WONDER WHERE FROGS CAME FROM?
September 6, 2009
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
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.
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."
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.
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.
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.
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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