Graptolite

“ The graptolites are a group of extinct colonial animals which were very abundant during early Paleozoic time. They are characterized by a chitinous exoskeleton consisting of rows of cups or tubes which housed the living animal. These cups are attached to single or branching stalks (fig. 24) which in some forms were attached to sea weeds, rocks, or other foreign objects where they led a fixed existence. The stalks of the unattached graptolites grew on floats (fig. 24a) and these floating forms attained wide geographic distribution. ”

“ Sponge spicules, as may be guessed from their hardness, are common in the fossil state; there are also fossils of sea-anemone ancestors and fossils also of Hydra colonies. The last named, known as graptolites, look not unlike lead-pencil marks on the rock. They exist in many forms; Diplograptus has a stem with two rows of cups in which the little creatures lived long ages ago; Monograptus has but one row of cups upon its stem, whilst Didymograptus is a branched form. These fossils are by no means rare and are worth studying under the microscope. ”

“ In the fine shales of this formation Graptolites are very abundant. I collected these same bodies in great numbers in Sweden and Norway in 1835-6, both in the higher and lower shales of the Silurian system; and was informed by Dr. Beck of Copenhagen, that they were fossil zoophytes related to the genera Pennatula and Virgularia, of which the living species now inhabit mud and slimy sediment. The most eminent naturalists still hold to this opinion.
A species of Lingula is met with in the lowest part of the Llandeilo beds ”

“ Applying the method, in the first instance, to the highly plicated Silurian rocks of the south of Scotland, he found that by means of graptolites he was able to work out the structure of the ground. Each great group of strata was seen to possess its own graptolitic zones, and by their means could be identified not only in the original complex Scottish area, but in England and Wales and in Ireland. It was eventually ascertained that the succession of zones in Great Britain could be recognized on the Continent, in North America and even in Australia. ”

“ The range of the Graptolites in time extends from the Cambrian to the Carboniferous. The Dendroidea alone, however, have this extended range, the Graptoloidea becoming extinct at the close of Silurian time. Both groups make their first appearance together near the end of the Cambrian; but while in the succeeding Ordovician and Silurian the Dendroidea are comparatively rare, the Graptoloidea become the most characteristic and, locally, the most abundant fossils of these systems. ”

“ And in the center division, about Innerleithen, the intrusive traps constitute a marked and interesting feature, more particularly as they there assume a subcrystalline granite structure, and convert the sedimentary [152] deposits into hard flinty slates, or Lydian stone. The organic remains are not abundant: they are scattered, too, at wide intervals, but still sufficiently characteristic of the formation. They consist of graptolites, encrinites, trilobites, and several genera of shells. ”

“ As regards the modes of reproduction among the Graptolites little is known. In the Dendroidea, as already pointed out, the bithecae were possibly gonothecae, but they have been interpreted by some as nematophores. In the Graptoloidea certain lateral and vesicular appendages of the polypary in the Lasiograptidae have been looked upon as connected with the reproductive system; and in the umbrella-shaped synrhabdosomes already referred to, the common centre is surrounded by a ring of what have been regarded as ovarian capsules. ”

“ GRAPTOLITES, an assemblage of extinct zoophytes whose skeletal remains are found in the Palaeozoic rocks, occasionally in great abundance. They are usually preserved as branching or unbranching carbonized bodies, tree-like, leaf-like or rod-like in shape, their edges regularly toothed or denticulated. Most frequently they occur lying on the bedding planes of black shales ”

“ Graptolites are known from rocks that range from Cambrian to Mississippian in age, and they are among the most important guide fossils for Ordovician and Silurian rocks.
Graptolites have been reported from Cambrian rocks in central Texas and from the Ordovician of west Texas (fig. 24) . The most abundant of these occur in certain Ordovician rocks in the Trans-Pecos area where they are common fossils in certain formations.
Subphylum Vertebrata
The vertebrates are the most advanced of all chordates. ”

