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Chapter XVIII — THE SILURIAN (UPPER SILURIAN) PERIOD | Index | Chapter XX — THE MISSISSIPPIAN (EARLY CARBONIFEROUS) PERIOD |
[p. 559]
Early in fche Devonian period, the sea covered the present area of land to some such extent as shown in Fig. 407. During the period, there were changes in the relations of land and water, some of which were of great importance in their effects on the life of the period.
The Devonian system, like its predecessors, is conveniently divided into a Lower, a Middle, and an Upper portion. The subdivisions now recognized in New York (the state where the Devonian is best developed and known) are as follows :
The subdivisions in the last two columns are not applicable in detail to regions remote from New York. Of the above formation names, Helderberg (or Helderbergian), Oriskany, Onondaga (Corniferous), Hamilton, Portage, and Chemung, have widest application outside the state, but other local names are used instead of these in many places.[p. 560]
[p. 561]
Devonian of the East
The Lower Devonian. The known Helderbergian series is confined largely to the eastern part of the continent. It is known (1) in Maine and farther northeast, (2) in the Appalachian belt, and (3) in the lower Mississippi basin (Fig. 407) . It is largely limestone, 300 to 600 feet thick in eastern New York and Pennsylvania, but thinner to the west.
The Oriskany formation is best known in the northern Appalachian region, but beds of the same age have a wide though not well determined distribution farther west. They are also present in the northeast (Gaspe, New Brunswick, and Nova Scotia). Where best known, the Oriskany is made up chiefly of coarse sandstone. From the vicinity of Cumberland, Md., where the formation has a thickness of a few hundred feet, it thins to the northeast and the southwest, and loses its most distinctive faunal characteristics.
The Middle Devonian. The most important formations of the Middle Devonian are the Onondaga and Hamilton, which are more wide-spread than the Lower Devonian formations. In the east, the Onondaga formation is underlain by clastic beds (Esopus and Schoharie) the equivalents of which* have not been differentiated with certainty west of the Appalachian region. They appear to be shore formations, and the lower beds of the Onondaga limestone may have been in process of deposition over the eastern interior while the Esopus and Schoharie beds were accumulating in the Appalachian belt. If this is the case, the conditions for limestone accumulation were presently extended eastward.
The Onondaga limestone is found from New York to the Mississippi (Fig. 408). It rests on Silurian beds,[1] often with little evidence of unconformity. The epicontinental sea in which the limestone was formed was relatively clear and shallow, as shown by the composition of the rock and the character of the fossils it contains. In many places the limestone is rich in coral, and locally the coral-reef structure is as perfectly shown as in the reefs of modern times. This is true, for example, at the rapids of the Ohio near Louisville. The formation is not a thick one, rarely more than 100 to 200 feet.[p. 562]
[p. 563]
The equivalent of the Ulsterian series occurs east of the Appalachians in northern New England and Canada, having a distribution similar to that of the Helderberg. The formation occurs also on the west side of James Bay (south end of Hudson Bay), and the remnants here may have been connected formerly with one another, and with the equivalent formation of the interior of the United States.[2] The distribution of such formations as this and the Niagara (p. 539) give some idea of the extent to which erosion has removed formations from regions which they once covered.
Following the Onondagan epoch of clear seas, conditions changed so as to give origin to deposits of mud where limestone had been accumulating. These mud beds and their equivalents, now consolidated, constitute the Marcellus and Hamilton formations of New York (p. 559). In the interior, the equivalents of the two formations are commonly grouped together under the name Hamilton, or given local names. In the east, shale is the most common rock, but in the west, there is a good deal of limestone.
Considerable areas in the southern and northwestern parts of the Mississippi basin which had been land in the earlier part of the period appear to have been submerged in this epoch, for the Hamilton formation probably overlaps its predecessor in these directions, resting on Silurian beds. The spread of the sea at this time, beginning perhaps a little earlier, appears to have submerged areas in the south (southern Appalachians and areas farther west) which had been land since the close of the Ordovician, and perhaps opened up connection between the interior sea and the Gulf of Mexico, allowing shallow-water species of animals to migrate into the Mississippi basin from the south. The Cincinnati arch may have been land throughout the Hamilton epoch, though this cannot be affirmed. If the Hamilton formation once overspread this arch, it has been removed.
