Studies of the fossil record show that extinctions in Earth's history vary from a disappearance of a species (an extinction), to the disappearance of entire lineages and populations within regional communities or globally (a mass extinction). Paleontologist have scoured outcrop areas and made extensive collection of fossils. Their investigations have have revealed information about the appearance, changes, and extinction of many species. In many cases, they have made detailed analysis of fossil population and distributions across a region where rock layers of a particular age are preserved—one example involves extensive in sedimentary rock formations like the Triassic-age Chinle Formation in Painted Desert region of Arizona that contain an abundance of well preserved fossils (Figure 2.11).
Figure 2.11. Outcrop area of the Triassic-age Chinle Formation in the Painted Desert, Arizona is an example of an ideal study area that has an abundance of fossils preserved in many layers of strata over a large region.
The changes in species structure of an ecological community over time is called ecological succession. Ecological succession takes place on time scales ranging from decades (such as what happens to forest community after a massive wildfire or catastrophic superstorm) or even millions of years during an ice age or a mass extinction event. Figure 2.12 shows an interpretation of the changes in the species populations in within an ancient ecosystem over time as revealed by fossils preserved is successive layers of sedimentary strata. Changes in ancient species populations and ecosystems can be inferred from the abundance the fossil preserved (or missing), the character of the fossils themselves, and sometimes information can be inferred from the sediments surrounding fossils or trace fossils in the sedimentary layers investigated in a study area. Studies show that species appear, populations grow, and then decline and vanish, sometimes returning, or are often replaced by other species that either have out-competed them, or simply replaced them when climate changes or other processes occurred that changed an ecosystem community setting over time.
Figure 2.12. Population changes in a local ecosystem over time (select species and total population of all species). Interpretations like this may be made from exhaustive studies of fossil collections from an area like in Figure 2.11.