“ This budding of the polyp-bearing thecae gives to the polypary or colony the appearance of a fret-saw, with the teeth directed away from the sicula.
The habit of the earlier graptolites was to branch repeatedly, as in Clonograptus, or to show a compound leaf-like structure as in Phyllograptus. Later « 124 » on the many-branched forms had their branches reduced until, as in Didymograptus, there were only two branches. Sometimes the branches opened out to direct the thecae upwards, the better to procure their food supply. ”

“ The little teeth on the sides of these were cells, inhabited probably by polyps, like those represented in the modern Sertularia in Fig. 36. Some of them were probably attached to the bottom. In others the branches radiated from a central film which may have been a hollow vesicle or float, enabling them to live at the surface of the water (Fig. 38) . These Graptolites are specially characteristic of the Upper Cambrian and Lower Silurian. ”

“ The remains of marine organisms are abundant and varied. Graptolites flourished as in the preceding period, but the forms characteristic of the Ordovician gave place early in the Silurian to the single-axis type (Monograptidae) which prevailed until the close of the period (Rastrites, Monograptus, Retiolites and Cyrtograptus) . As in the Ordovician rocks, the graptolites have been largely employed as zonal indicators. Trilobites were important; the genera Calymene, Phacops and Encrinurus attained their maximum development ”

“ From Tasmania a Diplograptus has been recorded, but the particular horizon and locality are uncertain.
Silurian Graptolites, Victoria.—
In the Silurian shales at Keilor, in Victoria, Monograptus is a common genus, and Cyrtograptus and Retiolites australis (Fig. 75 F) also occur. ”

“ For ten years (1840–1850) he made a detailed study of these rocks, engaging workmen specially to collect fossils, and in this way he obtained upwards of 3500 species of graptolites, brachiopoda, mollusca, crustacea (particularly trilobites) and fishes. ”

“ It is only when a species begins to secrete a shell or a spicule or a carapace or a lime-supported stem, and so put by something for the future, that it goes upon the Record. But in rocks of an age prior to those which bear any fossil traces, graphite, a form of uncombined carbon, is sometimes found, and some authorities consider that it may have been separated out from combination through the vital activities of unknown living things. ”

“ Other indirect evidences are the pre-Cambrian limestones, iron ores, and graphite deposits, since such minerals and rocks have been formed in later times by the help of organisms. If the carbonate of lime of the Algonkian limestones and marbles was extracted from sea water by organisms, as is done at present by corals, mollusks, and other humble animals and plants, the life of those ancient seas must have been abundant. Graphite, a soft black mineral composed of carbon and used in the manufacture of lead pencils and as a lubricant, occurs widely in the metamorphic pre-Cambrian rocks. ”

“ Finally, it is deserving of notice that, if Laurentian graphite indicates vegetable life, it indicates this in vast profusion. That incalculable quantities of vegetable matter have been oxidized and have disappeared we may believe on the evidence of the vast beds of iron-ore; and, in regard to that preserved as graphite, it is certain that every inch of that mineral must indicate many feet of crude vegetable matter.
It is remarkable that, in ascending from the Laurentian, we do not at first appear to advance in evidences of plant-life. ”

“ In some of the uppermost Archaean rocks there have been recently discovered a few undoubted traces of fossil animals, but with this exception no fossils are known throughout the great mass of Archaean strata. It is true that some authorities regard the beds of graphite and other rocks as a proof of the abundance of plant life, but this supposition is not supported by any convincing evidence. ”

“ Accepting the validity of the argument, it follows that vegetation must have been extremely abundant in the Laurentian and Huronian ages on account of the presence in them of enormous deposits of iron ores, as on Lake Superior, in the Adirondacks, Missouri, etc. Some of the beds are hundreds of feet in thickness.
The presence of graphite, or plumbago, in the Eozoic rocks is by many regarded as a still stronger argument for the former existence of vegetation. As graphite is nearly pure carbon, it is easy to believe that it has accumulated from the remains of plants. ”