The conditions for the origin of the Hamilton shales would seem to be met if the surrounding lands (Appalachia and lands north of the interior sea), after standing low while the Onondaga limestone [p. 564] was making, were more elevated, or less protected by vegetation, or subjected to more concentrated or spasmodic precipitation during the Hamilton epoch. Under the earlier conditions, the land formations would have been undergoing decay, but the products of the decay might not have been removed; under the latter, there would have been opportunity for the transportation of the products of decay. Even during the general period of shale formation, however, limestone (often shaly) was making in some places, as in the Mississippi basin.
[p. 565]
In the east, the Hamilton (including Marcellus) formation is very thick, being 1,500 to 5,000 feet thick in Pennsylvania, where it is mainly clastic. Its thickness over the interior, where it contains more limestone, is much less.
The Upper Devonian. The Upper Devonian series has a distribution (Fig. 410) similar to that of the Middle, though it is more wide-spread, especially west of the Mississippi. On the whole, the Upper Devonian is more distinct from the Middle than the Middle is from the Lower, and is somewhat closely connected with the lower part of the succeeding system. An uncomformity appears at the base of the Upper Devonian in some places, and the series overlaps all other Devonian formations, resting on Ordovician beds in others.[3]
The Senecan series of New York consists of various thin marine formations (p. 559), chiefly clastic, all of which bear evidence of shallow-water origin.
The Chautauquan series possesses some exceptional features. The Chemung formation of New York is very like the Portage, though more sandy, and even conglomeratic locally. It ranges in thickness from 950 feet near Lake Erie, to 1,500 feet in the vicinity of Cayuga Lake. Much greater thicknesses are attained in Pennsylvania, but to the westward the formation thins rapidly.
In the Catskill region, there is a series of red shales and sandstones, the Catskill formation, which appear to be, in a general way, the time-equivalents of the Chemung. In some places the Catskill beds may represent less than the full Upper Devonian, and in others more. The Catskill formation is poor in fossils, and such as occur are partly, if not wholly, of fresh- and brackish-water forms. Hence it is inferred that the Catskill region was so far shut off from the ocean as not to afford the conditions necessary for marine life. The redness of the formation is a feature which marks many other formations made in inclosed or partially inclosed basins.
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The Catskill formation has a thickness of 3,000 feet in New York and twice that amount in Pennsylvania. Local beds of sandstone (the Oneonta of central New York), seemingly like the Catskill in origin, occur outside the Catskill region, suggesting that similar conditions of deposition existed now and then farther west.
The thickness of the Upper Devonian in central and western New York approaches 4,000 feet, and is even more in Pennsylvania and Maryland.[4] In Ohio the same series (Black, or Ohio shale [5]) has a maximum thickness of 2,600 feet, and thins notably to the north and west, being but a few hundred, and often but a few score feet thick in Indiana, Illinois, Iowa, southern Michigan, Kentucky, and Tennessee. Diverse names are applied to the series and its subdivisions in various localities.
Although the implication of the preceding paragraphs is that the Devonian formations of the East are chiefly marine, it should be added that some of the formations, as the Portage and some of the beds of Maine, contain considerable numbers of land plants, and are presumably, not altogether marine.
West of New York and Pennsylvania the Upper Devonian beds have few commonly recognized subdivisions, or, in most places, none at all, and the eastern names are not in general use.
Devonian of the West
The Devonian system, so far as known, is absent from the larger part of the Great Plains within the United States, and this great expanse of territory was probably land during the period. No Devonian beds are found about the older formations in Dakota [p. 568] or Wyoming, in those places where the succession of formations has been studied in detail. The Helderberg is present in the Arbuckle Mountains of Oklahoma,[6] and probably in southwestern Texas.[7] The Devonian system has little development in the Rocky Mountains, but is somewhat wide-spread between the Rockies and the Sierras, though its outcrops are not extensive. In some places, as about Globe, Arizona, the system is much faulted and affected by igneous rock;[8] in others it is bounded by unconformities, both below and above,[9] while in still others its limits are not sharply defined. The system in the west has not been generally subdivided, and where subdivisions have been made, they have not been correlated with those of the east. In the Great Basin region, Onondagan types of fossils are found, and in overlying beds, fossils corresponding to the Eurasian (rather than to the east American) faunas, have been recognized. Hamilton types, with great vertical range, also occur. The testimony of the fossils of the Basin region is to the effect that it was not connected with the eastern interior sea in such a way as to allow the free intermigration of marine life.
