Geology Reference
In-Depth Information
commonplaceminerals—mostoftencalciumcarbonateorsilica.Suchaninnovationmeant
a lot in the struggle for survival, for predators would rather eat a soft-bodied morsel than
waste energy breaking a tough mineralized exoskeleton. Soon it was make your own shell
or die. The resulting fossil record is amazingly rich, marking a time when sedimentary lay-
ersbecamepackedwithlifelikeremains—atimethathasbeencalledtheCambrian“explo-
sion.”
Explosion
is a misleading moniker. This was no sudden transformation; it took many
millions of years for “biomineralization” to catch on. A few sponges with hardened spines,
preserved in the fossil-rich Doushantuo formation of southern China's Guizhou province,
may have learned the trick as far back as 580 million years. By about 550 million years
ago, at the tail end of the Ediacaran Period, a variety of wormlike creatures had learned to
craft carbonate minerals into tube-shaped protective homes on the ocean floor.
The first recognizable shelly fauna, though small and fragile, appeared around the world
in rocks about 535 million years old. (I recall a special undergraduate field trip to Nahant,
on the Massachusetts coast just north of Boston, to collect these rare fossils. The brisk sea
air, the waves breaking on the picturesque rocky shore, the gorgeous white puffy clouds,
and the blue ocean were all memorable—the scrappy weathered fossils, barely visible to
the naked eye, not so much.)
The real “explosion” occurred a few million years later, roughly 530 million years ago,
when all manner of shelled animals rather suddenly came on the scene. An evolutionary
arms race ensued. Armored predators and armored prey assumed larger and larger dimen-
sions. Teeth and claws arose, as did bony protective plating and sharp defensive spines.
Eyes became mandatory in the cutthroat world of the teeming Paleozoic oceans. As count-
less generations of shelled creatures lived and died, their carbonate bioskeletons contrib-
uted to massive, resistant limestone layers, which nobly decorate the last half-billion years
ofEarthhistory.Stupendousfossil-packedcarbonatecliffsandridgesdottheglobe,domin-
ating the landscape in dozens of countries, forming the tallest peaks of the Canadian Rock-
ies and the china-white cliffs of Dover, even capping the summit of Mount Everest.
Of all the Cambrian evolutionary innovations, wide-eyed sea creatures called trilobites
are the most prized, the most photogenic. At this point, a disclaimer is required. I love
trilobites. I unearthed my first nearly complete specimen not far from my boyhood home
in Cleveland, Ohio, at the age of seven or eight, and I've been collecting them ever since.
My cache now holds more than two thousand pieces, all of which are being donated to the
Smithsonian. (You can see some of the best specimens in the Sant Ocean Hall at the Na-
tional Museum of Natural History.) So I'm biased.
Though the initial rise of biomineralization was gradual, at 530 million years ago life
with hard parts seems suddenly to have been everywhere. All manner of leggy trilobites
and striated clams, nutlike brachiopod shells and delicate fanlike bryozoa, porous sponges
