The Nature of Fossils

Mosquito in amber, fossilized tree resin from the Cretaceous Period approximately 92 million years old. (Courtesy: S. Kurth)

Among the most intriguing objects on our planet are rocks shaped like organisms. They are called "fossils" which means "things dug up" in Latin". For example, from the Jurassic deposits of southern Germany,given time and luck, one can collect an entire stony zoo of what looks like marine animals. Some of these are readily recognized as resembling sponges, corals, bivalves (mussels and clams and such), snails, sea urchins, brittle stars, lamp shells and other types of creatures commonly found along the sea shore. Others, more rarely found, look like parts of fishes or reptiles (teeth, vertebrae). Very lucky collectors have found entire fish, diving reptiles (ichthyosaurs) and horseshoe crabs, even something that looks like a cross between a bird and a reptile.

But the most common type of fossil, from the bottom to the top of the sequence, are coin- to fist-sized spiral-shaped objects resembling a tightly wound ram's horn. The locals, for centuries, called these objects "shtah shnegli" which means "stone snails". Geologists call them "ammonites", after the ram's horns of the Egyptian god Amun, spelled Ammon by the Greeks. (As an aside: the word ammonia and its derivatives has the same origin: ammonium salts were originally obtained from horns of sheep and deer.) Another common fossil is a cigar-shaped item, broken on one end and pointed on the other, which makes it look like the armed end of a spear. People once referred to this as a thunder bolt, with a view to the activities of the thunder god Thor. The "bolts" are commonly seen in ravines along unpaved farm roads throughout the landscapes of the lower Jurassic, especially after rainstorms when the dust and dirt has been washed off. The geologists call them "belemnites". Ammonites and belemnites are now known to be remains of cephalopods, that is, mollusks with tentacles and a relatively large brain (the largest among invertebrates). Both ammonites and belemnites are extinct; living relatives are the chambered Nautilus and the cuttlefish Sepia. The ornamentation on the ammonite shells changes subtly from one layer to the next, as one explores the sequence of the Jurassic deposits, from the black shales at the bottom, through the brown sandstones in the middle, up to the white limestone layers at the top. The changes in morphology are recognizable from one quarry to another, and even all over western Europe.

Belemnite fossil from the late Jurassic Period, approximately 200 million years old. (Courtesy: British Geological Survey)

The subtle differences in the morphology of ammonites, then, allow the stratigraphic correlation of equivalent layers over long distances. (They do not allow such correlation into the Jurassic of the Colorado Plateau, because here we have Navajo Sandstone, which is a deposit made of dune sand in a terrestrial environment. No marine animals, no ammonites.)

The nature of fossils as the altered remains of once living organisms is said to have been recognized by certain Greek philosophers. If so, there was apparently little effect on Aristotle's thinking. Also, any such insights did not inform the world view of the leading 16th century naturalists such as the German mining engineer Georg Bauer "Agricola" (1494-1555) and the Swiss physician Conrad Gesner (1516-1565), both of whom published on the subject.

Gesner's volume was the first well-illustrated book on fossils. For Gesner and his contemporaries, fossils were strange objects formed within a vigorous Earth, delightful and marvelous manifestations of God's creativity, and with powers worth exploring and putting to use. (Such powers are ascribed to gems, even today, by popular superstitions, with links to astrology.)

Only a hundred years later was the close resemblance between certain fossils and living organisms recognized as demonstrating that there is a causal connection.

The Danish naturalist Nicolaus Steno (aka: Niels Stensen, 1636-1686), while working at the Experimental Academy in Florence, Italy, received an enormous shark's head for dissection from a catch by local fishermen. He realized the great similarity of the shark's teeth with the much larger fossils called "tongue-stones" that he had seen in Denmark. He decided that these fossils, "Glossopetrae", were the teeth of a super-size shark. Steno published these findings in 1667.

Two years earlier, the English physicist Robert Hooke (1635-1703) had already drawn attention to the fact that the fine structures of charcoal and of fossilized wood, when observed through a microscope, have much in common, and suggested that buried wood turns to stone through "petrifying water", that is, precipitation of mineral matter from a solution. This is today the accepted view.

Ammonite fossil from the late Jurassic Period, approximately 200 million years old. (Courtesy: British Geological Survey)

At the time, both Hooke and Steno were fighting the commonly accepted concept of a vital force suffusing the Earth and responsible for making fossils within the rock. It took till the end of the 17th century to lay that to rest. In the last years of that century a distinguished naturalist (a member of the Royal Society), published a treatise on fossils denying their organic origin. He found it confusing that many fossils had no known living representative and that they differed from one rock layer to another. Accepting the views of Hooke and Steno implied that there had been extinction, and that the world had not been created perfect. Not a likely scenario in the world view then prevailing.

Once it was understood that fossils represented vanished organisms, they could be used to reconstruct history. The first history so reconstructed was a Great Flood (inspired by Biblical testimony) that washed marine organisms up onto land, where they could turn into fossils. Steno liked the idea, but he went further. He also suggested that the organic remains had been deposited within regular horizontal beds, in a normal marine environment. Thus, their sequence describes the history of life in the sea. Later, the strata were deformed and up-lifted through mountain-building processes. This is one reason we find fossils high up in the mountains.