II
LIFE IN THE EARLY CAMBRIAN
I
IN the old Chaldean fable of the descent of Ishtar into Hades, to recover her lost Tammuz, at each successive gate of the lower regions she is stripped of some of her ornaments and garments, till at length she has to appear naked and unadorned in the presence of the lord of the Nether World. So in our descent from the surface on which men live, through the successive rocky layers of the earth's crust, we leave behind, one by one, all the higher forms of life with which we are familiar; but there still remain to us our six groups of aquatic invertebrates, in the guise, it is true, of species and genera now unknown in a living state, yet well represented as far down as the lower part of the Cambrian. Let us now suppose that we take our stand on the shores of the Cambrian sea, or cast our dredge into its waters in search of these old animals; though we can only actually do so by painfully hammering and chiselling them out of their rocky tombs, and this often in fragments which must be put together before we can fully realize the forms and structures of the animals to which they belonged.
We may pause here, however, to remark that neither the geographical nor climatal conditions of the earth at this early time were similar to these with which we are now familiar. The marine animals of the Cambrian have left their remains in beds of sediment, which now constitute rocks forming parts of our continents remote from the sea, and much elevated above its level, showing that large areas, then under the ocean, are now dry land; while there is no good evidence that the sea and land have changed places. The facts rather indicate that the continents have extended their area at the expense of the ocean, which has, however, probably increased in depth. In evidence of these statements, I need only mention that some of the oldest rocks in the Scottish and Welsh hills, in Scandinavia, in Russia and in Bohemia, are rich in Cambrian marine fossils.
Fig. 1.—Olenellus Thompsoni, Hall.
A characteristic Trilobite of the Lower Cambrian in North America. After Walcott and specimen in Peter Redpath Museum.
In America, in like manner, such rocks are found on the flanks of the Appalachians, in New Brunswick, and in Newfoundland, in the table-land of Colorado and in the Rocky Mountains. In point of fact, a map of the Northern Hemisphere at this period would show only a limited circumpolar continent with some outlying islands to the south of it, and shallows stretching across the northern part of the areas of the present Atlantic and Pacific Oceans. The great ocean, however, thus extending over most of the temperate and tropical parts of the Northern Hemisphere, was probably also more muddy and shallow than that of modern times. The surface temperature of this vast ocean was also, it is probable, more uniform than that of the modern sea, while even its profounder depths or abysses would have more earth-heat than at present. Thus we may, without hesitation, affirm that in this early age the conditions for the introduction of swarming marine life of low grade, and its extension over the whole earth, were at a maximum.
Let us inquire, then, what these old Cambrian seas actually produced, more especially in the early portions of that ancient and probably protracted time.
The most highly organized type of which we have any certain evidence is that of the Crustacea, the group to which our modern lobsters and crabs belong, and its most prominent representatives are the trilobites (Figs. [1], [2]), so called from the three lobes into which the body is divided. These creatures are indeed remarkable for the twofold property of bilateral symmetry, and fore and aft jointed structure, both based on the number three. From front to rear we have a large head, usually with well-developed eyes and oral organs, a middle or thoracic part composed of a series of movable segments, and a tail-piece sometimes small, sometimes nearly as large as the head. Transversely, the body is divided into a central and two lateral lobes, which can be seen in the head, the thorax, and usually in the tail as well. The organization of these animals must have been as complex as that of most existing Crustaceans. Their nerve system must have been well developed; a vast number of muscles were required to move the different parts of the trunk, and the numerous and complex limbs which have been observed in some of the species, and no doubt were possessed by all. Their digestive and circulatory organs must have been in proportion to the complexity of their locomotive organs.
Fig. 2.—Triarthrus Becki, Green.
A Trilobite of primitive type, showing its limbs and antennæ. (After Beecher.)
