What Are Fossils and Where Are They Found the Most?

Fossils tell us the world over about life on Earth before humans existed — they can help us understand what the climate was like, what creatures lived on the planet, and what caused significant changes in their evolution.

Understanding how fossils are formed, where they're best preserved, and where they are found is imperative for experts to piece together their mysteries.

Fossils offer us a glimpse into a past. They are the remains or traces of organisms that existed in the geological past, encapsulating a moment in time that ranges from a few thousand years to an astonishing several billion years old. 

When paleontologists hunt for fossils, they can encounter two different types. The first, "body fossils," are the remaining parts of the animal, like shells, horns, bones, and teeth. The second, "trace fossils," are the tracks and traces organisms may have left behind, like burrows or prints in the ground.  

Fossils aren't merely the remnants an organism leaves behind. These remnants undergo a process called "fossilization," which changes their composition. After an organism dies, if the carcass is buried by mud, sand, or rock sediments, it's protected from scavengers and weathering. After some time, its skin and flesh decompose and wear off, and small mineral particles from the sediment seep into the pores of the skeleton that's left until it's completely filled with minerals.

This process — called permineralization — turns the bones into a stone copy so they're preserved as fossils.  

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There are loads of fossils to be found worldwide, but not nearly as many as animals roaming the planet. Why? Because most ancient organisms never got a chance to fossilize — they rotted in the open air, scavengers picked them apart, or water swept them away. 

Some scientists hypothesize less than 8 to 10 percent of animals make it to fossilization, and others argue that as little as less than one-tenth of 1 percent. "Whatever the percentage, we know it must be low just given what is stacked against an organism being fossilized," says Matthew Gibson, curator of natural history at The Charleston Museum in South Carolina. 

Becoming a fossil is no easy feat — that's why the percentage is so low. The organism must be quickly buried to keep other organisms from feeding on it. Ideally, they're buried in an anaerobic environment to stop microbes from breaking it down, and they must survive the long process of mineralization without being uncovered and destroyed. It's paradoxical here, but for something to survive millions of years, it must survive the very first minutes after its death.

"Then, even if it survives all that, the fossil would have to have remained relatively secure for millions of years until someone actually finds it," says Gibson.

Although there are three different types of rocks — igneous rock, metamorphic rock, and sedimentary rock — almost all fossils are found specifically in sedimentary rock, that's because, for fossilization to happen, sediments like organic material, sand, silt, and mud have to settle around the remaining bones.  

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Different types of organisms are likely to have fossilized in different types of environments, but the most crucial aspect of fossilization is a quick burial in a place where the organism is least likely to be scavenged, according to Gibson. That's why an organism has more chances of being preserved in environments like water systems, rivers, and the ocean, which are constantly moving sediment, rather than out in the open land.

"Fossils can be found all over the world, although sometimes it may not be as obvious," says Gibson. "It's all a matter of understanding the geology and, in turn, how that translates to those past environments."

Some of the oldest fossils discovered are of microbes preserved in arid western Australia. It's easier to find plant fossils in areas that were once swampland. In coastal environments that once were oceans themselves, marine animal fossils like shark teeth and whale bones can be found miles inland.

Gibson once helped excavate a giant sloth in Goose Creek, South Carolina. "It was probably the most surprising location I have ever worked in; this particular sloth was uncovered in a drainage ditch for a soccer field," says Gibson. "This meant that restrooms and soda machines were only a few yards away. Certainly very convenient."

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All seven continents have been home to fossil discoveries, but some countries are more prolific than others. 

The "Hell Creek Formation," found between Montana, Wyoming, and North and South Dakota, has produced most of the fossils in the U.S., including mammals, dinosaurs, fish, and amphibians. Utah, too, has one of the most complete fossil records on the planet, starting 2 billion years ago and featuring everything from mollusks to trilobites.

China has been a treasure trove of dinosaur fossil discoveries. In Yunnan province, the Chengjiang Fossil Site is a UNESCO World Heritage site for preserving an exceptional array of hard and soft tissues from 530 million years ago. 

In the Qingjiang River, paleontologists have discovered over 100 species of prehistoric animals — more than half new two science — from a burst in biodiversity during the Cambrian Period. Near the city of Liaoning, paleontologists identified the first feathered dinosaur, Sinosauropteryx. 

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Yes, there is fossil evidence of continental drift. Fossils were also key to helping scientists figure out that the world used to be one whole landmass, the supercontinent Pangea, which slowly split up into seven continents.

In the late 1800s, several geologists and climatologists first started noticing on maps that continents were shaped in ways that seemed to fit, like pieces of a puzzle. In 1915, German geologist Alfred Wegener published The Origin of Continents and Oceans, in which he detailed his hypothesis of "the continental drift."

Wegener suggested that mountain ranges with identical rocks could be found on opposite sides of the Atlantic Ocean — indeed, the Appalachians and the mountains of Eastern Greenland and Great Britain are made of the same rock. Fossils of the same types of extinct animals and plants could be found across these same rocks all over the seven continents, but there was no way they could have traveled across oceans to spread out like that.

Read More: Continental Drift: A Revolutionary Theory That Was Once Considered Pseudoscience

What really drove the point home for Wegener, though, were the fossils of an ancient fern called Glossopteris, which thrived during the Permian and Triassic periods and, although extinct, was preserved till modern day with fossils of its imprint, and some organic remains.

These fossils can be found across all of the continents — spanning the most landmasses overall — but Glossopteris's seeds were too heavy to have been blown across oceans by the wind. "Given the current distance between the continents today, it was incredibly unlikely that the seeds could have traveled over the oceans to create such a disjunct range," says Gibson.

And since fossils don't exist in a vacuum, when it's said these fossils span across the continents, it means their rock layers do as well. "It is a combination of this biological and geological evidence that suggests at one point the continents were fused," says Gibson.

Initially, between the 1910s and 1950s, Wegener was not taken too seriously. Although he had a lot of evidence, he didn't have an explanation as to what led the pieces of land to split apart. It was only as recently as the 1960s that geophysicists used theories about seismological forces to confirm Wegener's hunch.

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