How Do Scientists Determine the Age of Dinosaur Bones?

By: Tracy V. Wilson  | 

The 67-million-year-old Tyrannosaurus rex skeleton known as Sue
The 67-million-year-old Tyrannosaurus rex skeleton known as Sue stands on display at Union Station on June 7, 2000, in Washington, D.C.
Mark Wilson/Newsmakers/Getty Images

When paleontologist Mary Schweitzer found soft tissue in a Tyrannosaurus rexfossil, her discovery raised an obvious question -- how the tissue could have survived so long? The bone was 68 million years old, and conventional wisdom about fossilization is that all soft tissue, from blood to brains, decomposes. Only hard parts, like bones and teeth, can become fossils. But for some people, the discovery raised a different question. How do scientists know the bones are really 68 million years old?

Today's knowledge of fossil ages comes primarily from radiometric dating, also known as radioactive dating. Radiometric dating relies on the properties of isotopes. These are chemical elements, like carbon or uranium, that are identical except for one key feature -- the number of neutrons in their nucleus.

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Atoms may have an equal number of protons and neutrons. If, however, there are too many or too few neutrons, the atom is unstable, and it sheds particles until its nucleus reaches a stable state. Think of the nucleus as a pyramid of building blocks. If you try to add extra blocks to the sides pyramid, they may stay put for a while, but they'll eventually fall away. The same is true if you take a block away from one of the pyramid's sides, making the rest unstable. Eventually, some of the blocks can fall away, leaving a smaller, more stable structure.

The result is like a radioactive clock that ticks away as unstable isotopes decay into stable ones. You can't predict when a specific unstable atom, or parent, will decay into a stable atom, or daughter. But you can predict how long it will take a large group of atoms to decay. The element's half-life is the amount of time it takes for half the parent atoms in a sample to become daughters.

To read the time on this radioactive clock, scientists use a device called a mass spectrometer to measure the number of parent and daughter atoms. The ratio of parents to daughters can tell the researcher how old the specimen is. The more parent isotopes there are -- and the fewer daughter isotopes -- the younger the sample. The half-life of the isotope being measured determines how useful it is at dating very old samples. Once all the parents have become daughters, there's no more basis for comparison between the two isotopes. Scientists can't tell whether the clock ran down a few days or millions of years ago. This means that isotopes with a short half-life won't work to date dinosaur bones.

The short half-life is only part of the problem when dating dinosaur bones -- researchers also have to find enough of the parent and daughter atoms to measure. Read on to see what it takes to date a fossil and what volcanic ash has to do with it.

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Dating Sedimentary Rock

An eagle flies over the Grand Canyon in Arizona, April 5, 2007. You can see the layers of sedimentary rock
An eagle flies over the Grand Canyon in Arizona, April 5, 2007. You can see the layers of sedimentary rock
GABRIEL BOUYS/AFP/Getty Images

The most widely known form of radiometric dating is carbon-14 dating. This is what archaeologists use to determine the age of human-made artifacts. But carbon-14 dating won't work on dinosaur bones. The half-life of carbon-14 is only 5,730 years, so carbon-14 dating is only effective on samples that are less than 50,000 years old. Dinosaur bones, on the other hand, are millions of years old -- some fossils are billions of years old. To determine the ages of these specimens, scientists need an isotope with a very long half-life. Some of the isotopes used for this purpose are uranium-238, uranium-235 and potassium-40, each of which has a half-life of more than a million years.

Unfortunately, these elements don't exist in dinosaur fossils themselves. Each of them typically exists in igneous rock, or rock made from cooled magma. Fossils, however, form in sedimentary rock -- sediment quickly covers a dinosaur's body, and the sediment and the bones gradually turn into rock. But this sediment doesn't typically include the necessary isotopes in measurable amounts. Fossils can't form in the igneous rock that usually does contain the isotopes. The extreme temperatures of the magma would just destroy the bones.

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So to determine the age of sedimentary rock layers, researchers first have to find neighboring layers of Earth that include igneous rock, such as volcanic ash. These layers are like bookends -- they give a beginning and an end to the period of time when the sedimentary rock formed. By using radiometric dating to determine the age of igneous brackets, researchers can accurately determine the age of the sedimentary layers between them.

Using the basic ideas of bracketing and radiometric dating, researchers have determined the age of rock layers all over the world. This information has also helped determine the age of the Earth itself. While the oldest known rocks on Earth are about 3.5 billion years old, researchers have found zircon crystals that are 4.3 billion years old [source: USGS]. Based on the analysis of these samples, scientists estimate that the Earth itself is about 4.5 billion years old. In addition, the oldest known moon rocks are 4.5 billion years old. Since the moon and the Earth probably formed at the same time, this supports the current idea of the Earth's age.

You can learn more about fossils, dinosaurs, radiometric dating and related topics by reading through the links below.

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Originally Published: Jan 15, 2008

Dinosaur Bones FAQs

Where were the oldest fossils found?
The oldest fossils, microscopic in nature, were discovered in a 3.5 billion-year-old rock in Western Australia.
Are dinosaurs millions of years old?
At the end of the Cretaceous Period, dinosaurs went extinct. That was about 65 million years ago. Dinosaurs are believed to have walked the Earth for close to 165 million years.
How is fossil age calculated?
Fossil age is determined using two methods, relative dating and absolute dating. In relative dating, fossils are compared to similar fossils and rocks, the ages of which are known. Absolute dating, on the other hand is used to calculate the precise age of fossils through radiometric dating. This process measures isotope decay inside the fossil or the rock to determine its exact age.
Has soft tissue even been found in dinosaur bones?
Yes. While soft tissue is rare because it decomposes, scientists have found intact soft tissue in dinosaur bones before. The famous case is when when American paleontologist Mary Schweitzer of North Carolina State University discovered collagen fibers in the fossilized leg bone of a T rex.

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More Great Links

  • Lewin, Roger. "Rock of Ages - Cleft by Laser." New Scientist. 9/28/1991. (1/2/2008) http://space.newscientist.com/article/mg13117884.900-rock-of-ages--cleft-by-laser-if-you-want-to-date-a-rockget-a-laser-thats-the-message-for-geologists-interested-in-the-preciseageof-anything-from-moon-rocks-to-hominid-fossils-.html
  • Museum Victoria. "Determining the Age of Rocks and Fossils." (1/2/2008) http://museumvictoria.com.au/prehistoric/what/fossilage.html#absolute
  • University of California at Berkeley. "Understanding Evolution for Teachers: Radiometric Dating." (1/2/2008) http://evolution.berkeley.edu/evosite/evo101/IIE1aAtomicclocks.shtml
  • USGS. "Geologic Time." Version 1.2 (1/2/2008) http://pubs.usgs.gov/gip/geotime/
  • USGS. "How do geologists date rocks? Radiometric dating!" (1/2/2008) http://wrgis.wr.usgs.gov/docs/parks/gtime/radiom.html
  • USGS. "The Age of the Earth." (1/2/2008) http://wrgis.wr.usgs.gov/parks/gtime/ageofearth.html

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