The Story of the Earth and Man

by Sir John William Dawson

Of the fishes of the Mesozoic we need only say that they were very abundant, and consisted of sharks and ganoids of various types, until near the close of the period, when the ordinary horny-scaled fishes, such as abound in our present seas, appear to have been introduced. One curious point of difference is that the unequally lobed tail of the Palæozoic fishes is dropped in the case of the greater part of the ganoids, and replaced by the squarely-cut tail prevalent in modern times.

In the sub-kingdom of the Mollusca many important revolutions occurred. Among the lamp-shells a little Leptaena, no bigger than a pea, is the last and depauperated representative of a great Palæozoic family. Another, that of the Spirifers, still shows a few species in the Lower Mesozoic. Others, like Rhynchonella, and Terebratula, continue through the period, and extend into the Modern. Passing over the ordinary bivalves and sea-snails, which in the main conform to those of our own time, we find perhaps the most wonderful changes among the relatives of the cuttle-fishes and Nautili. As far back as the Silurian we find the giant Orthoceratites contemporary with Nautili, very like those of the present ocean. With the close of the Palæozoic, however, the Orthoceratites and their allies disappear, while the Nautili continue, and are reinforced by multitudes of new forms of spiral chambered shells, some of them more wonderful and beautiful than any of those which either preceded or followed them. Supreme among these is the great group of the Ammonites,—beautifully spiral shells, thin and pearly like the Nautilus, and chambered like it, so as to serve as a float, but far more elaborately constructed, inasmuch as the chambers were not simply curved, but crimped and convoluted, so as to give the outer wall much more effectual support. This outer wall, too, was worked into ornamental ribs and bands, which not only gave it exquisite beauty, but contributed to combine strength to resist pressure with the lightness necessary to a float. In some of these points it is true the Gyroceras and Goniatites of the Palæozoic partially anticipated them, but much less perfectly. The animals which inhabited these shells must have been similar to that of Nautilus, but somewhat different in the proportion of parts. They must have had the same power of rising and sinking in the water, but the mechanical construction of their shells was so much more perfect relatively to this end, that they were probably more active and locomotive than the Nautili. They must have swarmed in the Mesozoic seas, some beds of limestone and shale being filled with them; and as many as eight hundred species of this family are believed to be known, including, however, such forms as the Baculites or straight Ammonites, bearing to them perhaps a relation similar to that of Orthoceras to Nautilus. Further, some of the Ammonites are of gigantic size, one species being three feet in diameter, while others are very minute. The whole family of Ammonitids, which begins to be in force in the Trias, disappears at the end of the Mesozoic, so that this may be called the special age of Ammonites as well as of reptiles.

Further, this time was likewise distinguished by the introduction of true cuttle-fishes, the most remarkable of which were those furnished with the internal supports or “bones,” known as Belemnites, from a fancied resemblance to javelins or thunder-bolts, a comparison at least as baseless as that often made in England of the Ammonites to fossil snakes. The shell of the Belemnite is a most curious structure. Its usual general shape is a pointed cylinder or elongated cone. At top it has a deep cavity for the reception of certain of the viscera of the animal. Below this is a conical series of chambers, the Phragmacone; and the lower half of the shell is composed of a solid shelly mass or guard, which, in its structure of radiating fibres and concentric layers, resembles a stalactite, or a petrified piece of exogenous wood. This structure was an internal shell or support like those of the modern cuttle-fishes; but it is difficult to account for its peculiarities, so much more complex than in any existing species. The most rational supposition seems to be that it was intended to serve the triple purpose of a support, a float, and a sinker. Unlike the shell of a Nautilus, if thrown into the water it would no doubt have, sunk, and with the pointed end first. Consequently, it was not a float simply, but a float and sinker combined, and its effect must have been to keep the animal at the bottom, with its head upward. The Belemnite was therefore an exceptional cuttle-fish, intended to stand erect on the sea-bottom and probably to dart upward in search of its prey; for the suckers and hooks with which its arms were furnished show that, like other cuttle-fishes, it was carnivorous and predaceous. The guard may have been less ponderous when recent than in the fossil specimens, and in some species it was of small size or slender, and in others it was hollow. Possibly, also, the soft tissues of the animal were not dense, and it may have had swimming fins at the sides. In any case they must have been active creatures, and no doubt could dart backward by expelling water from their gill chamber, while we know that they had ink-bags, provided with that wonderfully divided pigment, inimitable by art, with which the modern Sepia darkens the water to shelter itself from its enemies. The Belemnites must have swarmed in the Mesozoic seas; and as squids and cuttles now afford choice morsels to the larger fishes, so did the Belemnites in their day. There is evidence that even the great sea-lizards did not disdain to feed on them. We can imagine a great shoal of these creatures darting up and down, seizing with their ten hooked arms their finny or crustacean prey. In an instant a great fish or saurian darts down among them; they blacken the water with a thick cloud of inky secretion and disperse on all sides, while their enemy, blindly seizing a few mouthfuls, returns sullenly to the surface. A great number of species of Belemnites and allied animals have been described; but it is probable that in naming them too little regard has been paid to distinctions of age and sex. The Belemnites were for the most part small creatures; but there is evidence that there existed with them some larger and more formidable cuttles; and it is worthy of note that, in several of these, the arms, as in the Belemnites, were furnished with hooks as well as suckers, an exceptional arrangement in their modern allies. It is probable that while the four-gilled or shell-bearing cuttles culminated in size and perfection in the Ammonitids of the Mesozoic, the modern cuttles of the two-gilled and shell-less type are grander in dimensions than their Mesozoic predecessors. It is, however, not a little singular that a group so peculiar and apparently so well provided with means, both of offence and defence, as the Belemnites, should come in and go out with the Mesozoic, and that the Nautiloid group, after attaining to the magnitude and complexity of the great Ammonites, should retreat to a few species of diminutive and simply-constructed Nautili; and in doing so should return to one of the old types dating as far back as the older Palæozoic, and continuing unchanged through all the intervening time.

