Before we begin to study dinosaurs, we need some basic background information. This chapter will ensure that you have the basic knowledge required to understand the terms and processes discussed in the chapters of this curriculum, as it pertains to Geologic Time. We start with the Geologic Time Scale. Although some students may already be familiar with the time scale, it is being included here so that all students will be familiar with it.
Geologic Time Scale
The geologic time scale provides a system of chronologic measurement relating stratigraphy to time that is used by geologists, paleontologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of the Earth.
Evidence from radiometric dating indicates that the Earth is about 4.570 billion years old. The geological or deep time of Earth's past has been organized into various units according to events which took place in each period. Different spans of time on the time scale are usually delimited by major geological or paleontological events, such as mass extinctions. For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous–Tertiary extinction event, which marked the demise of the dinosaurs and of many marine species. Older periods which predate the reliable fossil record are defined by absolute age.
Each era on the scale is separated from the next by a major event or change.
Terminology
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| Segments of rock (strata) in chronostratigraphy | Periods of time in geochronology | Notes |
|---|---|---|
| 4 total, half a billion years or more | ||
| 12 total, several hundred million years | ||
| tens of millions of years | ||
| millions of years | ||
The largest defined unit of time is the supereon, composed of eons. Eons are divided into eras, which are in turn divided into periods, epochs and ages. The terms eonothem, erathem, system, series, and stage are used to refer to the layers of rock that correspond to these periods of geologic time.
Geologists further break down "periods" into three units, called Upper, Middle, and Lower. These may also be represented as Early, Middle, and Late. For example, Lower Cretaceous and Early Cretaceous are synonymous. This is because as you look at a rock layer that represents the Cretaceous, the oldest rocks are on the bottom. These oldest rocks are the "lowest" in the rock unit.
Geologic units from the same time but different parts of the world often look different and contain different fossils, so the same period was historically given different names in different locales. For example, in North America the Lower Cambrian is called the Waucoban series that is then subdivided into zones based on succession of trilobites. In East Asia and Siberia, the same unit is split into Tommotian, Atdabanian, and Botomian stages. A key aspect of the work of the International Commission on Stratigraphy is to reconcile this conflicting terminology and define universal horizons that can be used around the world.
In the curriculum you will come across the term "Index Fossil." The identification of rock strata by the fossils they contained, pioneered by William Smith, Georges Cuvier, Jean d'Omalius d'Halloy, and Alexandre Brogniart in the early 19th century, enabled geologists to divide Earth history more precisely. It also enabled them to correlate strata across national (or even continental) boundaries. If two strata (however distant in space or different in composition) contained the same fossils, chances were good that they had been laid down at the same time.
The "Age of the Dinosaurs" is that period of the time scale called the Mesozoic Era. The Mesozoic started 251 million years ago (Ma), and ended 65.5 Ma. There are three major periods of the Mesozoic, known as the Triassic, Jurassic, and Cretaceous.
| Mesozoic | Cretaceous | Late | 99.6 |
| Early | 145.5 | ||
| Jurassic | Late | 161.2 | |
| Middle | 175.6 | ||
| Early | 199.6 | ||
| Triassic | Late | 228.0 | |
| Middle | 245.0 | ||
| Early | 251.0 | ||
Please note the times in the blocks at right are the start times for that segment. Also note that the Cretaceous is divided into only two segments, Early and Late, which omits the "middle" portion. Although this is the new accepted division for the Cretaceous, much of the literature related to dinosaurs contains references to three divisions. This curriculum references the three division format.
Of importance to dinosaur study is the further breakdown of the geologic time scale into stages. You will see these stages (ages) listed in the dinosaur descriptions. For example, the Stegosaurus lived from 155 - 150 Ma, from the Kimmeridgian Age to the Tithonian Age, of the Jurassic Period. Many of these ages will become very familiar to you.
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