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The person who rendered this good service to palaeontology was Nicolas Steno, professor of anatomy in Florence, though a Dane by birth. Collectors of fossils at that day were familiar with certain bodies termed "glossopetrae," and speculation was rife as to their nature.

It seems to this noble author that the glossopetrae should be classed in the animal kingdom, because, being burnt, they are changed into cinders as bones, before they are reduced into a calx or ashes, whilst calcined stones are immediately reduced into a calx.

Add to this, that the substance is different in different parts of the glossopetrae; solid at the point, less solid at the root, compact at the surface, porous and fibrous in the interior: besides, the polished surface, contrary to the custom of nature, which forms no stone, whether common or precious, is polished; and, lastly, the figure that varies different ways, as well as the size, being found great, broad, triangular, narrow, small, very small, pyramidal, straight, curved before, behind, to the right and to the left, in form of a saw with small teeth, furnished with great jags or notches, and frequently absolutely pyramidal without notches; all these particulars favour his opinion.

In the first half of the seventeenth century, Fabio Colonna had tried to convince his colleagues of the famous Accademia dei Lincei that the glossopetrae were merely fossil sharks' teeth, but his arguments made no impression.

He further says, that the roots of the glossopetrae are often found broken in different ways, which is an evident argument that they have not been produced by nature, in the place they are digged out of, because nature forms other fossils, figured entirely in their matrix, without any hurt or mutilation.

There were callaides shot away from the mountains with slings, carbuncles formed by the urine of the lynx, glossopetrae which had fallen from the moon, tyanos, diamonds, sandastra, beryls, with the three kinds of rubies, the four kinds of sapphires, and the twelve kinds of emeralds. They gleamed like splashes of milk, blue icicles, and silver dust, and shed their light in sheets, rays, and stars.

Even that feat of palaeontology which has so powerfully impressed the popular imagination, the reconstruction of an extinct animal from a tooth or a bone, is based upon the simplest imaginable application of the logic of Steno. A moment's consideration will show, in fact, that Steno's conclusion that the glossopetrae are sharks' teeth implies the reconstruction of an animal from its tooth.

Fifty years later, Steno re-opened the question, and, by dissecting the head of a shark and pointing out the very exact correspondence of its teeth with the glossopetrae, left no rational doubt as to the origin of the latter.

Some are elliptic, but, for the greater part round: some represent an hemisphere, others a segment, others an hyperbola. The glossopetrae are naturally of a conic figure, representing acute, obtuse, regular, and irregular cones.

Hence, since the glossopetrae are altogether similar to sharks' teeth, they must have been produced by sharklike fishes; and since many fossil shells correspond, down to the minutest details of structure, with the shells of existing marine or freshwater animals, they must have been produced by similar animals; and the like reasoning is applied by Steno to the fossil bones of vertebrated animals, whether aquatic or terrestrial.