The Story of the Earth and Man

by Sir John William Dawson

The Lingulæ are especially interesting as examples of a type of beings continued almost from the dawn of life until now; for their shells, as they exist in the Primordial, are scarcely distinguishable from those of members of the genus which still live. While other tribes of animals have run through a great number of different forms, these little creatures remain the same. Another interesting point is a most curious chemical relation of the Lingula, with reference to the material of its shell. The shells of mollusks generally, and even of the ordinary lamp-shells, are hardened by common limestone or carbonate of lime: the rarer substance, phosphate of lime, is in general restricted to the formation of the bones of the higher animals. In the case of the latter, this relation depends apparently on the fact that the albuminous substances on which animals are chiefly nourished require for their formation the presence of phosphates in the plant. Hence the animal naturally obtains phosphate of lime or bone-earth with its food, and its system is related to this chemical fact in such wise that phosphate of lime is a most appropriate and suitable material for its teeth and bones. Now, in the case of the lower animals of the sea, their food, not being of the nature of the richer land plants, but consisting mainly of minute algæ and of animals which prey on these, furnishes, not phosphate of lime, but carbonate. An exception to this occurs in the case of certain animals of low grade, sponges, etc., which, feeding on minute plants with siliceous cell-walls, assimilate the flinty matter and form a siliceous skeleton. But this is an exception of downward tendency, in which these animals approach to plants of low grade. The exception in the case of Lingulaa is in the other direction. It gives to these humble creatures the same material for their hard parts which is usually restricted to animals of much higher rank. The purpose of this arrangement, whether in relation to the cause of the deviation from the ordinary rule or its utility to the animal itself, remains unknown. It has, however, been ascertained by Dr. Hunt, who first observed the fact in the case of the Primordial Lingulæ, that their modern successors coincide with them, and differ from their contemporaries among the mollusks in the same particular. This may seem a trifling matter, but it shows in this early period the origination of the difference still existing in the materials of which animals construct their skeletons, and also the wonderful persistence of the Lingulæ, through all the geological ages, in the material of their shells. This is the more remarkable, in connection with our own very slender acquaintance with the phenomenon, in relation either to its efficient or final causes.

Before leaving the Lingulæ, I may mention that Mr. Morse informs me that living specimens, when detached from their moorings, can creep like worms, leaving long furrows on the sand, and that they can also construct sand-tubes wherein to shelter themselves. This shows that some of the abundant “worm burrows” of the Primordial may have been the work of these curious little shell-fishes, as well as, perhaps, some of the markings which have been described under the name of Eophyton, and have been supposed, I think incorrectly, to be remains of land plants.

In addition to Lingula we may obtain, though rarely, lamp-shells of another type, that of the Orthids, These have the valves hinged along a straight line, in the middle of which is a notch for the peduncle, and the valves are often marked with ribs or striae. The Orthids were content with limestone for their shells, and apparently lived in the same circumstances with the Lingulæ; and in the period succeeding the Primordial they became far more abundant. Yet they perished at an early stage of the world’s progress, and have no representatives in the modern seas.