The system is said to be 8,000 feet thick in parts of Nevada,[10] and 2,400 feet in the Wasatch Mountains; but in the Yellowstone Park,[11] it is only 160 feet thick, and not divisible into distinct formations. In the western interior generally, limestone is the dominant formation.
Devonian formations are known in both northern and southern California, and may be present in many places where the rocks are metamorphosed past identification. In the Klamath Mountains, the Devonian (chiefly Middle) is much disturbed, and contains igneous rocks (tuffs and lava flows). The Devonian system is also [p. 569] abundantly represented in widely separated parts of Alaska.[12] The Devonian faunas f the coastal region, like those of the Great Basin, are Eurasian in their affinities.
Middle Devonian in the northwest. A considerable area of Devonian which has sometimes been called Hamilton is found in the basin of the Mackenzie River and southward to Manitoba.[13] The great arm of the sea in which the Devonian of this area accumulated appears to have extended as far south as northern Missouri (Fig. 409). Whether this arm of the sea antedated the Hamilton epoch is uncertain.
The fossils of this northwestern Devonian are different from those of the Hamilton formation of the east (Illinois to New York) , and if the beds of the two regions were contemporaneous, as they may have been, they seem to have been deposited in waters which were not connected. The union was probably prevented by a narrow belt of land running south-southwest from Wisconsin to Missouri, somewhat as shown in Fig. 409. Till late in the Hamilton, this land seems to have constituted a barrier between the eastern interior sea and a northwestern sea which stretched from Missouri on the southeast, through the Mackenzie basin to the Arctic Ocean on the north. Toward the end of the Hamilton epoch, this barrier seems to have been removed sufficiently to allow the waters and the fife on opposite sides to mingle freely.
Areas Where the Devonian Comes to the Surface
While the Devonian system is widely distributed it does not appear at the surface over large areas. The reasons for its limited exposures are substantially the same as those for the limited exposures of earlier systems, and have been explained.
The absence of the Devonian strata in certain situations is significant. Thus between Iowa and Indiana, Devonian formations do not appear at the surface between the Silurian on the north and the Carboniferous on the south. The absence of Devonian beds [p. 570] here might indicate either that the deposits of the Carboniferous period extended farther north than those of '. ie Devonian, concealing the latter, or that Devonian beds, once deposited north of the present border of the Carboniferous system, were worn away before the deposition of the latter. Of these alternatives the latter is probably correct, for near Chicago a small remnant of Devonian sediment has been found in a fissure in the Niagara limestone, as illustrated by Fig. 411. The limestone was apparently fissured before the Devonian sediments were deposited upon it. Portions of the sediments fell into an open fissure, carrying with them distinctive fossils (fish teeth). In this protected position, the fossils escaped removal.
Igneous rocks. Igneous rocks have little representation in most parts of the well-known Devonian of the continent, but in Nova Scotia, New Brunswick, and Maine, and at some points in the west, there are igneous rocks which appear to be of Devonian age. In many places in the west, the Devonian strata have been affected by dikes and intrusions of later times.
Duration. The Devonian period was probably one of the shorter periods of the Paleozoic, though its duration cannot now be stated more definitely.
Close of the Devonian
The general period of quiet which had prevailed during the period seems not to have come to an end at its close. Only in the eastern part of the continent, in Maine, Nova Scotia, New Brunswick, and the adjacent region to the north do the Devonian strata appear to have been notably disturbed at the close of the period. Here they are overlain unconformably by the Lower Carboniferous. Elsewhere the formations of the younger system rest on those of the older without stratigraphic break. There is, indeed, some reason for associating the Upper Devonian, as here outlined, with the succeeding system.
