Palaeontology

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PAOEONTOLOGY. B Y ERNEST A . ELLIOTT, F . Z . S . , F . E . S . , & c .

subject (palaios, ancient ; onta, beings ; logos, study) is the science which treats of all sentient beings, vegetable and animal, that have inhabited our globe during any past period.

It has been incorrectly termed the " science of the rocks," for that properly applies solely to Geology. The word ' rock ' in this connection includes all materials composing the crust of the Earth; gravel, sand and mud are thus just as much ' rock ' as the actual hard masses from which they have been derived.

No complete record of these ancient biological forms can ever be arrived at, since the many changes in the crust of the Earth have destroyed much, and many are buried at depths inaccessible to us. What we do know is derived from Fossils (Lat. fossus, dug up), which may be either actual portions of animal or vegetable organisms, such as shells, corals, bones, wood, bark or leaves; and generally there is included in the term " any body, or the traces of the existence of any body, whether animal or vegetable, which has been buried in the earth by natural causes " (Lyell). Thus we have to include casts or moulds of shells and footprints of various animals in sand or mud. Another form of Fossil is where the original organic body has been replaced by some mineral substance ; in some such cases, not only the external form but, the internal delicate structure is retained. A familiar example is the fossil or ' silicified' wood, wberein the microscope reveals the minutest vessels characterizing the original ligneous tissue ; such wood must have slowly decayed in water holding in Solution silica, or flint ; as each particle decayed, it was replaced by a particle of flint tili ultimately the whole became silicified. Many other substances may take the place of silica, such as lime or iron pyrites which is very common in wood in the Gault, oxide of iron, sulphur, malachite, managnese, talc, &c.

One of the commonest Fossil forms is where lime is replaced by silica, as in shells and corals ; the more soluble carbonate of lime is dissolved and replaced by the barely soluble flint.

The reverse appears to have obtained in certain Sponges that have a siliceous skeleton, and this has been partly or wholly replaced by lime ; it has been found, too, that the portions remaining siliceous retain the original structure, whereas the other part is composed of crystaline calcite, devoid of other structure.

For the purpose of Palaeontology all rocks may be divided into two great classes :—(1) Igneous Rocks which are formed within the earth by the action of heat, effectually destroying all trace of animal and vegetable life ; and (2) sedimentary or THIS

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Aqueous Rocks formed at the surface of the Earth, and owing their structure to the action of water ; this second section m a y be again divided into divisions that are Mechanically formed and Chemically formed.

Mechanically-formed Rocks are those of which we have proof that their particles have been mechanically transpiorted to their present site ; they are all derived from the abrasion of pre-existing Rocks, and bence are often called ' Derivative.'

They are cbiefly Arenaceous, consisting of grains of flint or silica such as sand or sandstone, grit and most conglomerates ; or eise Argillaceous, containing clay or hydrated Silicate of alumina ; here belong clays, shales, marls and most sorts of flag-stones.

On the other hand, Chemically-formed Rocks have been erected by chemical agencies, most frequently through the medium of living animals or plants ; of first importance among these is the CHALK. Carbonate of lime is soluble in water holding in Solution a certain amount of carbonic acid gas, and is present in all natural waters. Many aquatic animals and some aquatic plants have the power of separating the lime thus held in Solution : in this w a y sbeH-fish, crustaceans and other creatures, build up their exoskeletons. Chalk is composed of innumerable hard parts and shells of marine animals, some whole and some fragmentary, e.g. Foraminifera and Globigerina. This chalk bas been deposited at tbe bottom of a sea; and similar formation is found below the Atlantic and Pacific oceans in the Constitution of " Atlantic Ooze," a kind of sticky mud which, when dried, strongly resembles greyisb chalk : in such mud live and die numerous sponges, sea-lilies and other small things, whose harder parts are preserved to us in chalk as fossils.

LIMESTONE, also, is essentially of organic origin ; some limestones consist entirely of the skeletons, more or less broken, of certain marine animals cemented together b y carbonate of lime, as in the case of the ' Crinoidal limestones ' and the wellknown ' Encrinite m a r b l e ' ; others again are totally built up of the skeletons of Corals, and such can be paralleled b y the accretions still in course of formation into ' Coral reefs.' The ' Nummulitic limestone ' is a mass, sometimes attaining a thickness of thousands of feet, entirely made up of shells of Foraminifera, varying in size from that of a small pea to that of a florin ; and some of tbe calcareous Algce, e.g. the Corallines and Nullipores, m a y form extensive accumulations of lime like that of the ' Leitha-Kalk ' of the Tertiary series. Phosphate of lime occurs in the bones of vertebrate animals and, to a less extent, in the Invertebrata; as is found in the Lingula among brachiopod Shell-fish, Conularia and Theca among pteropod Shell-fish, and very generally among Crustacea. Some of the nodules worked for agricultural purposes from the Upper

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Greensand may be organisms, such as Sponges that are infiltrated with phosphate of lime; and all are probably of organic origin.

F L I N T or silex is found in minute quäntities dissolved in seawater, and certain plants and animals construct from it their flinty skeletons : thus the microscopic Polycystina, closely related to Foraminifera, differ from tbem chiefly in forming their shells from flint in place of lime. Diatoms are minute and mostly fresh-water creatures, secreting a flinty skeleton : the ' tripoli ' of Bohemia, largely used for polishing purposes, and the ' infusorial earth ' of Virginia are composed entirely of Diatoms, forming in some places beds of thirty feet in thickness. Flints themselves are largely formed by infiltration of, or by imbedding, organic creatures ; some are fßll of Foraminifera, others mainly consist of Sponges or Sea-urchins.

