EarthParts #34 - Fossils
Channel: Earth.Parts
Date: 2018-11-21

Fossils are the remains of ancient life forms that have become trapped and partially preserved in sedimentary rock. If a carcass can be buried in sediment before rotting, it might become a fossil. 

Limestone rock easily preserves fossils of marine life, especially life that produces shells of calcium carbonate, calcium phosphate or silica. Limestone is mostly composed of myriads of planktonic, CaCO3 shells of tiny coccolithophores and foraminifera, but can also include fossils of coral-building organisms and other ocean life. 

Shale can preserve fossils to a fine scale of detail, and carcasses of plants and animals that become buried in fine marine silt can be sequestered from oxygen quickly enough to remain intact through burial. Over time fossils form in shale as the rock is compressed and flattened, and dead tissue renders down to black carbon residue.

A world-class example of shale that preserves fossils of soft-bodied animals from the ocean is the Burgess Shale, which outcrops in Alberta, Canada. The Burgess Shale provides a rare glimpse into life among the coral reefs of the middle Cambrian Period. 

Sandstone is poor for fossil preservation because the grains are typically too large, and tend to destroy fossils as compaction converts sand into sandstone. However, some exceptions exist such as the sandstone impression-fossils of ancient sea life found in Australia's Ediacara hills. Ediacaran fossils pre-date even the Cambrian Period, and are all soft-bodied with no hard-mineral parts. 

Fossils can be preserved by permineralization, where dissolved minerals in groundwater precipitate into void spaces in bone or wood and fill them with silica, opal, pyrite or other minerals. So-called "petrified wood" is an example of woody plant material that has been permineralized with silica or agate. 

The semi-precious gemstone amber is fossilized tree resin or sap, and can include samples of its contemporary environment such as insects, pollen, leaf material, even the feathers of dinosaurs.