[p. 571]
Economic Products
Gas and oil. The Upper Devonian is the chief source of oil and gas in western Pennsylvania[14] and southwestern New York, and is one of the sources in West Virginia.[15] The Middle Devonian is the oil-producing series of Ontario. Within the regions of their occurrence, oil and gas are more likely to be found under low anticlines than in other positions, apparently for the reason that anticlines furnish an inverted basin capable of holding these light substances against the pressure of the heavier subterranean water which tends to force them to the surface. In all cases there must apparently be an impervious bed or combination of formations above, to prevent the escape of the oil and gas. In this there is a certain similarity to the conditions requisite for artesian wells, but with the difference that the artesian wells receive their supplies from above and must be closed below, while the oil and gas wells receive their supplies from below and must be closed above.
The lower part of the Chattanooga shale of central Tennessee is the horizon of black phosphates, which are of some importance commercially.[16]
The Foreign Devonian
The continent of Europe. At the close of the Silurian there seem to have been more considerable geographic changes in Europe than in America, for the Devonian system is there more commonly [p. 572] unconformable on its base. This is especially true in the northwestern part of the continent.
During the progress of the period, the European continent was progressively submerged, for the Middle and Upper Devonian formations are more wide-spread than the Lower (Fig. 413). In this respect, the phenomena of Europe are in partial correspondence with those of America, though the overlap of the Middle and Upper Devonian of Europe is greater than in America.
The British Isles. In the British Isles the Devonian system has two phases. The first is found in southwestern England in the area which gave the system its name (Devonshire). The system here is thick and of marine origin. Igneous rocks are often associated with the sedimentary. Valuable ore-bearing veins occur, as in Devon and Cornwall.
The second phase of the Devonian is the Old Red Sandstone, widely distributed in Great Britain and Ireland and found at some points on the continent. Concerning the history of this Sandstone there has been much difference of opinion, but it is commonly held to have been deposited in a series of inclosed or nearly inclosed basins containing lakes or inland seas, the waters of which were fresh or brackish. It is further believed that crustal warpings gave the sea occasional access to these basins. In general the strata are poor in fossils. Some of those present are fresh-water species, and others are land species; but marine species occur at some horizons. It is not improbable that some parts of this singular sandstone are of subaerial, rather than subaqueous origin. The Old Red Sandstone has some features like those of the Catskill formation of America.
The Old Red Sandstone of the British Isles has at its maximum a thickness of more than 20,000 feet,[17] but this includes much igneous rock. There is an unconformity in the series, and the upper division contains conglomerates of such a character as to have raised a question concerning the existence of glaciers in Great Britain in Devonian times.
West central Europe. The Devonian of Germany is remarkable for the proportion of igneous rock interbedded with the sedimentaries. [p. 573] The igneous rock occurs in many separate beds, showing that there were many periods of igneous activity separated by intervals of quiet. In much of central-western Europe the Devonian strata have been metamorphosed, and their structure is often complex. In not a few places, especially where the sedimentary rocks have been invaded by igneous rocks, mineral veins have been developed, and from them large quantities of iron, tin, copper, and other metals have been obtained.[18]
Russia. The Devonian of Russia is made up of beds of arenaceous and calcareous rocks, the former containing fossils related to those of the Old Red Sandstone, the latter containing fossils of a marine fauna. The Lower Devonian appears to be wanting in much of Russia, and the Middle and Upper parts of the system are generally unconformable on subjacent formations.
[p. 574]
Other continents. The Devonian system has a wide distribution in Siberia and China, and is known at many points in southern Asia. It occurs in North and South Africa, and in the Falkland Islands. It has great thickness (10,000 feet) in New South Wales, and has been recognized in Victoria. Rocks of this age are the oldest known formations of the North Island of New Zealand. The system has considerable development in South America, and carries an indigenous fauna akin to the Hamilton fauna of North America. So far as identified in South America, the Devonian beds are referable to the upper part of the Lower Devonian, and to the lower part of the Middle.
Climatic Conditions
Certain evidences of great diversity of climate, or of variations of climate during the period, are not at hand. The Old Red Sandstone and the Catskill formation perhaps point to aridity, but this can hardly be affirmed. In formations thought to be Devonian, evidences of glaciation have been reported from South Africa,[19] but this evidence is perhaps not conclusive.
