How do we know?
↻ One of the pages with a moving picture
Some fossils are clues to how old a rock is.
- An animal dies and is quickly buried by mud or sand, safe from scavengers.
- Soft parts rot away. Hard parts like bones and teeth last longest.
- Water with minerals seeps into tiny holes in the bones, slowly turning them to stone.
- Millions of years later, erosion wears the rock away and bones poke out.
Index fossils are clues to age. They come from living things that lived only for a short time long ago, so finding one tells scientists how old a rock is.
Fossils can be found in big numbers. At Dinosaur National Monument, about 1,500 dinosaur bones fill one rock wall, and visitors can see them all inside a hall. SUE the T. rex, found in South Dakota in 1990, is 90% complete.
Turning to stone does not only happen to bones. Logs buried over 200 million years ago became almost solid quartz at Petrified Forest in Arizona.
Wow number
1,500 dinosaur bones fill one rock wall at Dinosaur National Monument. Visitors can see them all inside a hall!
Myth or fact?
Most animals that die become fossils. Myth. Most rot or get eaten first.
Some fossils are poop. Fact. Fossil dung, called coprolite, shows what animals ate.
Think about it
What would help your footprint in mud last millions of years?
Where in the world?
South Dakota, USA where SUE the T. rex was found in 1990. In Petrified Forest, Arizona, logs buried over 200 million years ago became almost solid quartz — 45.0283° N, 102.0394° W.
Where these facts come from
Every number on page 24, and who measured it
This page explains how fossils form, and why most living things never become fossils.
How Fossils Form — The U.S. National Park Service
nps.gov/subjects/fossils/how-fossils-form.htm
We checked it on 31 October 2026
This page explains how minerals fill up old bones.
Permineralization and Replacement (U.S. National Park Service) — The U.S. National Park Service
nps.gov/articles/000/permineralization-and-replacement.htm
We checked it on 31 October 2026
This page tells us how fossils get uncovered over time.
Laws, Regulations, & Policies — The U.S. National Park Service
nps.gov/subjects/fossils/fossil-protection.htm
We checked it on 31 October 2026
This page also explains how fossils are made.
How Is a Fossil Made? — The U.S. National Park Service
nps.gov/zion/learn/nature/fossils-made.htm
We checked it on 31 October 2026
This page tells us about the wall of about 1,500 dinosaur bones.
Quarry Exhibit Hall — The U.S. National Park Service
nps.gov/dino/planyourvisit/quarry-exhibit-hall.htm
We checked it on 31 October 2026
This page tells us where SUE the T. rex was found.
How well do you know SUE? — The Field Museum in Chicago, home of SUE
fieldmuseum.org/blog/how-well-do-you-know-sue
We checked it on 31 October 2026
This page tells us how old the stone logs at Petrified Forest are.
Petrified Wood — The U.S. National Park Service
nps.gov/pefo/learn/nature/petrified-wood.htm
We checked it on 31 October 2026
This page tells us how fossils help date rock layers.
Geologic Time: Index Fossils — The U.S. Geological Survey, the government's Earth scientists
pubs.usgs.gov/gip/geotime/fossils.html
We checked it on 31 October 2026
This page tells us about trace fossils like footprints and fossil poop.
Trace Fossils — The U.S. National Park Service
nps.gov/subjects/fossils/trace-fossils.htm
We checked it on 31 October 2026
Has this changed?
How fossils form is not going to change.
New fossils are found all the time, because erosion keeps wearing rock away and uncovering more.
Words from this page
- fossil
- The remains or marks of a living thing from long ago, preserved in rock.
- erosion
- When wind, water or ice slowly wear rock away.
- coprolite
- Fossil dung.
- trace fossil
- A fossil that shows what an animal did, like a footprint, instead of its body.
Try it!
Make a track like a fossil footprint.
1. Flatten some playdough. 2. Press a toy animal’s feet across it, like it is walking. 3. Let it dry. Can a friend tell which animal made it?
Why: fossil footprints are trace fossils. They show what an animal did, not its body.
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