Geology Reference
In-Depth Information
the extra oomph by growing larger—much larger. Most dramatic were the giant insects,
exemplified by monster dragonflies with two-foot wingspans. The increased oxygen also
enhancedatmosphericdensityandmadeflyingandglidingthatmucheasier.Otheranimals
undoubtedly migrated to previously uninhabitable higher elevations with thickening air
they could now breathe in.
For a span of tens of millions of years, life on the Pangaean supercontinent flourished.
The climate was benign, resources were plentiful, and life evolved with abandon. But then
rather suddenly, mysteriously, 251 million years ago, life collapsed, in the most calamitous
extinction event in Earth history.
The Great Dying and Other Mass Extinctions
For the past 540 million years, the fossil record has piled up. It speaks of profligate biolo-
gicalinvention—hundredsofthousandsofknownfossilspeciesofcoralsandcrinoids,bra-
chiopodsandbryozoa,clamsandsnails,nottomentionthevastnumberofdifferentmicro-
scopic animals. Specialists estimate diversity in excess of twenty thousand known species
of trilobites, with dozens more described every year. Given that trilobites inhabited Earth
for only about 180 million years (between about 430 and 250 million years ago), that's an
average ofanewtrilobite species every fewthousand years. Taking all the rich diversity of
fossil life into account, several new species must have appeared on average every century
for more than 500 million years.
What's not so immediately obvious from the fossil record are a few stark episodes
of mass death, the sudden extinction of millions of species. It's relatively easy to spot
something new, and paleontologists are not immune to the temptation of describing “the
first” or “the earliest” appearance of a significant taxon or trait. The first plant, the first
amphibian, the first cockroach, and the first snake (albeit with tiny vestigial hind legs) all
havemadethefossilnews.OnerecentpapereventrumpetedthediscoveryofEarth'soldest
knownfossil penis (fromafour-hundred-million-year-oldspider)—yet another remarkable
find from the Rhynie chert.
Loss is harder to recognize in the fossil record. Extinctions require meticulous teasing
out of fossil diversity layer by layer, time interval by time interval, across the globe. Dec-
ades of effort have paid off in the documentation of five great mass extinctions—five
hellish times over the past 540 million years when Earth has suffered the loss of more than
half its species. As more data accumulate, it seems there may have been as many as fifteen
other less severe mass extinction episodes as well.
It's not easy to document the sudden loss of species from the fossil record. Given the
many advances and retreats of the oceans, the opening and closing of shallow seas, the
slowing of sedimentation during cool periods, and the irreversible losses owing to erosion,