Figure 2, borrowed from Beecher,[4] shows the limbs of a species, not of the Lower Cambrian, but of a somewhat later formation. There can be no doubt, however, that those of earlier species were equally perfect, more especially as Triarthrus is an animal of an old type approaching to extinction in the age succeeding the Cambrian, and its representatives in the earlier and palmy days of the family could not have been inferior in organization. These creatures swarmed in every sea in the Cambrian period, and were represented by a great number of species, some of them of large size, others very small; some many-jointed, others few-jointed, and with a great variety of tubercles, spines, and other ornamental and protective parts. If we ask for their affinities and place in the great group of Crustacea, the answer must be that, while in some points allied to the higher forms, they approach most nearly to those which occupy a medium position in the class, and are, in fact, a composite type, presenting points of structure now distributed among different groups. If we ask for affinities with lower groups, we have to reply that their nearest allies in this direction are the bristle-footed marine worms; but there is a vast gap, both in the Cambrian and Modern seas, between any of these worms and the Crustacea, which, either as embryos or as adults, have any resemblance to them.
[4] American Journal of Science, 1896.
The Trilobites, after appearing in a great variety of generic and specific forms, and playing a most important part in their time, were not destined to continue beyond the Carboniferous period, and before that time they were beginning to give place to the Limuli, King-crabs, or Horseshoe-crabs, a few species of which continue on our coasts until the present time. In this limited duration the Trilobites present a strange contrast to certain shrimp-like Crustaceans, their contemporaries (the Phyllopods), which very closely resemble some still extant, and the same remark applies to swarms of little bivalve Crustaceans (Ostracods), which are still represented by hosts of modern species both in the sea and in the fresh waters. There is, however, a remarkable group of shrimp-like Crustaceans, represented in the modern world by only a few small species, which in the Cambrian age attained greater size, and constitute a very generalized type combining characters now found in lower and higher groups of Crustacea.
Hymenocaris vermicauda of Salter ([Fig. 3]) may serve to illustrate one of these primitive forms.
Fig. 3.—Hymenocaris vermicauda, Salter.
A Lower Cambrian Shrimp of generalized type. (After Salter.)
In point of fact, as Dr. Henry Woodward has shown in an able presidential address delivered to the Geological Society in 1895, at the base of the Lower Cambrian we still have several distinct groups of Crustacea; and if with some we were to hold them as traceable to one original form or to a worm-like ancestor, we must seek for this far back in those pre-Cambrian rocks in which we find no Crustaceans whatever. There is, it is true, no good reason to demand this; for whatever the cause, secondary or final, which produced any form of Crustacean in the Lower Cambrian, it might just as well have produced several distinct forms. Evolutionists seem to be somewhat unreasonable in demands of this kind, for any cause capable of originating a new form of living being, might have been operative at the same time in different localities and under somewhat diverse conditions, and may also have acted at different times. All imaginary lines of descent of animals are more or less subject to this contingency; and this may partly account for the great diversity in the lines of affiliation presented to us by evolutionists, which may in part have a basis in fact in so far as distinct varietal and racial forms are concerned, but may just as likely be entirely fallacious in the case of true species. In any case, in the lowest rocks into which we can trace Crustacea, we have already probably five of the orders into which their successors in the modern seas are divided by zoologists; and this is certainly a singular and suggestive fact, the significance of which we shall be better prepared to understand at a later stage of our investigation.
Allied in some respects to the Crustacea, though much lower in grade, are the marine Worms—a great and varied host—usually inhabiting the shallower parts of the ocean; though the 330 species collected by the Challenger expedition show that they also abound in those greater depths to which voyagers have only recently had access. Sea-worms seem thus to be able to live in all depths, as well as in all climates; and in accordance with this they abound in the oldest rocks, which are often riddled with the holes caused by their burrowing, or abundantly marked on the surfaces of the beds with their trails.
The great province of the Mollusca, in which, for our present purpose, we may include some aberrant and rudimentary Molluscoids, is now best known to us by its medium types, the univalve and bivalve Shell-fishes; the higher group of the Cuttle-fishes and Nautili, though not uncommon, being much less numerous, and one at least of the lower groups, the Lamp-shells or Brachiopods, being represented in the modern world by but few forms. The extension of the Mollusks backwards into the Cambrian is remarkable as being on the whole meagre in comparison with that of the Crustaceans, and as presenting only in small numbers the types most common in later times. One or two shells, and perhaps some tracks, represent the highest group: some forms resembling the floating species of Sea-snails, and a very few ordinary bivalves represent the types best known in the modern seas; while the Brachiopods, and probably some still simpler forms, are in great comparative excess. The individual specimens are also of small size, as if these creatures were but insinuating themselves on the arena of life in insignificant and humble forms. So far as yet known, the lowest groups supposed to be allied to the Mollusks, the Ascidians or Sea-squirts, and the Sea-mosses (Polyzoa), do not appear; but they may have been represented by species which possessed no hard parts capable of preservation.
