Geology Reference
In-Depth Information
Whatever the causes, the Great Dying left a staggering hole in Earth's biodiversity. It
took thirty million years to recover, but recover it did. And, in a theme repeated after every
extinctionevent,lossledtoopportunity.Anewera,theMesozoic,sawnewfaunaandflora
evolve to fill the vacant niches.
Dinosaurs!
A successful publisher once advised me that if I wanted to sell lots of science topics, I
shouldwriteaboutoneoftwopopulartopics:blackholesordinosaurs.(Thepublishereven
went so far as to include “black holes” in the title of one of my topics that had absolutely
nothing to do with black holes.)
So here goes. Dinosaurs came on the scene about 230 million years ago as beneficiaries
of the end-Paleozoic mass extinction. These fascinating reptiles started slow and small but
diversified and radiated into every ecological niche over a span of more than 160 million
years. For a time after the Great Dying, dinosaurs competed side by side with large amphi-
bians, but another significant extinction event 205 million years ago, coincident with an-
othermegavolcanoepisode,wipedoutmostnondinosaurvertebrates.Adinosaurexplosion
followed.
Dinosaurs are only the most arresting and charismatic of the Mesozoic Era fauna. By
farthecommonest fossilsfromthetime aretheelegantly coiled marine cephalopods called
ammonites. If I hadn't grown up in the vicinity of trilobite-rich Paleozoic rocks, if I had
beenraisedinsteadintheMesozoiclandsofSouthDakota,Iprobablywouldhavecollected
ammonites. Their shells are stunningly beautiful, with their spiral symmetry and iridescent
surfaces. These segmented cephalopods, distant ancestors of the chambered nautilus, fea-
ture exquisite shell ornamentations called sutures that once separated each interior cham-
ber from the next. Unlike trilobites, ammonite shells can't provide a realistic picture of the
complete animals. The big protruding head, with its large eyes and ten suckered tentacles,
has long since decayed. What remains is just the protective armored home of a much more
interestingcreature.For160millionyears,ammonitesevolvedanddiversifiedintheMeso-
zoic seas.
The Mesozoic Era saw many other important biological developments. The flowering
plants first appeared then. So did the first true mammals. And as with every other signi-
ficant chunk of Earth history, there were many changes in geography and topography to
accompany these developments in the living world. Pangaea began to break up, and the
Atlantic Ocean was born. Atmospheric oxygen levels continued dropping to a dangerously
low 15 percent, only to rebound to roughly the present value of 21 percent. Sea levels fell
androseoverandoveragain,thoughthere'snoevidenceforanysignificantglaciation dur-
ing the Mesozoic—nothing to rival even the ice age that ended the Paleozoic.