I. The Marine Faunas
When the sea partially withdrew from the North American continent near the close of the Silurian period, the shallow-water faunas were restricted to limited bodies of water about the continental border. There appears to have been a want of free communication between these, and the life of each developed different aspects according to the conditions of each embayment. When the sea again advanced upon the land in the early Devonian from these different embayments, the advance from each carried toward the interior its own somewhat peculiar fauna. The early Devonian life therefore consisted of the expansions of these provincial faunas. They invaded the continent more or less simultaneously, but they reached the interior more or less successively. The following [p. 575] faunas have been recognized: (1) the Helderberg, (2) the Oriskany, (3) the Onondaga (Corniferous), (4) the Southern Hamilton, and (5) the Northwestern Hamilton fauna. They reached the interior in the order named. As each in turn came in contact with the previous fauna, there was a commingling and conflict of the two, resulting in the destruction of some species and the adjustment of others to one another. The final result was the development of a new, composite fauna from the survivors.
The Helderberg fauna. The Helderberg fauna seems to have developed from the late Silurian fauna in the embayment at the mouth of the St. Lawrence and on the border of the adjacent continental shelf, and perhaps also on the border of southern Europe. The fauna appears to have found a way into the Appalachian valley-trough and thence to have spread westward and northward as far as the advancing waters of the time permitted. Perhaps there was access to the interior also from embayments on the southern coast. The fauna seems never to have occupied more than the eastern part of the great interior region. The fauna of similar date in southern Europe (Hercynian) had much in common with the Helderberg fauna of America, but both differed markedly from the earlier Devonian faunas of the more northern latitudes of Europe and America.
The main features of the Helderberg fauna were a marked development of the mollusks, great numbers of the molluscoids, an erratic tendency of the trilobites, a scantiness of crinoids and corals, and a notable absence of fishes. Some of these features were doubtless due to the physical conditions of the originating tract where there was a muddy, partly calcareous bottom. So far as like conditions prevailed in the interior to which the fauna emigrated, its original characters were retained, as shown by the abundance of mollusks and molluscoids, and the fewness of corals and crinoids. The gastropods (p, Fig. 414) came into prominence, especially in the rather inferior capulid type. Certain pelecypods of the winged type (aviculids, o, Fig. 414) were also conspicuous. The brachiopods surpassed all other classes in numbers but did not develop such distinct peculiarities as the gastropods and pelecypods. The trilobites showed a tendency to sport, taking on strange forms, one of which is illustrated in Fig. 414. The crinoids lost the prominence they had in the Silurian and fell to an insignificant place in the fauna, probably because the physical conditions were unfavorable. For like reasons, probably, the corals were relatively few. The cystoids suffered less but were not abundant. Lower forms were present but not prominent, except that the hydrozoan Stromatopora became common and formed thick beds. It is notable that fish remains were almost entirely absent from the Helderberg deposits, although present in the preceding and succeeding formations.
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[p. 577]
The Oriskany fauna. The Oriskany fauna followed the Helderberg into the interior apparently by a similar route. Its place of [p. 578] origin is not known with certainty, but it probably had its main habitat along the Atlantic coast, for it was a sand-loving fauna and was probably distributed along the arenaceous tracts of the Devonian coasts. It was bound by many ties to the Helderberg fauna, but contained distinctive features, implying a separate origin in part. Brachiopods (Fig. 415) formed a leading type, some of which were notable for their great size, lengths or breadths of 2 to 4 inches being reached. Though less numerous than the brachiopods, the mollusks were abundant and showed some distinctive characters. The capulids were the leading gastropods as before, and some were exceptionally large. Some also began to bear spines, a feature which became more conspicuous later. The pelecypods were not very abundant, but some were phenomenally large. So many forms of unusual size among different orders imply congenial conditions of life. The cephalopods, so powerful in Silurian times, were reduced to a single species, so far as the known record shows. Trilobites were not plentiful, and corals, crinoids, cystoids, and bryozoans were rare, as might be expected from the sandy nature of the formation. Remains of fishes have not been found. On the whole, the Oriskany fauna was essentially an assemblage of well-fed mollusks and molluscoids, with but a sprinkling of other forms.
