How do scientists determine the age of fossils?

A number of methods are used today to date fossils. Most of the methods are indirect meaning that the age of the soil or rock in which the fossils are found are dated, not the fossils themselves. The most common way to ascertain the age of a fossil is by determining where it is found in rock layers. In many cases, the age of the rock can be determined by other fossils within that rock. If this is not possible, certain analytical techniques are often used to determine the date of the rock layer.

One of the basic ways to determine the age of rock is through the use of radioactivity. For example, radioactivity within Earth continuously bombards the atoms in minerals, exciting electrons that become trapped in the crystals structures. Using this knowledge, scientists use certain radiometric techniques to determine the age of the minerals, including electron spin resonance and thermoluminescence. By determining the number of excited electrons present in the minerals and comparing it to known data that represents the actual rate of increase of similar excited electrons the time it took for the amount of excited electrons to accumulate can be calculated. In turn, this data can be used to determine the age of the rock and the fossils within the rock.

There are other methods for determining fossil age. For example, uranium- series dating measures the amount of thorium-230 present in limestone deposits. Limestone deposits form with uranium present and almost no thorium. Because scientists know the decay rate of uranium into thorium-230, the age of the limestone rocks, and the fossils found in them, can be calculated from the amount of thorium-230 found within a particular limestone rock.

What are molds and casts?

Molds and casts are types of fossils. After burial, a plant or animal often decays, leaving only an impression of its hard parts (and less often, soft parts) as a hollow mold in the rock. If the mold is filled with sediment, it can often harden, forming a corresponding cast.

Fossils are not always bones. These fish fossils are not actually bones, but rather imprints the fish made in the soil. (iStock).

What are trace fossils?

Not all fossils are hardened bones and teeth, or molds and casts. There are also fossils that are merely evidence that creatures once crawled, walked, hopped, burrowed, or ran across the land. Trace fossils are just that: the traces of a creature left behind, usually in soft sediment like sand or mud. For example, small animals bored branching tunnels in the mud of a lake bed in search of food; and dinosaurs hunted for meals along a river bank, leaving their footprints in the soft sand. Similar to the fossil formation of hard parts, the footprints and tunnels were filled in by sediment, then buried by layers of more sediment over millions of years, eventually solidifying. Today we see the results of this long-ago activity as trace fossils. Many originators of trace fossils are unidentifiable in other words, there are no hard fossils of the creatures left in the area, just their tracks. Some of the most famous trace fossils are those of dinosaurs tracks (for example, in Culpepper, Virginia, and near Golden, Colorado), and human-like footprints (for example, in east Africa), which were all found in hardened sediment.

There is a difference between tracks and trails, too: tracks are generally the traces of distinct footprints, whereas trails may have been produced by an animal dragging its feet or some other appendage as it moved. Tracks, therefore, are more distinctive, and different animals can be distinguished by their own particular footprints. Trails can seldom be associated with a particular animal.

What can some trace fossils of dinosaur tracks tell us?

Numerous fossilized dinosaur footprints, called a trackway, indicate much about dinosaurs speeds. One such trackway is located north of Flagstaff, Arizona, on the

Navajo Reservation. This site was first discovered by Barnum Brown of the American Museum of Natural History in the 1930s, but had been lost until recently. It includes an example of a running dinosaur that left tracks with an 8-foot (2.4-meter) space between the right and left prints; from these prints, scientists calculated that the dinosaur ran at speeds of 14.5 miles (23.3 kilometers) per hour one of the faster dinosaurs known. The record for fastest dinosaur, however, is presently held by a Jurassic carnivore that left a 16-foot (5-meter) gap between the right and left tracks in a Glen Rose, Texas, trackway. The calculated speed of this dinosaur was about 26.5 miles (42.8 kilometers) per hour, much faster than the speediest human.

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