This leads us to the consideration that while all the Crustacea necessarily possess some kind of crust or external skeleton, the Mollusks are very different in this respect. While some of them have ponderous shells, others even of the highest forms are quite destitute of such protective parts. This again leads to a curious question respecting the armature of the Trilobites. Some of these, even of the larger species, have strong and formidable spines, like those of the King-crabs and other modern Crustaceans. Now in the modern species we know these organs to be intended to defend their possessors against the attacks of fishes more swift and powerful than themselves. But what enemies of this kind had the Trilobites to dread? Yet species a foot or more in length presented great bayonet-like spines.
Fig. 4.—Ctenichnites ingens, Matthew.
A slab with markings of aquatic animals. From specimen in Peter Redpath Museum.
All that we know on this subject is that on the surfaces of the Lower Cambrian rocks there are in some places complicated and mysterious tracks or scratches, which seem to have been produced when the rock was in the state of soft mud, by large and swiftly swimming animals possessing some sort of arms or similar appendages ([Fig. 4]). Matthew has ingeniously suggested that they may have been large Mollusks allied to the modern gigantic Squids which still abound in the ocean, that they may have been sufficiently powerful to prey on the Trilobites, and, being swift swimmers, would have found them a helpless prey but for their defensive spines. Yet such large Mollusks might have perished without leaving any remains recognisable in the rocks, except what may be termed their hand-writing on clay. A few small examples of the shell-bearing species of these highest Mollusks, however, appear in the Cambrian, and in the succeeding ages they become very abundant and attain to large dimensions, again dwindling toward modern times. It would thus seem that for some unknown reason the highest and lowest Mollusks may have been locally plentiful, but the intermediate types were rare.
The much lower group of Echinoderms, or Sea-urchins and Sea-stars, curiously enough puts in but a small appearance in the Early Cambrian, being represented, as far as yet known, by only one embryonic group, the Cystideans. A little later, however, Feather-stars became greatly abundant, and a little later still the true Star-fishes and Urchins. The aberrant group of the Sea-slugs seems, so far as known, to be of more modern origin; but most of these animals are soft-bodied, and little likely to have been preserved.
The great group of the coral animals, so marked a feature of later ages, is scarcely known in the oldest Cambrian, except by some highly generalized forms[5] ([Fig. 5]). There are, however, small Zoophytes referable to the lower type of Hydroids, and markings which are supposed to be casts of stranded Jelly-fishes. If, with some naturalists, we regard the Sponges as very humble members of the coral group (Cœlenterata), then we have a right to add them to its representatives in the lowest Cambrian; but perhaps they had better be ranked with the next and lowest group of all—the Protozoa.
[5] Dr. G. J. Hinde has carefully studied these forms, and also similar species occurring in Lower Cambrian beds in different parts of North America, Spain, Sardinia, and elsewhere. See note in the Appendix, and Journal Geol. Society of London, vol. xlv. p. 125.
Fig. 5.—Archæocyathus profundus, Billings.
Possibly a Coral of generalized type from the Lower Cambrian of L'Anse à Loup, Labrador. A small specimen.
Fig. 6.—Structures of A. profundus (magnified).
From specimens in Peter Redpath Museum.
(a) Lower acervuline portion. (b) Upper part, with three of the radiating laminæ and section of pores, (c) Portion of lamina, with pores, the calcareous skeleton unshaded.
These are the humblest of all the inhabitants of the sea, presenting very simple, jelly-like bodies with few organs, but sometimes producing complex and beautiful calcareous and siliceous coverings or tests. Animals of this type have been found in the Lower Cambrian, though not in such vast multitudes as in some later formations. There are also in the Cambrian some large, laminated, calcareous bodies (Cryptozoon of Hall), to be noticed more fully below, and which have recently been traced in still lower deposits even below the lowest Cambrian (Figs. [7], [8]). These have some resemblance to the layer-corals or stromatoporæ of the Silurian and Ordovician, which are by many regarded as the skeletons of coral animals of a low type; but the microscopic structure of Cryptozoon rather allies it with some of the larger forms of Protozoa found higher up in the series of formations. We shall have to discuss this later in connection with still older fossils.