The Onondaga fauna. The Onondaga fauna was distinguished from the two preceding faunas by hosts of marine fishes of welldeveloped divergent types. From this time on fishes were abundant in the epicontinental waters of America and Europe, and doubtless ranged widely over the seas. A notable feature of the Onondaga formation consists of thin layers (“bone-beds”) made up almost wholly of the plates, teeth, spines, etc., of the fishes, whose numbers must have risen into the millions. The fragmentary nature of the remains, however, makes their classification difficult, but among them were arthrodirans (joint-neck) in abundance, sharks of several orders, ganoids (crossopterygians) and doubtless other forms. The necks of the arthrodirans were so joined to their bodies as to give their heads vertical motion, a rare feature among fishes. The sharks included those which had cutting and piercing teeth, as well as those which had pavement teeth adapted to crushing shell fish.
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The ancient ganoids usually had cartilaginous skeletons and bonyscales, while the modern teleosts have bony skeletons and membranous scales. The fishes of the time seem to have been more fully clothed with spines and defensive armor than those of recent times. Compared with the existing species, they were doubtless heavy, clumsy, and sluggish. From the degree of development already attained, it may be inferred that the ancestors of these fishes had been living for a long time in the originating tract, probably somewhere in the north, for, as noted, they do not appear in the Helderberg and Oriskany faunas of the southeast. It is not improbable that fish were inhabitants of the northern seas from Silurian times onward. Some of the American species of arthrodirans and sharks have been found in Spitzbergen, and others in Germany.
Another significant feature of the Onondaga fauna was the profusion of corals. From the rapids of the Ohio at Louisville, more than 200 species have been collected, embracing both the simple cup form (a,Fig. 416) and the compound type. Some of the cup corals attained a length of 18 inches and a diameter of 3 inches, but the range in size was great, small and large forms intermingling. The reef-building habit attained greater development than in the Silurian times, the reef at the rapids of the Ohio being the most famous example. In view of the abundance of the corals, it is rather singular that the crinoids were rather few, since both forms usually find clear, warm waters congenial. The crinoids, however, do not appear to have lost their vitality, for they appeared in abundance later. Very likely they may have been depressed by some hostile organic influence. Cystoids have not been found. They were far down their declining curve toward extinction. Blastoids were present (b,Fig. 416) but other echinoderms have not been found. Brachiopods formed an important part of the fauna, many of them being large and giving evidence of congenial conditions. In contrast with the Helderberg and Oriskany faunas, cephalopods were abundant. It will be remembered that in the primitive types of the cephalopods, the septa of the shells were plane or symmetrically curved, and that their juncture with the outer shell was a simple curve. In the Onondaga epoch one form had septa which [p. 580] were abruptly bent, and suture lines which were lobed (i, Goniatites,Fig. 416). This was the first notable step in a remarkable series of crumplings of the septa which developed later. Gastropods, similar to those of the earlier Devonian faunas, were present and the spines of the shells had now become pronounced among the capulids, which perhaps signifies the necessity of defense against the abundant fishes and cephalopods. Pelecypods were also abundant, and many of them resembled those of the Helderberg and Oriskany faunas, from which they were doubtless descended. Trilobites were present in more than half a hundred species, a notable increase over the number known in the same region in the preceding epoch. The high degree of ornamentation of many of the species was a conspicuous feature. Barnacles were present, as were also annelids, sponges, hydrozoans, bryozoans, and protozoans. All of these played inconspicuous parts, though in the economy of the whole they were doubtless not unimportant.
[p. 581]
Reviewing these features, it may be said that certain of the forms were clearly the descendants of species that came into the interior sea from the eastward as members of the Helderberg and Oriskany faunas. At the same time, there were prominent elements of the fauna, particularly the host of fish, cephalopods, and corals, which seem, with equal clearness, to have come in from some other source. A very characteristic development of these latter elements is found along the Straits of Mackinac, while to the north, in the James Bay basin, less than 300 miles away, there was a fauna of similar aspect. This suggests a connection between these localities, along which these new elements of the fauna migrated; but there is no positive evidence of a former connection across the intervening tract. It is known, however, that the formation has been, in part at least, removed from this tract. On the whole the striking features of the fauna seem to be most readily explained by supposing that there was a generating tract to the north,[20] either on the American or European continent, and that from this source migration into the interior sea took place as the waters from both the north and the south extended themselves upon the face of the continent. As the result of the invasion, some part of the Oriskany fauna, which already occupied the interior sea, was driven out or destroyed, while the rest intermingled with the northern invaders. In further support of this conclusion, there is no evidence that any of the Onondaga species supposed to have come from the north found their way into South America, while, on the other hand, certain of the Oriskany species lived there and certain of the Hamilton species, which appear to have come in from the south a little later, also lived in South America.
