Fossils and Relative Time: The Law of Faunal Succession
The text above explains how geologists can put rocks in time order from oldest to youngest. The job may be quite difficult, because some outcrops are covered by soil, water, or other rocks, because offsets on faults may be so large that rocks on opposite sides can’t be matched back up easily (remember the bear in Cade's Cove way back in the Great Smoky Mountains), etc. But the job can be done, tested, and found to be correct.
When this job is done, a remarkable result emerges. Many rocks contain fossils, and different rocks contain different fossils. Arranging the rocks in order reveals that the fossils also are arranged in order. Rocks of similar age from similar environments have similar fossils; rocks with a greater age difference also have a greater difference in their fossils. The younger a fossil is, the more it resembles things living today. This scientific result, that fossils succeed one another in a definite and recognizable order, is called the Law of Faunal Succession. “Law” here is not something passed in Congress; a scientific “law” such as this one is a successful summary of an immense number of different observations, and works very well in explaining and predicting many different things in many places and times.
The Law of Faunal Succession was developed in England by a 17th-century canal engineer/geologist named William Smith. He faced practical problems; digging in some rocks is easier than in others, some rocks hold canal water better than others, etc. If he crossed a river into a region where all the rocks were buried beneath soil and thus difficult to study, he wanted to know whether the rocks under that soil were the ones he wanted to build in, or whether the ones he wanted were younger or older, higher or lower, in the pile of rocks. Sometimes, a few fossils would be evident in a streambed or elsewhere. (He even checked on fossils that local farmers were using as weights for balances to weigh cheese!). He found that the fossils made his job easier by providing shortcuts for putting things in order.
Much later, faunal succession figured in the development of our understanding of biological evolution. But, remember that the Law of Faunal Succession is like so much good science—a practical idea that gives useful results and helps humans do things successfully.
The Geologic Time Scale
| Era | Period | Age since end of period(Years) |
|---|---|---|
| Cenozoic = New Life (Age of Mammals) | Quaternary, Tertiary | 0 (present time) |
| Mesozoic = Middle Life (Age of Dinosaurs) | Cretaceous, Jurassic, Triassic | 65 million years ago |
| Paleozoic = Old Life (Age of Shellfish) | Permian, Pennsylvanian, Mississippian, Devonian, Silurian, Ordovician, Cambrian | 225 million years ago |
| Precambrian = Before Cambrian (Age of Algae) | None Listed | 570 million years ago |
Geologists learned to put rocks and fossils in order long before calendar-year ages could be assigned accurately (how many years old is that?). Lacking accurate year counts, the early geologists agreed on a “shorthand” for describing ages. Time is divided into big chunks, called “eras,” which are divided into smaller chunks, or “periods,” and on into still smaller chunks. The big chunks (which you would be wise to know) are, from youngest to oldest, the Cenozoic (= new life), the Mesozoic (= middle life), and the Paleozoic (= old life).
The oldest period in the Paleozoic is called the Cambrian, and the rocks older than it are called the Precambrian. As the Precambrian includes most of the history of the Earth, more and more workers are using “-zoic” terms for pieces of it (see the Enrichment). The periods are mostly named for places where the rocks are well-exposed and where they were studied by early geologists, such as the Cambrian for Cambria (also known as Wales, a region to the west of England in the United Kingdom), Devonian for Devonshire in southwestern England, and the Pennsylvanian for Pennsylvania (Europeans usually lump the Mississippian and Pennsylvanian together and call that lump the Carboniferous, but the USA has such wonderful deposits of this age that we divide the Carboniferous to allow better time resolution). The youngest two periods are the Paleogene (“old generation”) and Neogene (“new generation”). In older reference works, you may see the Paleogene and the first part of the Neogene referred to as the Tertiary, with the more-recent part of the Neogene called the Quaternary; science progresses, but old books and old web sites often are not updated.
Life appeared early in the Precambrian, and multicelled organisms were present in the Precambrian. The Cambrian began with a great diversification of organisms with shells. Fossils older than the Cambrian are relatively rare because soft body parts usually are not fossilized; when shells appeared, they accumulated to make limestone layers, so fossils “suddenly” (over a few million years) became common. (But thanks to a lot of work, many fossils are known from the Precambrian now!). The Paleozoic and the Mesozoic ended with mass extinctions that probably killed a majority of organisms on Earth (we’ll return to this soon). The mass extinction at the end of the Mesozoic was caused by a meteorite impact. The mass extinction at the end of the Paleozoic was caused by global warming and chemical changes linked to a vast outpouring of carbon dioxide and other materials from the largest known flood basalt event in geological history.