“ If there were no growth of vegetation at this far-away period to give rise to these deposits of graphite, we are compelled to ask ourselves whether, perchance, there did not then exist conditions of which we are not now cognisant on the earth, and which allowed graphite to be formed without assistance from the vegetable kingdom. ”

“ At another place in the same district a bed of graphite from ten to twelve feet thick, and yielding twenty per cent. of the pure material, is worked. When it is considered that graphite occurs in similar abundance at several other horizons, in beds of limestone which have been ascertained by Sir W. E. Logan to have an aggregate thickness of 3500 feet, it is scarcely an exaggeration to maintain that the quantity of carbon in the Laurentian is equal to that in similar areas of the Carboniferous system. ”

“ In correspondence with this, while in some Laurentian graphitic rocks, as, for instance, in the compact graphite of Clarendon, the carbon presents a curdled appearance due to segregation, and precisely similar to that of the bitumen in more modern bituminous rocks, I can detect in the graphitic limestones occasional fibrous structures which may be remains of plants, and in some specimens vermicular lines, which I believe to be tubes of Eozoon penetrated by matter once bituminous, but now in the state of graphite. ”

“ Extensive supplies of graphite are found in rocks of the Laurentian age in Canada. In this formation nothing which can undoubtedly be classed as organic has yet been discovered. Life at this early period must have found its home in low and humble forms, and if the eozoön of Dawson, which has been thought to represent the earliest type of life, turns out after all not to be organic, but only a deceptive appearance assumed by certain of the strata, we at least know that it must have been in similarly humble forms that life, if it existed at all, did then exist. ”

“ Graphite, both in the natural and artificial form, contains impurities such as silica, iron oxide and alumina, but the natural form has a much greater percentage of these foreign materials, in some cases as high as 40%. Graphite is usually mixed with other pigments, such as red lead and sublimed blue lead, thus serving better as a paint coating. The difference in physical appearance of the various carbon pigments is interesting, as each pigment has characteristics of its own. In graphite we find a great tendency toward agglomeration or massing of particles. ”

“ We have indications, however, of a still more ancient vegetation, in the carbonaceous shales and thick beds of graphite far down in the Middle Laurentian, since there is no other known agency than the vegetable cell by means of which carbon can be extracted from the atmosphere and fixed in the solid state. These great beds of graphite, therefore, imply the existence of abundance of vegetable life at the very commencement of the era of which we have any geological record. ”

“ The apparent absence of fossils from the highly crystalline limestones did not seem to offer a proof in the negative, any more than their undiscovered presence in newer crystalline limestones where we have little doubt they have been obliterated by metamorphic action; while the carbon which, in the form of graphite, constitutes beds, or is disseminated through the calcareous or siliceous strata of the Laurentian series, seems to be an evidence of the existence of vegetation, since no one disputes the organic character of this mineral in more recent rocks. ”

“ They do not essentially differ from recent gravel deposits. But in course of time the action of percolating water assisted by pressure tends to convert gravels into firm masses of conglomerate by depositing carbonate of lime, silica and other substances in their interstices. Gravels are not usually so fossiliferous as finer deposits of the same age, partly because their porous texture enables organic remains to be dissolved away by water, and partly because shells and other fossils are comparatively fragile and would be broken up during the accumulation of the pebbles. ”

“ But the absence of any associated rocks which bear distinct signs of having originally been ancient sediments makes one hesitate to refer the graphite to an organic origin, and possibly the carbonaceous matter may have been produced from quite another source than that of plant life. ”

“ All of this graphite occurs in extremely old Precambrian graphite schist rocks that we now see at the surface in this part of Texas. It is believed that the schists were once ancient sedimentary rocks, such as shales, which contained organic matter. Long ago, great forces below the earth’s surface altered these rocks. When this happened, the organic material that they contained changed into the mineral we know as graphite.
Graphite has a number of uses. It is mixed with clay to make the pencil lead that we use for writing. ”