PALAEONTOLOGY, the science which deals with the re mains of animals and plants found buried in the rocks. For con venience it is now customary to use the term only for the study of fossil animals, and to deal with plants under the head of palaeobotany (q.v.). In essence however, the two form one science distinguished from zoology and botany by the fact that the organisms with which it deals are not all of the same age, but cover, incompletely it is true, the history of life in the world from a time not long after its appearance, when organisms capable of being preserved first arose.
The preservation of any animal as a fossil depends on its burial in a sediment which is not subsequently destroyed by denudation.
In the sea, the remains of marine animals are continually ac cumulating on the sea-bottom. They may lie in the place where they fall or, if exposed to the action of currents, be carried for considerable distances and when the speed of the current becomes insufficient to move them, be laid down, often in association with many other objects of the same area and weight. Thus it is common to find great accumulations of fossil shells all of approxi mately the same size, which are not necessarily in the place where they actually lived. On the other hand shells of burrowing mol luscs may be found as they were in life. The remains of animals even if of such relatively permanent structure as shells are soon destroyed if they lie for long exposed to the action of the sea, and they can only be preserved if they are deposited at a place where mud or sand is being laid down. Such districts are comparatively rare and of local occurrence. They may be found chiefly where the sea-bottom off a coast is sinking. Certain special areas such as coral reefs provide exceptionally favourable condi tions for the preservation of marine fossils.
On land the possibilities of preservation are much smaller. The remains of a land fauna are chiefly preserved either in the sedi ments which form on the bottom of lakes, or perhaps more com monly in arid areas where the general level of the land-surface is rising, because of the distribution of dust and sand over it by wind action. Under these circumstances the skeleton of a dead animal may become covered by a pile of blown sand, and may, if it escape the action of floods and wind, remain buried for geological periods. One exceptional method of preservation, more important in the case of plants than animals, is in the detritus of forests growing on a coastal plain at or below high-water mark, as do mangroves to-day. If this plain be situated in a sinking area, great masses of deposits (now represented by coal-fields) may be built up, and in them animals and plants will be abun dantly preserved.
The nature of the preservation of an individual fossil depends greatly on its own chemical and physical composition, and on that of the surrounding sediments. The calcareous shells of mol luscs and brachiopods, if buried in a mud which becomes con verted into a clay, may be preserved indefinitely in their original form, because such rocks hinder the passage of water, which, when freely allowed, will either dissolve the shell completely or bring about a re-arrangement, often into crystals, of the molecules of which it is composed. In some cases such clays, especially if they become impregnated with petroleum, may preserve traces of the soft parts of the animal, in the form of a carbonaceous film made from its material exactly as coal has arisen from plant materials.
The most remarkable of such fossils are those from the Middle Cambrian of Burgess pass, British Columbia, described by C. D. Walcott. Here the external form even of slender soft processes of worms has been perfectly preserved and even something of their internal anatomy can be made out from colour differences in the carbonaceous films which represent them.