Many rock deposits comprise CARBON in varying quäntities, and most of this is certainly organic. Silurian shales, containing Graptolites but no plants, have a small amount of carbon, derived from the decay of the Zoophytes ; but the ' bituminous shales ' of Caithness are oily from the decomposition of the very numerous Fish imbedded in them. The great bulk of carbon comes from plants ; it may occur as anthracite, coal, or lignite which is also termed ' brown coal,' wherein the wood retains more of its original form and renders it coal-in-themaking. Still older is G R A P H I T E or black-lead, found as far back as in the Laurentian formation ; while proof of its organic origin is impossible because all trace of structure has become obliterated, there is little doubt it is an altered coal. To the Coal Measures, as they are called, we are indebted for a large quantity of plant remains, as well as less frequent fossilised animals.

We will now briefly glance at such creatures as we may expect to encounter in a fossil State.

Among P R O T O Z O A the Foraminifera, Radiolaria and Spongida develop structures of lime or flint, which have been abundantly preserved ; but the Gregarinidce and infusorial animalculas, the Monera and Amceba possess, with few exceptions, no hard parts to leave a trace. Among CCELENTERATA the fresh-water Polvpes, oceanic Hydrozoa, jelly-fishes, sea-anemones and the Ctenophora are destitute of secreted lime ; so also are the Lucernarida, yet impressions left by stranded bodies of these animals have been detected in the Solenhofen lithographic slate. Corals and most Hydroid Zoophytes have hard parts capable of preservation, and the former are extremely abundant in a fossil state ; as also are the long-extinct Graptolites. In A N N U L O I D A tape-worms such as Scolecida, being entirely soft,

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have not been preserved and we have no geological evidence of their existence in former times, though it seems very probable.

H O L O T H U R I D J E possess only a miminum of calcareous matter, so traces of them, also, are few ; but on the contrary the tough E C H I N O D E R M A T A are very completely fossilised. Of A N N U L O S A such lower forms as Leeches and Earth-worms comprise no hard structure, but the latter have left traces in the form of their burrows or tracks ; and the ' tubicolar ' Annelides are known by the preservation of their tubes. The terrestrial classes of M Y R I A P O D A , A R A C H N I D A and Insects have their chitinous exoskeletons preserved but partially, usually when either accidentally drowned or imbedded in vegetable remains, as in amber : CRUSTACEANS, baving a stout outer covering, are fairly abundant. Among the MOLLUSCA we find the Tunicaries, sea-slugs and Nudibranchiata destitute of hard parts, and thence unknown to palseontologists ; but shells of Mollusca are the most abundant fossils of all, though those of land species are not often met with and then usually in the case of specimens that have been blown or fallen into water.

Among V E R T E B R A T A , again, we note the preponderance of aquatic or amphibious creatures ; Fishes have been found as far back as the Upper Silurian rocks ; Amphibians began before the Carboniferous period, wherein they reached their greatest development; true Reptiles first appear in the Permian deposits, and the so-called ' age of Reptiles' is Mesozoic. Footmarks, apparently left by Birds, are found in the Trias, but the earliest undoubted bird-fossils occur in Jurassic strata ; and the London Clay has yielded some of these creatures. On the other hand, among Mammalian fossils the majority are terrestrial, and comparatively scarce. It is to be expected that they, being the most highly organised products, would make their appearance at a late period ; and the earliest known comes from the Upper Trias : Microlestes antiquus, a form apparently allied to the Marsupiais. Some occur in the lower Oรถlite, more in the Middle Purbeck, but then comes a gap, for none are yet discovered in Cretaceous strata ; though, from the base of Eocene rocks up to the present day, numerous Mammalian remains occur in all formations.

Palseobotany shows us that traces of Algce turn up as far back as the Lower Silurian strata ; Ferns begin in the middle of Silurian times, and forms that hardly differ from those now existing are found in the Devonian rocks ; while Equisitacece and Club-mosses first appear far down in the Palseozoic period.

Sigillarice, that are thought to be coniferous, occur also in the Devonian rocks ; in the Carboniferous ones we find other Sigillaria, ferns, Lepidodendron and the calamites, along with a good representation of coniferae, a few flowering-plants and.

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from the Scotch beds, Pothocites. Both ferns and calamites continue in the Permian rocks, but here the Sigillaroids have disappeared ; in the Trias ferns and conifers are abundant, as also are Cycadaceous plants ; and Calamites are now represented by true Equisitites.

The Jurassic and Lower Cretaceous deposits yield Cycads, ferns and conifers, of which the first is predominant; and in the Upper Cretaceous we have the earliest appearance in any quantity of the Angiospermous Exogens, that is, plants having their seeds in a seed-vessel : here, too, are the Oak, Birch and many other familiar genera of trees.

Hence we see that, roughly speaking, the Palasozoic period may be called the " Age of Acrigens " ; the Secondary, including Lower Cretaceous, the " Age of Gymnosperms " ; and the Upper Cretaceous, with Tertiary times, the " Age of Angiosperms."

It then follows that Palaeontology introduces us to the kindred sciences of Geology, Mineralogy, Zoology and Botany.

We are enabled to trace the gradual development of vegetable and animal forms, from the lowest to the most highly organized types, though necessarily there must be many gaps in the chain, which will never be completely filled.