Many things can be learned from fossils. We can learn when different plants and animals lived, what kinds of organisms existed together, and what ancient plants and animals looked like and how they might have lived.

By figuring out how old fossils are, we can determine when certain species of plants and animals existed and which might have existed in the same time and place in the past. This helps us to figure out how they might have interacted and even helps us to guess at what ancient food chains might have been like. By putting together fossils, we are able to make good estimates of what ancient creatures might have looked like. We can also use this information to guess how they lived. For example, by putting together a T-Rex skeleton, we can guess that it walked on two legs and we can also guess that it ate meat because of its sharp teeth.

Radioactive dating tells us an absolute age for fossils and rocks. We determine how much of a radioactive element is left in a rock or fossil and compare it to the amount that should have originally been there and calculate the age of the fossil based on the calculations.

When using radioactive dating, we use half lives to date a fossil. Certain radioactive types of elements, such as carbon-14, decay at a specific rate. A half life is how long it takes for half of the original type to decay, which means only half as much will be left. Carbon-14 has a half life of 5730 years, so if a fossil only has half as much carbon-14 as we would expect it to then it must be about 5700 years old. We can figure out how much carbon-14 a fossil should contain by looking at how much carbon-12, a non-radioactive substance that stays the same over the years, is in the fossil. Living things have a certain amount of carbon-14 compared to carbon-12, so as the carbon-14 decays there is less and less carbon-14 compared to carbon-12.

Eras are large periods of time and are made up of smaller periods of time.

An era, like the Mesozoic era, is a very large period of time. The Mesozoic era is also made up of three shorter periods, the Cretaceous, Jurassic, and Triassic time periods. You could think of an era like America and the periods like states. The periods are smaller units that make up the larger era. There are also several eras, just like there is more than one country.

Fossils form when dead organisms (or parts of them) are buried in rock or impressions are preserved in rock.

Often fossils are created when organisms are washed into a lake and settle on the bottom. As more and more dirt, rock, sand, and other sediment washes into the lake and also settles on the bottom the organism becomes buried. Over many years more and more sediment builds up over the spot where the organism is buried. Eventually, the weight on top of it squishes the soft material the organism is buried in and turns it into sedimentary rock. An organism might also be preserved very quickly if they are buried in hot volcanic ash that quickly turns to rock, or they might be buried in hot, dry sand in the desert and be very well preserved. Over time the original tissue decays and is slowly replaced by minerals that turn the original creature into stone slowly. Other fossils may form when wet sand or clay has an impression made in it and then dries: this leaves fossils like footprints, imprints of leaves, or even the outline of dinosaur skin or feathers.

The Cenozoic Era is often called the Age of Mammals. It began about 65 million years ago.

65 million years ago the dinosaurs died out, along with many other species. After this, mammals quickly began to expand. More and more mammal species appeared and took important roles in ecosystems. Because of this, the era after 65 million years ago, the Cenozoic, is sometimes called the Age of Mammals.

Relative dating is now more accurate than many years ago because there are more fossils to compare to. 100 years ago a paleontologist might have had four or five fossils that they could reliably compare new fossils to for relative dating. Now, we now the ages of many fossils so it is easier to find the relative age of new fossils that we find.

In Darwin’s time, a paleontologist might have been only able to say that a new fossil was relatively older than Fossil A because it was a foot lower than it in the rock and also relatively younger than Fossil B because it was a foot higher than it in the rock (remember that fossils that are higher in the rock are younger and fossils that are lower are older). Now, we can say that a new fossil is older than Fossil C, which we know to be 70 million years old from radioactive dating, because it is 1 inch lower, and relatively younger than Fossil D, which we know to be 75 million years old, because it is 1 inch higher than that fossil. So we could guess that the new fossil is between 70-75 million years old, which is very close in geologic time!