Fossil Patterns in Time

Article excerpt

Close your eyes and visualize the world 70 million years ago. What do you see? As a science teacher familiar with geologic time and the likely scenery of different geologic eras, you may envision wide-ranging herds of many kinds of dinosaurs and few, if any, small mammals hiding in the shadows. Included in this vision of the age of dinosaurs might be the sail-backed Dimetrodon, plate-backed stegosaurs, huge long-necked sauropods (such as Brachiosaurus), swimming ichthyosaurs and plesiosaurs, flying pterosaurs, three-horned Triceratops, and T. rex.

However, many may be surprised to learn that only four of these are actually considered dinosaurs (stegosaurs, Brachiosaurus, Triceratops, and T. rex). Furthermore, only Triceratops and T. rex were abundant 70 million years ago (mya), while the sauropod dinosaurs, pterosaurs, plesiosaurs, and ichthyosaurs were all nearly extinct, and the stegosaurs and premammalian Dimetrodon had completely died out much earlier (Munsart 1993; Scotchmoor et al. 2002).

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Why do so many people have this vague and muddled sense of past life? This is due in part to our lack of real experience with deep time. The various efforts by textbooks and teachers using timescales to provide some sense of the vastness of geologic time never connect with time periods familiar to most people. In addition, most textbook treatments of geologic time and ancient life-forms fail to clearly show the patterns of change in life over the ages--the first appearance of different groups, how their populations changed over time, and when they went extinct (Figure 1). As a result, most of us tend to lump "prehistoric life" into a composite collection of life-forms, most of which we don't see today, -with little sense of how or when these changed over time. And this has led to some widespread misconceptions.

It's no wonder, then, that when people read about the "Cambrian explosion," they get the impression that all the major groups of animals "suddenly" appeared, and included the first appearances of fishes, amphibians, reptiles, birds, and mammals. Both assumptions are false. These false impressions of that early time are due primarily to the incompleteness of our knowledge--until recently. With the latest studies of many more fossils of that time, and more precise age-dating strategies, we can now see that the earliest animals in most major animal groups began to diversify through several series of transitional stages (the phyla of today, not classes) from the beginning of the Cambrian (about 542 mya) through a 17-million-year period before the supposed "explosion"--about 525 mya (Maloof et al. 2010). When considered along with the following "explosion" period (the next 5 to 6 million years), it turns out that this emergence of most animal phyla was actually a gradual process, spanning some 20 to 30 million years--nothing close to an explosion. Furthermore, the major groups (classes) of vertebrates that we all know--fishes, amphibians, reptiles, birds, and mammals--made their first appearances over the next 350 million years!

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Purpose of the lesson

After students have completed this activity, they should be able to do the following:

* show how a linear timescale works--one that ultimately relates the unfamiliar to the familiar. Specifically, students should know that if we equate one year of their lives to the thickness of a $1 bill, when this scale is extended, a million years would equal the length of a football field, and 500 million years would equal 500 football fields (about 30 miles), which they have likely traveled in their region.

* recognize, based on the fossil record, the gradual, stepwise pattern of the major groups of vertebrate animals making their first appearance several tens of millions of years apart over 350 million years.

This lesson works well as an introduction to Earth history, when students are asked to consider time periods of millions and billions of years. …