In many parts of the Primordial ocean the muddy bottom swarmed with crustaceans, relatives of our shrimps and lobsters, but of a form which differs so much from these modern shell-fishes that the question of their affinities has long been an unsettled onfi with zoologists. Hundreds of species are known, some almost microscopic in size, others a foot in length. All are provided with a broad flat horseshoe-shaped head-plate, which, judging from its form and a comparison with the modern king-crabs or horseshoe-crabs, must have been intended as a sort of mud-plough to enable them to excavate burrows or hide themselves in the slimy ooze of the ocean bed. On the sides of this buckler are placed the prominent eyes, furnished with many separate lenses, on precisely the same plan with those of modern crustaceans and insects, and testifying, as Buckland long ago pointed out, to the identity of the action of light in the ancient and the modern seas. The body was composed of numerous segments, each divided transversely into three lobes, whence they have received the name of Trilobites, and the whole articulated, so that the creature could roll itself into a ball, like the modern slaters or wood-lice, which are not very distant relatives of these old crustaceans.[F] The limbs of Trilobites were long unknown, and it was even doubted whether they had any; but recent discoveries have shown that they had a series of flat limbs useful both for swimming and creeping. The Trilobites, under many specific and generic forms, range from the Primordial to the Carboniferous rocks, but are altogether wanting in the more recent formations and in the modern seas. The Trilobites lived on muddy bottoms, and their remains are extremely abundant in shaly and slaty beds, though found also in limestone and sandstone. In the latter they have left most curious traces of their presence in the trails which they have produced. Some of the most ancient sandstones have their surfaces covered with rows of punctured impressions (Protichnites, first footprints), others have strange series of transverse grooves with longitudinal ones at the side (Climactichnites, ladder footprints); others are oval burrows, marked with transverse lines and a ridge along the middle (Rusichnites, wrinkle footprints). All of these so nearly resemble the trails and tracks of modern king-crabs that there can be little doubt as to their origin. Many curious striated grooves and bifid marks, found on the surfaces of Primordial beds, and which have been described as plants, are probably only the marks of the oral organs or feet of these and similar creatures, which passed their lives in grubbing for food in the soft, slimy ooze, though they could, no doubt, like the modern king-crabs, swim when necessary. Some still more shrimp-like creatures, Hymenocaris, which are found with them, certainly had this power.

[F] Woodward has recently suggested affinities of Trilobites with the Isopods or equal-footed crustaceans, on the evidence of a remarkable specimen with remains of feet described by Billings.

A lower type of annulose or ringed animal than that of the Trilobites, is that of the worms. These creatures cannot be preserved in a fossil state, except in the case of those which inhabit calcareous tubes: but the marks which their jointed bodies and numerous side-bristles leave on the sand and mud may, when buried under succeeding sediments, remain; and extensive surfaces of very old rocks are marked in this way, either with cylindrical burrows or curious trails with side scratches looking like pinnate leaves. These constitute the genus Crusiana, while others of more ordinary form belong to the genus Arenicolites, so named from the common Arenicola, or lobworm, whose burrows they are supposed to resemble. Markings referable to seaweed also occur in the Primordial rocks, and also some grotesque and almost inexplicable organisms known as Oldhamia, which have been chiefly found in the Primordial of Ireland. One of the most common forms consists of a series of apparently jointed threads disposed in fan-like clusters on a central stem (Oldhamia antiqua). Another has a wider and simpler fan-like arrangement of filaments. These have been claimed by botanists as algæ, and have been regarded by zoologists as minute Zoophytes, while some more sceptical have supposed that they may be mere inorganic wrinklings of the beds. This last view does not, however, seem tenable. They are, perhaps, the predecessors of the curious Graptolites, which we shall have to represent in the Silurian.

Singularly enough, Foraminifera, the characteristic fossils of the Laurentian, have been little recognised in the Primordial, nor are there any limestones known so massive as those of the former series. There are, however, a number of remarkable organisms, which have usually been described as sponges, but are more probably partly of the nature of sponges and partly of that of Foraminifera. Of this kind are some of the singular conical fossils described by Billings as Archæocyathus, and found in the Primordial limestone of Labrador. They are hollow within, with radiating porss and plates, calcareous in some, and in others with siliceous spicules like those of modern sponges. Some of them are several inches in diameter, and they must have grown rooted in muddy bottoms, in the manner of some of the deep-sea sponges of modern times. One species at least of these creatures was a true Foraminifer, allied, though somewhat distantly, to Eozoon. In some parts of the Primordial sandstones, curious funnel-shaped casts in sand occur, sometimes marked with spiral lines. The name Histioderma has been given to some of these, and they have been regarded as mouths of worm-burrows. Others of larger size have been compared to inverted stumps of trees. If they were produced by worms, some of these must have been of gigantic size, but Billings has recently suggested that they may be casts of sponges that lived like some modern species imbedded in the sand. In accordance with this view I have represented these curious objects in the engraving, On the whole, the life of these oldest Palæozoic rocks is not very abundant; but there are probably representatives of three of the great subdivisions of animals or, as some would reckon them, of four the Protozoa, the Radiata (Cœlenterata), the Mollusca, and the Annulosa. And it is most interesting thus to find in these very old rocks the modern subdivisions of animals already represented, and these by types some of them nearly allied to existing inhabitants of the seas I have endeavoured in the engraving to represent some of the leading forms of marine life in this ancient period.