The Crustaceans of the Mesozoic had lost all the antique peculiarities of the older time, and had so much of the aspect of those of the present day, that an ordinary observer, if he could be shown a quantity of Jurassic or Cretaceous crabs, lobsters, and shrimps, would not readily recognise the difference, which did not exceed what occurs in distant geographical regions in the present day. The same remark may be made as to the corals of the Mesozoic; and with some limitations, as to the star-fishes and sea-urchins, which latter are especially numerous and varied in the Cretaceous age. In short, all the invertebrate forms of life, and the fishes and reptiles among the vertebrates, had already attained their maximum elevation in the Mesozoic; and some of them have subsequently sunk considerably in absolute as well as relative importance.

In the course of the Mesozoic, as indicated in the last chapter, there had been several great depressions and re-elevations of the Continental Areas. But these had been of the same quiet and partial character with those of the Palæozoic, and it was not until the close of the Mesozoic time, in the Cretaceous age, that a great and exceptional subsidence involved for a long period the areas of our present continents in a submergence wider and deeper than any that had previously occurred since the dry land first rose out of the waters.

Every one knows the great chalk beds which appear in the south of England, and which have given its name to the latest age of the Mesozoic. This great deposit of light-coloured and usually soft calcareous matter attains in some places to the enormous thickness of 1,000 feet. Nor is it limited in extent. According to Lyell, its European distribution is from Ireland to the Crimea, a distance of 1,140 geographical miles; and from the south of France to Sweden, a distance of 840 geographical miles. Similar rocks, though not in all cases of the precise nature of chalk, occur extensively in Asia and in Africa, and also in North and South America.

But what is chalk? It was, though one of the most familiar, one of the most inscrutable of rocks, until the microscope revealed its structure. The softer varieties, gently grated or kneaded down in water, or the harder varieties cut in thin slices, show a congeries of microscopic chambered shells belonging to the humble and simple group of Protozoa. These shells and their fragments constitute the material of the ordinary chalk. With these are numerous spicules of sponges and silicious cell-walls of the minute one-celled plants called Diatoms. Further, the flinty matter of these organisms has by the law of molecular attraction been collected into concretions, which are the flints of the chalk. Such a rock is necessarily oceanic; but more than this, it is abyssal. Laborious dredging has shown that similar matter is now being formed only in the deep bed of the ocean, whither no sand or mud is drifted from the land, and where the countless hosts of microscopic shell-bearing protozoa continually drop their little skeletons on the bottom, slowly accumulating a chalky mud or slime. That such a rock should occur over vast areas of the continental plateaus, that both in Europe and America it should be found to cover the tops of hills several thousand feet high, and that its thickness should amount to several hundreds of feet, are facts which evidence a revolution more stupendous perhaps than that at the close of the Palæozoic. For the first time since the Laurentian, the great continental plateaus changed places with the abysses of the ocean, and the successors of the Laurentian Eozoon again reigned on surfaces which through the whole lapse of Palæozoic and Mesozoic time had been separated more or less from that deep ocean out of which they rose at first. This great Cretaceous subsidence was different from the disturbances of the Permian age. There was at first no crumpling of the crust, but merely a slow and long-continued sinking of the land areas, followed, however, by crumpling of the most stupendous character, which led at the close of the Cretaceous and in the earlier Tertiary to the formation of what are now the greatest mountain chains in the world. As examples may be mentioned the Himalaya, the Andes, and the Alps, on all which the deep-sea beds of the Cretaceous are seen at great elevations. In Europe this depression was almost universal, only very limited areas remaining out of water. In America a large tract remained above water in the region of the Appalachians. This gives us some clue to the phenomena. The great Permian collapse led to the crumpling-up of the Appalachians and the Urals, and the older hills of Western Europe. The Cretaceous collapse led to the crumpling of the great N.W. and S.E. chain of the Rocky Mountains and Andes, and to that of the east and west chains of the south of Asia and Europe. The cause was probably in both cases the same; but the crust gave way in a different part, and owing to this there was a greater amount of submergence of our familiar continental plateaus in the Cretaceous than in the Permian.

