Fossils and Geologic Time

What is a fossil?

Fossils come in two big groups.

Why most living things never fossilize

When a plant or animal dies, scavengers eat it, bacteria rot it, and wind and water break up what is left. Soft bodies like jellyfish and worms almost always disappear. For a fossil to form, the remains usually need to be buried quickly, in mud at the bottom of a lake or sea, or under sand or volcanic ash, so they are protected. Animals with hard parts, like shells and bones, have the best chance. So the fossil record is like a photo album with most of the pages missing. Scientists know they are seeing only a small fraction of everything that ever lived.

How fossils form

Mary Anning of Lyme Regis

Mary Anning was born in 1799 in Lyme Regis, a small town on the south coast of England. The cliffs there are made of layers of limestone and shale about 200 million years old, from the Early Jurassic, and they are packed with fossils. Every winter storm knocks down more cliff and uncovers new ones. Mary’s family was poor, and they collected fossils, which people called “curiosities,” to sell to tourists.

The sea dragon

In 1811, when Mary was 12, her brother Joseph spotted the skull of a strange animal in the cliffs. About a year later, in 1812, Mary dug out the rest of the skeleton, more than 5 meters long. It was a sea reptile with a long, toothy snout and huge eyes, later named Ichthyosaurus (“fish lizard”). It was not the first ichthyosaur fossil ever found, but it was one of the first to be studied and described by scientists.

More discoveries

• 1823: Plesiosaurus. Mary found the first complete skeleton of a Plesiosaurus, a sea reptile with a tiny head, a very long neck, and four flippers. It looked so odd that the famous French scientist Georges Cuvier at first suspected it was a fake. It was real. The Geological Society in London held a meeting to discuss it, but Mary was not invited.
• 1828: a flying reptile. She found a pterosaur, a flying reptile, later named Dimorphodon. It was the first British pterosaur.
• Coprolites. Mary noticed stony lumps often found inside ichthyosaur skeletons. When she broke them open, she found fish bones and scales. They were fossil dung, and they showed what these animals ate. She helped pioneer the study of coprolites.

Credit where it is due

Mary Anning taught herself geology and anatomy, and scientists traveled to Lyme Regis to learn from her. But as a woman, and as someone from a poor, working family, she was shut out of the scientific world. Wealthy collectors often bought her fossils and described them without naming her. She could never join the Geological Society of London. Women were not elected as Fellows of the society until 1919, more than 70 years after her death.

Mary Anning died in 1847. The next year, the president of the Geological Society, Henry De la Beche, honored her in his yearly address. Today the University of California Museum of Paleontology quotes a description of her as “the greatest fossilist the world ever knew,” and in 2022 a statue of her was unveiled in Lyme Regis.

Putting rocks in order: relative dating

Relative dating tells you which rock or event came first, without giving an age in years. It is like knowing your cousin is older than you without knowing her birthday. Geologists use a few simple rules, many first written down by the Danish scientist Nicolas Steno in 1669.

The most famous unconformity is at Siccar Point in Scotland, where James Hutton saw tilted gray layers with red sandstone lying across their worn-down tops in 1788. You can read that story on the Rocks and the Rock Cycle page. Hutton realized how much time it takes to lay down rock, tip it up, wear it flat, and lay down more on top. Earth had to be very, very old.

Index fossils

Around 1800 the English surveyor William Smith noticed that each rock layer held its own set of fossils, always in the same order, even far apart. That means fossils can be used to match up layers in different places.

The most useful are index fossils. A good index fossil was widespread, so it is found in many places, common, so it is easy to find, and short-lived as a species, so it marks a narrow slice of time. Trilobites, sea creatures related to crabs and insects, and ammonites, coiled relatives of squid, are classic index fossils, because their species changed quickly over time. If you find the same kind of ammonite in a cliff in England and in a canyon in another country, the two layers formed at about the same time.

Putting dates on rocks: radiometric dating

To give rocks an age in years, scientists use atoms that act like clocks. Some kinds of atoms are radioactive: they slowly break down into other atoms at a steady rate that does not change with heat or pressure. The original atom is the parent, and the new one is the daughter.

The time it takes for half of the parent atoms to change is called the half-life. After one half-life, half the parent atoms are left. After two, a quarter. After three, an eighth. By measuring how much parent and daughter are in a sample, scientists can count how many half-lives have passed.

How old is Earth?

The oldest known rocks on Earth are about 4 billion years old, and tiny zircon crystals from Western Australia are about 4.4 billion years old. Earth itself is older still. By comparing lead and uranium in ancient rocks and in meteorites, which formed at the same time as Earth, scientists have found that Earth is about 4.54 billion years old, give or take about 1 percent.

The geologic time scale

Geologists split Earth’s history into named chunks, a bit like a calendar splits time into years, months, and days. The biggest chunks are eons. Eons are split into eras, and eras into periods. Many of the boundaries mark big changes in the fossils. The official chart is kept by the International Commission on Stratigraphy, and scientists update it as dates get more precise.

Mass extinctions

A mass extinction is when a large share of Earth’s species die out in a geologically short time. Two of the biggest mark the edges of the Mesozoic era.

• The end-Permian extinction, about 252 million years ago, was the worst in Earth’s history. Most species in the oceans died out, including the last trilobites. Huge volcanic eruptions in what is now Siberia are the main suspect.
• The end-Cretaceous extinction, 66 million years ago, wiped out about three-quarters of species, including all the dinosaurs except birds, along with the pterosaurs, plesiosaurs, and ammonites. An asteroid roughly 10 kilometers wide hit what is now the Yucatán Peninsula in Mexico, leaving the Chicxulub crater, about 180 kilometers across and now buried. Dust and soot blocked out sunlight, and Earth turned dark and cold.

Earth’s history in one year

4.54 billion years is too big to picture. So squeeze all of Earth’s history into one calendar year. Earth forms at midnight at the start of January 1, and today is the stroke of midnight at the end of December 31. At this scale, each day stands for about 12.4 million years, and each second for about 144 years.

All of written human history fits into the last minute or so of the year.

Credits

Written by Mary, Jugami’s Earth science teacher. The rock layers, fossil-forming steps, and time scale are original, hand-coded SVG made for Jugami. They are simplified sketches; only the time scale is drawn to scale.