Perhaps one of the most interesting discoveries in these rocks is that of rain-marks and shrinkage-cracks, in some of the very oldest beds—those of the Longmynd in Shropshire. On the modern muddy beach any ordinary observer is familiar with the cracks produced by the action of the sun and air on the dried surfaces left by the tides. Such cracks, covered by the waters of a succeeding tide, may be buried in newer silt, and once preserved in this way are imperishable. In like manner, the pits left by passing showers of rain on the mud recently left bare by the tide may, when the mud has dried, become sufficiently firm to be preserved. In this way we have rain-marks of various geological ages; but the oldest known are those of the Longmynd, where they are associated both with ripple-marks and shrinkage-cracks. We thus have evidence of the action of tides, of sun, and of rain, in these ancient periods just as in the present day. Were there no land animals to prowl along the low tidal flats in search of food? Were there no herbs or trees to drink in the rains and flourish in the sunshine? If there were, no bone or footprint on the shore, or drifted leaf or branch, has yet revealed their existence to the eyes of geologists The beds of the Primordial age exist in England, in Bohemia, in Sweden and Norway, and also in North America. They appear to have been deposited along the shores of the old Laurentian continent, and probably some of them indicate very deep water. The Primordial rocks are in many parts of the world altered and hardened. They have often assumed a slaty structure, and their bedding, and the fossils which they contain, are both affected by this. The usual view entertained as to what is called slaty structure is, that it depends on pressure, acting on more or less compressible material in some direction usually different from that of the bedding. Such pressure has the effect of arranging all the flat particles as scales of mica, etc. in planes parallel to the compressing surface. Hence, if much material of this kind is present in the sediment, the whole rock assumes a fissile character causing it to split readily into thin plates. That such yielding to pressure has actually taken place is seen very distinctly in microscopic sections of some slaty rocks, which often show not only a laminated structure, but an actual crumpling on a small scale, causing them to assume almost the aspect of woody fibre. Such rocks often remind a casual observer of decaying trunks of trees, and sections of them under the microscope show the most minute and delicate crumpling. It is also proved by the condition of the fossils the beds contain. These are often distorted, so that some of them are lengthened and others shortened, and if specimens were selected with, that view, it would be quite easy to suppose that those lengthened by distortion are of different species from those distorted so as to be shortened. Slaty cleavage and distortion are not, however, confined to Primordial rocks, but occur in altered sediments of various ages.

The Primordial sediments must have at one time been very widely distributed, and must have filled up many of the inequalities produced by the rending and contortion of the Laurentian beds. Their thicker and more massive portions are, however, necessarily along the borders of the Laurentian continents, and as they in their turn were raised up into land, they became exposed to the denuding action first of the sea, and afterwards of the rain and rivers, and were so extensively wasted away that only in a few regions do large areas of them remain visible. That of Bohemia has afforded to Barrande a great number of most interesting fossils. The rocks of St. David’s in Wales, those of Shropshire in England, and those of Wicklow in Ireland are also of great interest; and next to these in importance are, perhaps, the Huronian and Acadian groups of North America, in which continent—as for example in Nova Scotia and in some parts of New England—there are extensive areas of old metamorphic rocks whose age has not been determined by fossils, but which may belong to this period.