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Dead Things in Rocks

Dr. Ben Waggoner BIOL 1400

Starting in the 1500s, there was a surge of

interest among scientists in various funny-looking rocks that were found in many parts of the world—rocks that

looked something like living organisms, known as “figured stones” or fossils.

No less a personage than the artist Leonardo da Vinci speculated that these fossils must be the

remains of once-living things from ancient seas.

Since things are much more ancient than letters, it is no marvel if, in our day, no records exist of

these seas having covered so many countries. . . But sufficient for us is the testimony of things created in the salt waters, and found again in high mountains far from the seas.

But although some “figured stones”

looked like familiar living organisms. . .

others were much less like anything that was known to live. Some scholars argued that “figured stones” had

grown in the Earth by some natural power, and had never been

alive.

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Nicholas Steno �(1638 - 1686)

A Danish physician (and later a Roman Catholic priest; he was beatified in 1987) Steno was one of the first persons to think about how layers of rocks might form, and he devised a principle that geologists still use. . .

This is his drawing of the head and teeth of a dead shark, which he used to demonstrate that certain odd stones, called glossopetrae, were in fact the teeth of ancient sharks.

Steno proposed that if a “figured stone” or “fossilium” looked like a living organism under close inspection, that was because it once had been a living organism.

Many of the Earth's rocks are

sedimentary rocks; they were laid down by a fluid—usually

by water, as layers of mud, silt, and sand.

younger

older

Steno’s Law of Superposition: In a sequence of sedimentary rocks, the lowest layers are the oldest, and the highest are the youngest—unless something

has disturbed the rocks after they were formed.

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Ubehebe Crater, Death Valley, CA Drum Mts., Millard County, Utah

Steno’s Law says nothing at all about how old a rock sequence is, or how long it took to form. The layers

on the left, we now reckon, formed about 500 million years ago, over several million years. Those on the right formed a few thousand years ago, over a

period of a few weeks or months.�All it is is a statement of relative age. (Nopah Range, California)

Rock layers may be tipped off the horizontal. . .

or bent and deformed in various ways. . . (Haghios Pavlos, Crete, Greece) (China Ranch, southern Inyo County, California)

Rocks don’t have to form a continuous sequence, either. This line, where the angle of the rock layers

changes, represents a time gap.

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William Smith (1769-1839) •  A civil engineer and surveyor,

Smith came across numerous fossils during his work on canal surveying and mine engineering.

•  He realized that different layers contained different fossils. . .

•  . . . what was more: fossils always occurred in a predictable order of appearance. The order of fossils in sedimentary rocks was the same all over England. This is the “Law of Succession”.

Here’s an example. On the left is a diagram of rock layers. On the right are three different fossils called trilobites, with the arrows

showing where they appear. If you find these fossils anywhere in North America, you’ll find them in this order of appearance.

Another example, this time from northwestern

Arkansas. The rock layers around Fayetteville

(diagrammed on the right) contain fossils of

coiled shells (technically called ammonoids, on

the left). Different layers have different shells—

and the shells appear in the same order in rocks

all over the world.

Geologic Time Scale

•  Because fossils appeared in a predictable order, you can use them as markers of relative time.

• What’s more, you can define time periods based on what was living at the time.

•  This enables geologists to construct a relative time scale, define time periods, and name them.

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Working at the Museum of Natural History in Paris,

Cuvier documented something that bothered people at the time: many fossils were remains of

animals and plants that could no longer be found living anywhere in the world.

Cuvier documented extinction.

Georges Cuvier (1769-1832)

Cuvier compared this jaw of an ancient elephant-like beast

called a mammoth. . .

. . . with the bones of living elephants, such as the Indian

elephant shown here. . .

. . . and showed that they weren’t identical!

But mammoths were gigantic— the size of elephants! If they were still alive, we’d have found them by now.

Conclusion: The mammoth must have

GONE EXTINCT!

The same was true for the “Irish elk”, a giant

European deer (not really an elk, and not restricted to Ireland) whose bones

had been known for centuries. . . and for a

large and growing number of other fossils, everywhere that Cuvier

looked!

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What was more: the older fossils were, the less likely they were to resemble living organisms.

•  The youngest fossils included relatively familiar forms, like the long-horned bison at the top.

•  Older fossils included less familiar beasts, such as the vaguely rhino-like titanothere in the center.

•  And if you went back far enough in time, you found only bizarre reptile-like animals with no living counterparts at all.

The idea of an ancient Earth, which had housed strange extinct beasts, was increasingly accepted — especially as things like this turned up in the rocks. How could the huge and growing diversity of bizarre extinct lifeforms be crammed into 6000 years?

Fossil plesiosaur, discovered in the 1820s by Mary Anning at Lyme Regis, England