Fossil Patterns in Time

Flammer, Larry, Science Scope

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" …

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Publication information: Article title: Fossil Patterns in Time. Contributors: Flammer, Larry - Author. Journal title: Science Scope. Volume: 34. Issue: 6 Publication date: February 2011. Page number: 40. © 2009 National Science Teachers Association. COPYRIGHT 2011 Gale Group.

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