EVER WONDER WHERE FROGS CAME FROM?

by Whit Gibbons

September 6, 2009

When did amphibians first evolve on Earth? "The Rise of Amphibians: 365 Million Years of Evolution" (2009, Johns Hopkins University Press, $65.00) by Robert Carroll answers this and other questions about the ancestry of amphibians.

This is a remarkable book written by one of the world's outstanding paleontologists. Carroll has retired from McGill University in Montreal, Canada, but he has not given up scientific pursuits. He continues to examine the paths animals have taken in the past in order to understand what is with us on earth today. In "The Rise of Amphibians" he puts this prominent group of organisms in their place, from their appearance millions of years ago to their status in today's world.

Any modern biology student, whether interested in plants, animals, or microbes, would do well to read the first two chapters of this book. The author describes the history of earth, beginning with the planet's formation over 4 billion years ago and the appearance of self-replicating, living organisms 3.5 billion years ago. The use of radiocarbon dating, DNA analyses, and plate tectonics (involving the movement of the continental land masses) are discussed as part of the toolbox paleontologists use to reach conclusions about the geologic age of different life forms and what they looked like.

To be an expert in the field of paleontology requires having a thorough understanding of detailed anatomical analyses of body structures combined with a time and space sense of geological features. I admire Carroll's ability to present such information in an easy, readable fashion that any literate person can understand. For example, we should all be able to remember that the earliest amphibians appeared on earth 365 million years ago because, as the author points out, "it happens to coincide with the number of days in the year."

The oldest frog fossil known to paleontologists was discovered at a vertebrate fossil site in Arizona in the 1990s; the frog is believed to have lived 190 million years ago. The oldest fossil salamanders are from a site in China thought to be about 165 million years old. Finding salamander fossils is problematic because most species of salamanders are small and fragile. Thus their bones are much less likely to be captured in the fossil record. One fascinating account in the book is how the best preserved fossil salamanders, soft tissues as well as skeletons, were formed at a site in northern China and Inner Mongolia. The fossil beds were in lakes into which ash from erupting volcanoes was deposited for several million years, which created a stable layer of preserved creatures. Meanwhile, volcanic by-products in the form of toxic gases eliminated microorganisms that would otherwise have destroyed most of the fossil material, leaving an extraordinary and unprecedented record of fossil salamanders.

The book itself is a high quality production, with finely illustrated drawings of skulls, vertebrae, ribs, teeth, and long bones of more amphibians, past and present, than most of us imagined even existed. Such remains are the lifeblood of the professional paleontologist, and detailed illustrations are essential for a book intended for use at the college level.

For the amateur enthusiast, the book has a set of beautifully prepared color plates of extinct amphibians, the ancestors of modern frogs and salamanders, in their presumed native habitats. One scene depicts a giant, yellow, salamander-shaped monster with big teeth. The creature is shown alongside a stream dotted with moss-covered rocks and what I imagine to be a foot-long dragonfly hovering overhead. The animals' colors are of course imagined rather than known, but are certainly believable. A key to that particular plate explains that all the vertebrates shown in the drawing lived during the Upper Carboniferous geologic period in what is now Nova Scotia. Such verities add to the believability of the imagined scenes.

With regard to the future of amphibians, Robert Carroll notes the worldwide problem of declining amphibian populations and species extinctions. But he ends the book, whose story began 365 million years ago, on a hopeful note, at least for the amphibians. "Amphibians are tough survivors [and] . . . they may survive the end of us too."

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