Another remarkable indication of the nature of the Cretaceous subsidence, is the occurrence of beds filled with grains of the mineral Glauconite or “green-sand.” These grains are not properly sand, but little concretions, which form in the bottom of the deep sea, often filling and taking casts of the interior and fine tubes of Foraminiferal shells. Now this Glauconite, a hydrous silicate of iron and potash, is akin to similar materials found filling the pores of fossils in Silurian beds. It is also akin to the Serpentine filling the pores of Eozoon in the Laurentian. Such materials are formed only in the deeper parts of the ocean, and apparently most abundantly where currents of warm water are flowing at the surface, as in the area of the Gulf Stream. Thus, not only in the prevalence of Foraminifera, but in the formation of hydrous silicates, does the Cretaceous recall the Laurentian. Such materials had no doubt been forming, and such animals living in the ocean depths, all through the intervening ages, but with the exception of a few and merely local instances, we know nothing of them, till the great subsidence and re-elevation of the Cretaceous again allows them to ascend to the continental plateaus, and again introduces us to this branch of the world-making process.

The attention recently drawn to these facts by the researches of Dr. Carpenter and others, and especially the similarity in mineral character and organic remains of some of the deposits now forming in the Atlantic and those of the chalk, have caused it to be affirmed that in the bed of the Atlantic these conditions of life and deposit have continued from the Cretaceous up to the present time, or as it has been expressed, that “we are still living in the Cretaceous epoch.” Now, this is true or false just as we apply the statement. We have seen that the distinction between abyssal areas, continental oceanic plateaus, and land surfaces has extended through the whole lapse of geological time. In this broad sense we may be said to be still living in the Laurentian epoch. In other words, the whole plan of the earth’s development is one and the same, and each class of general condition once introduced is permanent somewhere. But in another important sense we are not living in the Cretaceous epoch; otherwise the present site of London would be a thousand fathoms deep in the ocean; the Ichthyosaurs and Ammonites would be disporting themselves in the water, and the huge Dinosaurs and strange Pterodactyls living on the land. The Italian peasant is still in many important points living in the period of the old Roman Empire. The Arab of the desert remains in the Patriarchal period, and there are some tribes not yet beyond the primitive age of stone. But the world moves, nevertheless, and the era of Victoria is not that of the Plantagenets or of Julius Cæsar. So while we may admit that certain of the conditions of the Cretaceous seas still prevail in the bed of the present ocean, we must maintain that nearly all else is changed, and that the very existence of the partial similarity is of itself the most conclusive proof of the general want of resemblance, and of the thorough character of the changes which have occurred.

The duration of the Cretaceous subsidence must have been very great. We do not know the rate at which the Foraminifera accumulate calcareous mud. In some places, where currents heap up their shells, they may be gathered rapidly; but on the average of the ocean bed, afoot of such material must indicate the lapse of ages very long when compared with those of modern history. We need not wonder, therefore, that while some forms of deep-sea Cretaceous life, especially of the lower grades, seem to have continued to our time, the inhabitants of the shallow waters and the land have perished; and that the Neozoic or Tertiary period introduces us to a new world of living beings. I say we need not wonder; yet there is no reason why we should expect this as a necessary consequence. As the Cretaceous deluge rose over the continents of the Mesozoic, the great sea saurians might have followed. Those of the land might have retreated to the tracts still remaining out of water, and when the dry land again appeared in the earlier Tertiary, they might again have replenished the earth, and we might thus have truly been living in the Reptilian age up to this day. But it was not so. The old world again perished, and the dawn of the Tertiary shows to us at once the dynasties of the Mammalian age, which was to culminate in the introduction of man. With the great Cretaceous subsidence the curtain falls upon the age of reptiles, and when it rises again, after the vast interval occupied in the deposition of the green-sand and chalk, the scene has entirely changed. There are new mountains and new plains, forests of different type, and animals such as no previous age had seen.