Geology Reference
In-Depth Information
and horn-shaped corals are preserved in layer after layer of sedimentary rocks across the
globe. In rock sequences from Montana to Morocco, you can lay your hand on the exact
layer—the exact sliver of history—when this startling invention of bioarmor really took
off.
Oneofthemostimpressivespotstostudytheabruptshiftfromsoft-bodiedtoshelledan-
imalsisnearthehistoricoasisvillageofTiout,nestledinthescenicfoothillsoftheAnti-At-
las Mountains in western Morocco. Many thousands of feet of carbonate sediments, stand-
ing almost vertically on end and exposed in the steep-walled valley of the Souss River,
provideacontinuousrecordoftheendoftheEdiacaranandthebeginningoftheCambrian.
Layer upon layer of the thin, reddish-brown limestone is utterly devoid of familiar fossils.
Youcanwalkamilealongtheriver'sgravel-strewn bed,whichisdrymostoftheyear,and
see little more than the occasional suggestion of a worm burrow.
Then suddenly, in a layer of limestone on a hillside above the village—a horizon that,
when viewed from a distance, seems no different from those above or below—the fossils
appear. The ancient
Eofallotaspis,
perhaps the earliest of all trilobites, marks the very be-
ginning of the Cambrian explosion. In layers a short distance above (that is, younger than)
those historic strata, new species are found: the distinctive, elliptical two-inch-long forms
of
Choubertella
and
Daguinaspis
. The latter species is by far the most common, but one of
the most productive and accessible outcrops lies smack in the middle of a saint's graves-
ite, a holy Muslim shrine. The small, round-domed, white structure is surrounded by a low
rockwall that'sfulloftrilobites. Itwouldn'tdoforvisiting geologists tobreakouthammer
and chisel, to disturb the quiet place. Local children seem exempt, however, and they sell
the “Tiout bugs” to tourists, tapping onyourcar window,holding uptheir freshly exhumed
wares.
“Hey mister, one hundred dirhams!” About twelve dollars.
I don't haggle. I buy them all.
Facies Change
For many years, my fossil collecting was focused on the bugs. It's hard to overstate the
thrill of cracking open a rock and finding a complete trilobite inside. Fishermen must get a
similar buzz when they hook a big fish, and poker players when they draw a full house; for
me, it's finding an exquisite animal that's been hidden for five hundred million years.
For years, the hunt was enough. Then as a senior undergraduate in the spring of 1970, I
took my first real paleontology course from the venerable Robert Shrock. For almost four
decades, Bob Shrock taught at MIT, and for almost twenty years following World War II,
he chaired the MIT department of geology and geophysics. He was a giant in the field,
with numerous classic publications, perhaps most notably
Index Fossils of North America
,
