Geological Palaeontology

Palaeogeography.

In addition to their value as time markers, fossils may throw light on the geographical conditions under which rocks were laid down, and on the palaeogeography of the world as a whole. Certain groups of animals are now, and, so far as our knowledge extends, always have been, restricted to salt water ; no echinoderm, for example, is ever found in rivers or lakes. Although a few forms (none capable of preservation as fossils) do occur in fresh water, the Coelenterata, as a group, are marine, and the occurrence of a coral is almost conclusive evidence that the rocks in which it was found were laid down under the sea. Brachiopods are equally marine, but Lingula and presumably some of its allies extend their range into brackish, though not into fresh water. In most cases, however, a phylum may have repre sentatives both in the sea and in fresh water, but even in these cases it will usually be found that many of its larger divisions are restricted to one or the other habitat. Thus the Mollusca have four great groups, the Solenogastres, Polyplacophora, Scaphopoda and the Cephalopoda, all of whose members are marine, whilst the other two, the Gastropoda and the Lamellibranchia, occur both in fresh and salt water, the former group also inhabiting the land.

The actual structure of the hard parts of an animal may indi cate whether it lived on land or in water, but can never present any direct evidence on the problem of the nature of the water in which it lived. Thus the whole geological use of a fossil as an indicator of the marine or fresh water origin of the rocks in which it is found depends on the accuracy of the determination of its affinities primarily with living forms whose habitat is known, and secondarily on its association with other organisms which are shown to have been marine by independent lines of evidence. But it is possible from the structure of a shell to determine some thing of its conditions of life. For example, those Mollusca which live between tide-marks on the sea-shore have to resist the full force of wave-action, and can only do so if they bury themselves in sand, lie concealed in cracks in rocks, or, like Purpura, have a shell so thick and massive as to stand hard blows. Forms like limpets, with their conical shell held down tightly to a fixed rock surface, escape displacement and destruction because of their shape. Lamellibranchs depend for the whole of their f ood supplies on suspended particles in water-current which is caused to pass over their "gills" by ciliary action. Thus a lamellibranch which lives buried in sand, must maintain contact with the sea water by a double tube long enough to stand out above the floor of the sea. The former presence of this great siphon is usually shown by a gaping hole left at the posterior end when the two valves of the shell are closed. By this means the habitat of cer

tain forms may be discovered from their structure.

Such organisms as live in deep water are not exposed to the risk of mechanical damage by currents. Their shells need be no more massive than is necessary for the maintenance of the shape of the animal, and hence as a whole deep-sea echinoids, lamelli branchs and brachiopods are characterized by the excessive tenuity of their skeletons sometimes so thin that the animal can scarcely support its own weight in air.

Fossils and Climate.

Appeal is often made to fossils in dis cussions as to the temperature under which certain formations have been deposited. For example, it is stated that such an extensive development of corals as occurs in the Coral Rag of Mid-Jurassic age, implies that this sea was at that time tropical. This con clusion is based entirely on the fact that at the present day coral reefs are only found in the tropics or some few hundred miles on each side thereof. This rests on an observational basis and there is nothing in the structure of the coral individuals to explain it. The coral Lophohelia, which forms great branching colonies two feet in height, lives in the cold deep waters of the Norwegian fjords, and off the coasts of Scotland. This animal and others which occur with it, differ in no consistent respects from allied tropical forms. Thus the mere occurrence of corals provides very unsubstantial evidence on which to found an interpretation of the temperature of past seas. If this doubt be present in Jurassic times then it is quite clear that abundance of corals in Palaeozoic rocks affords no evidence whatsoever as to the temperature, especially as the phylogenetic relationships between Palaeozoic and Mesozoic corals are quite uncertain. In the case of rocks of Pleistocene and late-Tertiary age in which the remains of living species are found, however, it is possible to use their evidence in following the course of temperature changes during the period of their deposition.

Most living species of marine animals have a distribution which is limited by temperature. Such fish as the haddock can live and breed only in waters whose upper and lower temperature limits are strictly defined, and it is reasonable to suppose that this dis ability has existed from the time of their origin. Thus the dis covery of such an animal as Haliotis (whose present northern limit is the Channel Islands) in late Tertiary rocks in England, would imply that there has since been a fall in temperature in the sea surrounding the British Isles. It is, however, clear that such evi dence must be used with caution and that it becomes more fragile the further back the species be traced.