds, and floodplains. Only a tiny percentage of living things became fossils. Most organisms decompose and nutrients are recycled into ground. 99% of all species are now extinct. We have discovered about 250,000 fossils of animals and plants

A fish returns to its birth place to spawn.

Having spawned the fish dies and shortly after sinks to the seafloor.

After several weeks, the soft body tissues have mostly decayed.

Techtonic activity causes nearby sediment to bury the fish.

Several months pass and all that remains of the buried fish is its skeleton.

As time passes, more sediment accumulates above the fish and the skeleton is gradually compressed and permineralised (mineral replacement)

Over time, the rock is distorted and uplifted by geological forces associated with continental movement, raising it above sea level.

The uplifted rock is exposed to weathering and gradually erodes away, eventually exposing the tip of the fish’s skull at the surface.

A paleontologist recognises the fish by the small area of skull exposed and begins to carefully extract the specimen.

A Pomognathus fish from Houghton Quarry (U.K.) – the skull is visible.

Relative dating estimates the time during which an organism lived. It compares the placement of fossils in layers of rock of a known age. Scientists infer the order in which species existed. If deeper, probably older.

Radiometric dating provides an accurate way to estimate the age of fossils. Radiometric dating (Absolute Dating) uses decay (break down of the atom) of unstable isotopes. Ex. Carbon – 14 Named for # of neutrons + protons (mass) Isotopes are atoms of an element that differ in their number of neutrons. Makes some of them unstable. neutrons protrons

Radiometric dating uses decay of unstable isotopes. A half-life is the amount of time it takes for half of the isotope to decay into a different element. Decay – release radiation and particles, causes the element to change form. The decay rate is equal to the half-life. Half-life of C-14 is 5730 years. Half of the sample has changed into N-14.

Radiocarbon Dating C-14 accumulates in organisms bodies through eating and breathing. When an organism dies, its intake of 14C stops, but the decay of C-14 continues. It is being changed into N-14 The fossil’s age can be estimated by comparing the ratio of C-12 to C-14. 12C is stable and doesn’t decay! The longer the organism has been dead, the larger the difference between the 12C and 14C.