Skeleton that the bird-like specimen (Aurornis xui) discovered in Yizhou Fossil & Geology Park, China.(Image credit: Thierry Hubin/IRSNB)
When animals, plants and other organisms die, they commonly decay completely. Yet sometimes, when the conditions are just right, they"re preserved as fossils.

You are watching: Fossils most often form when a dead organism is buried in sediments.

Several different physical and chemical procedures create fossils, follow to the brand-new York State geological Survey.

Freezing, drying and also encasement, such as in tar or resin, can develop whole-body fossils that maintain bodily tissues. These fossils represent the organisms as they were as soon as living, but these types of fossils are really rare.

The heat and also pressure from being hidden in sediment deserve to sometimes reason the organization of biology — consisting of plant leaves and also the soft body components of fish, reptiles and marine invertebrates — to relax hydrogen and also oxygen, leaving behind a residue of carbon.

This procedure — i beg your pardon is dubbed carbonization, or distillation — returns a in-depth carbon impression that the dead biology in sedimentary rock.

The many common method of fossilization is referred to as permineralization, or petrification. After an organism"s soft tissues decay in sediment, the hard parts — an especially the skeletal — space left behind.

Water seeps into the remains, and also minerals dissolved in the water seep into the spaces within the remains, wherein they type crystals. These crystallized minerals reason the remains to harden along with the encasing sedimentary rock.

In another fossilization process, called replacement, the mineral in groundwater change the minerals that comprise the bodily stays after the water totally dissolves the original hard parts of the organism.

Fossils also type from molds and also casts. If an organism totally dissolves in sedimentary rock, it can leave one impression the its exterior in the rock, referred to as an external mold. If the mold it s okay filled with various other minerals, it becomes a cast.

An inner mold forms when sediments or minerals to fill the interior cavity, such as a covering or skull, of one organism, and also the remains dissolve.

Organic remnants

In current years, researcher have uncovered that some fossils aren"t simply made the minerals. Fossil analyses have actually shown, for instance, that part retain organic material dated come the Cretaceous, a duration that lasted indigenous 65.5 million to 145.5 million year ago, and also the Jurassic period, which lasted from 145.5 million come 199.6 million years ago

Tests imply that this organic products belong to dinosaurs because they match details proteins indigenous birds, which developed from dinosaurs.

"It provided to be the no one thought it was feasible for any type of endogenous product — product that comes from the pet — can be left behind ~ the fossilization process," said Ken Lacovara, the dean that the college of Earth and Environment at Rowan college in brand-new Jersey. " that"s no really the case."

It"s unclear exactly how the organic product is preserved, but iron might help the proteins come to be cross-linked and also unrecognizable, or unavailable to the bacteria that would otherwise consume them, Lacovara said. (Formaldehyde functions in a comparable way, cross-linking the amino acids that comprise proteins, make them much more resistant to decay, mary Schweitzer, a molecule paleontologist at north Carolina State University, called Live Science.)

Another idea is "microbial masonry," Lacovara said. "It"s possible that the bacteria that originally chomped through the tissue are secreting minerals together a rubbish product that then hermetically seal a tiny bit of what remains behind," practically like a stone mason sealing turn off a structure, the told Live Science.

Moreover, sandstone — absent made the sand-size grains of minerals, sediments or not natural material — seems to be the best form of setting for maintaining organic product in fossils.

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"Sandstone is favor a bunch of volleyballs sitting on optimal of each other with large interstitial areas between them," Lacovara said. "So it seems prefer rapid decay can promote the conservation process. Perhaps we require the bacteria to get through fast and also to chomp through the sediment so that they have the right to sequester several of in the process."

Additional reporting by staff Writer Laura Geggel.

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Joseph Bennington-Castro is a Hawaii-based contributing writer because that Live Science and also He holds a master"s degree in science journalism from new York University, and a bachelor"s level in physics native the college of Hawaii. His work covers all locations of science, indigenous the quirky mating actions of different animals, to the drug and alcohol behavior of ancient cultures, to new advances in solar cell technology. On a more personal note, Joseph has had actually a near-obsession with video clip games because that as lengthy as he deserve to remember, and also is probably playing a game at this an extremely moment.