Ammonites make good index fossils.
An index fossil is formed by a plant or animal that lived during a limited geological timeframe. Its presence in a particular layer of sedimentary rock helps paleontologists and geologists determine the age of that layer and the relative ages of the layers above and below. A good index fossil needs to be abundant, easy to recognize and short-lived, and it needs to have a wide distribution.
Abundant
Ammonites, brachiopods, graptolites, nanofossils and trilobites make good index fossils. These fossils are found in large numbers in sedimentary rock layers.
Easy to Recognize
Index fossils have distinctive shapes. For example, ammonites have a ribbed spiral shell, while brachiopods look like clams. You can identify an index fossil just by looking at it.
Short-Lived
Different species lived at different geological times. This rapid evolution of species lets scientists more precisely date different rock layers. For example, if you find an ammonite from a species known to be from the Triassic period, then the rock layer it came from must be Triassic.
Wide Distribution
Marine animals make the best index fossils. Unlike land animals and plants, marine animals are found worldwide.
Tags: index fossil, good index, index fossils, make good, make good index, rock layers
