Palaeontology

When buried in sand a shell is exposed to the action of per colating waters which if acid are capable of dissolving it. Its preservation is therefore dependent either on the sealing down of the sand by such a layer of clay as will prevent the circulation of water, or on a solidification of the sand by a cementing together of the individual grains, usually by calcium carbonate, which happens sufficiently rapidly to form a solid case round the shell, which may then persist if the percolating water be completely saturated with calcium carbonate, altered only by the laying down of further calcite in its pores. In many cases however the shell itself is entirely removed in solution, and its former presence is shown by a cavity in the sandstone, which reproduces the surfaces of the shell in negative (often with complete perfection). When buried in a calcareous mud such as that which ac cumulates in the lagoon behind a coral reef, a shell often remains unaltered, and in a limestone which has been formed from such a mud, may be found in its original form, or changed only by the addition of more calcite. Under exceptional circumstances the material of an animal's skeleton may be removed and replaced by silica, sometimes as opal, by iron pyrites or other minerals.

Preparation of Fossils.

Before it can be studied, any fossil must be freed as completely as possible from the surrounding matrix. The technique of this operation varies in each individual case; bones and shells, preserved in clays, can often be entirely cleaned by washing. The chitinous skeletons of arthropods and graptolites are sometimes preserved unaltered in shales, sand stones or limestones, and can then be prepared by removing the rock with hydrofluoric or hydrochloric acid. The resulting prep aration can then in some cases be stained and mounted or cut into sections exactly as if it had formed part of a recent animal. Generally however the rock surrounding the fossil must be re moved mechanically by needles or chisels and hammers. Where the fossil is represented by a cavity whose walls preserve a mould of its surfaces, it is necessary to make casts in plaster of Paris or more usually in wax, plasticene or glue. For small complex objects such as the skull of a fish, it is often preferable to remove the bone and rely on such casts, rather than to attempt to pick away the stone and preserve the actual bone substance. Some times however both rock and bone are too hard to be attacked by steel tools, and too similar chemically to be prepared by solution in acid. In these cases it is possible to discover the struc ture by a method invented by W. J. Sollas. By this technique one surface of the specimen is ground to a flat face on which the fossil is shown in section. An enlarged photograph is then made of this ground surface. A layer of rock of a definite thickness is then removed by further grinding and another photograph is taken, and the process is repeated until the whole of the fossil is ground away, but the appearance of the sections made across it at definite distances apart is preserved in the series of photographs. Each section is reproduced by being cut out of a sheet of wax of appropriate thickness and these wax-plates when piled up in correct order reproduce perfectly the shape of the original fossil at any magnification required. This method, though extremely laborious, leads to results which can be attained in no other way. In some cases it may be necessary to make a thousand sections, photographs and wax-plates of a single object.

The preparation of the skeleton of a large vertebrate is usually carried out with a hammer and chisel, the rock being carved away from the bones until these are free : they can then be mounted so as to form a skeleton exactly as are the bones of a recent animal.

History.

The fossil remains of animals have long attracted attention. In a Neolithic grave at Dunstable, Beds., a skeleton was found surrounded by fossil echinoderms from the chalk, and a famous silicified tree-trunk acted as a pedestal in an Etruscan tomb. Fossil shells were repeatedly observed during mediaeval times and are referred to in such works as Agricola, "De Natura Fossilium," 1558. During the i8th century many works containing illustrations of fossils were published and some attempt was made to understand their structure and compare them with living forms. In one case a skeleton of Megatherium was actually set up in what was supposed to be its natural position in the museum at Madrid. The whole conception of palaeontology was, however, changed by the work of two men.

The first of these was William Smith (q.v.), a land-surveyor, who in 1791 when supervising the construction of the Kennet and Avon canal, observed that each of the well-marked formations represented in the Jurassic rocks in the neighbourhood of Bath, was characterized by the presence of fossils which could not be found in the beds above or below. Smith was able to show that it was possible by the aid of the contained fossils to trace the formations he had recognised near Bath, across the Midlands of England to the Yorkshire coast. These observations form the basis of stratigraphical geology and showed that there had been a succession of different faunas living in the English region during geological time.

Secondly, Georges, Baron Cuvier (q.v.) applied to the skeletons of fossil vertebrates the methods of study already developed for recent animals. In his great work Recherches sur les Ossements Fossiles, he gave most accurate and detailed accounts of the teeth and bones of fossil mammals and reptiles, endeavouring to build up by a comparison of many specimens the complete skeleton of a number of forms and to determine their systematic position in the animal kingdom by the methods of comparative anatomy.

Finally, from a study of the skeleton he attempted to infer some thing of the habits of the animals with which he dealt. His work was of the first importance because it showed that many fossil animals were of types that no longer exist. Previously when only extinct invertebrates had been studied, it was always possible to say that they still lived and would one day be found when the seas of the world had been completely explored. When not one but very many large extinct animals had been shown to have existed in France and elsewhere, this possibility became less plausible; even in 1824 the world had been so well exploited that it was most improbable that many large animals remained un known to zoologists. Indeed only two completely new types have since been discovered, the great panda and the okapi.

From William Smith and Georges Cuvier two different lines of palaeontological work have arisen. Until comparatively recently invertebrate fossils have chiefly been studied by men whose inter ests lay in stratigraphical geology and who used them purely as time-markers, just as potsherds are used by archaeologists. The type of work initiated by Cuvier has been carried on chiefly by zoologists and anatomists and has been concerned in the main with vertebrates.