Fig. 7.—Cryptozoon proliferum, Hall.
Portion of slab reduced in size. (After Hall.) See also [Fig. 59, p. 237].
Fig. 7a.—Portion of thin section of Cryptozoon proliferum (magnified × 50).
(a) Corneous layers, (a¹) One of these dividing, (b) Intermediate stroma with granules of calcite, dolomite and quartz, traversed by canals.
From a Micro-photograph by Prof. Penhallow.
[To face p. 39.
If now in imagination we cast our tow-net or dredge into the sea of the Lower Cambrian, we may hope to take specimens illustrative of all our six groups of invertebrate animals, and under several of them examples of more than one subordinate group. Of the Crustaceans we might have representatives of four or five ordinal groups, and of the Mollusca as many. These are the two highest and most complicated. In the four lower groups we would naturally have less variety, though it would seem strange, were it not for so many examples in later periods, that the dominant and highest groups should be most developed in regard to the number of their modifications.
Fig. 8.—Diagrammatic section of two Laminæ of Cryptozoon, showing the Canals of the intermediate space, or Stroma (magnified).
Specimen in Peter Redpath Museum.
Of the whole we might perhaps have been able to secure at least 200 species even in one locality. The likelihood is that if there had been a collecting expedition like that of the Challenger in Early Cambrian times, it could have secured thousands of specific forms representing all the above types, more especially as we probably know very little of the softer and shell-less animals of these old seas, and there is some reason to believe that these may have been in greater proportion than in the present ocean.
In illustration of the richness of some parts of the lowest Cambrian sea, I may refer here to the large and beautifully illustrated Memoir of Walcott on the Lower Cambrian, containing fifty folio plates of species collected in a few districts of North America; and, as a minor example, to the contents of a loose boulder of limestone of that age, found at Little Metis on the Lower St. Lawrence, under the following circumstances ([Fig. 9]):—
Fig. 9.—Lower Cambrian Fossils found in a few cubic inches of limestone in a conglomerate at Little Metis; viz., Trilobites of genera Olenellus, Ptychoparia, Solenopleura, Protypus; Brachiopod of genus Iphidea; Pteropod of genus Hyolithes; Gastropod, genus Stenotheca; Sponge, undetermined.
Along what is now the valley of the Lower St. Lawrence and the gulf of the same name, there seem to have been deposited in the oldest Cambrian or Olenellus period beds of limestone rich in shells of marine animals and fragments of these. These can be seen in place in some parts of Newfoundland, and here and there on the hills bounding the St Lawrence River; but for the most part they have been swept away by the sea when these districts were being elevated to form parts of the American land. Their ruins appear as boulders and pebbles in thick beds of conglomerate or pudding-stone, constituting portions of the Upper Cambrian and Lower Ordovician series, which now occupy the south coast of the Lower St. Lawrence. In one of these boulders, less than a foot in diameter, removed from its hard matrix and carefully broken up, I found fragments representing eleven different species, of which no less than eight were trilobites, one a gastropod, one a brachiopod, and one probably a sponge—and this forms an interesting illustration of the number of species sometimes to be found in a limited space, and also of the great prevalence of the Trilobites in these beds. The statistics of these groups for North America, as given by Walcott, show 165 species belonging to all the groups enumerated above, and of these the Trilobita constitute one-third of the whole; so that the Olenellus Zone, as it has been called from one genus of these Crustaceans, might well be named the reign of Trilobites, unless, indeed, as the indications already referred to seem to show, giant cuttle-fishes, destitute of shells, were then the tyrants of the sea, but are represented only by the markings of their long and muscular arms on the soft sea mud while dashing after their Crustacean prey. What I desire, however, chiefly to emphasize is, that in the lowest beds of the Cambrian we have evidence of sea-bottoms swarming with representatives of all the leading types of marine invertebrate life, and therefore seem to be still far from the beginning of living things, if that was a slow and gradual process, rather than a sudden or rapid series of events.
PRE-CAMBRIAN LIFE