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The Southern Hamilton fauna. At the beginning of the Hamilton epoch, there was a great influx of muddy material into the eastern part of the interior sea, while the formation of limestone continued as before in the western part. This appears to have been the result of crust al warping, which affected stream-erosion and inwash on the east, and closed the straits through which the Helderberg and Oriskany faunas had entered, while in the southwest, downward warping made a more open connection between the interior sea and the ocean in that direction. At any rate, it appears that a fauna whose relatives lived in South America, entered the interior sea, and, joining the resident Onondaga fauna, led to the evolution of the Southern Hamilton fauna. In the earlier stages, the resident clear-water fauna was thus forced to contend with the increasing turbidity of water on the one hand, and with the southern immigrants on the other. This led to a change of the resident fauna to fit the new conditions, and to the absorption and accommodation of the invaders. It was not so radical a transformation as that which attended the previous invasion from the north that gave rise to the Onondaga fauna. because then the invaders were the master type.
The fishes played a conspicuous part in the new fauna.[21] The arthrodirans reached their climax, and some of the species were among the largest and most formidable fish ever known. Dinichthys (Fig. 417) had an estimated length of 20 feet, and was armed with formidable mandibles 2 feet in length which, in-lieu of teeth, [p. 583] had cutting edges that closed shears-like, much like the mandibles of a turtle. The front part of the body was encased in heavy plates. Related types of less formidable aspect were also known. The sharks left abundant relics in the form of teeth and fin spines, some of the latter reaching a foot in length. In both classes, the devices of warfare make up nearly the whole record, and this doubtless correctly implies the state of affairs of the vertebrate kingdom.
Corals were affected adversely by the increased turbidity of the waters, but continued in diminished importance, especially in the western part of the interior. Crinoids were abundant locally, for certain beds of limestone are composed largely of their remains. Most of the genera were the same as those of the Onondaga epoch, but there were some new ones foreshadowing the remarkable development of the crinoids in the Mississippian period. The climax of the brachiopods was somewhere about this time. In [p. 584] articulate brachiopods still persisted as a minor factor, while the more highly differentiated forms attained wide diversity and high rank. The spirifers reached the greatest extension of their hingeline (j,Fig. 419) a feature peculiarly characteristic of the Hamilton epoch, and subordinately of the Devonian generally. Some specimens had the breadth along the hinge-line four or five times as great as the length of their shells. The Orthis family, once so prominent, had fallen to a lower place, while the productids made notable advances toward their great expansion in the Carboniferous period (Fig. 419).
[p. 585]
Some of the brachiopod species were closely related to species that lived in South America just before their appearance in the North American fauna, which supports the view that they were immigrants from the south. The muddy bottoms favored the mollusks. The goniatites increased in numbers and size (Fig. 419). Some of the species began to exhibit node-like ornamental expansions of the shell which became a conspicuous feature later. Some of the primitive straight cephalopods still persisted. The pelecypods especially were favored by the muddy bottoms and the number of recognized species approaches 200. A part of these were derived from Onondagan predecessors, while others were peculiarly Hamilton types, being immigrants or forms whose parentage is unknown. The gastropods were inferior to the pelecypods in number. Their genera were mostly Onondagan, though the species were largely new. The Hamilton trilobites were inferior to the Onondagan in numbers and ornamentation. Other crustaceans showed some increase of representation. At this time appeared the first known barnacles of the northern sessile type. In losing its pedicel and in fixing itself immovably on other objects it became degenerate, but it found a lowly place to which it has hung with wonderful persistence, not unlike the debased human class which it has come to typify.
The Northwestern Hamilton fauna. While the preceding fauna was developing in the eastern interior sea, another fauna was evolving on somewhat different lines in a sea which, advancing from the northwest, appears to have overspread a large tract of the northwestern interior. This sea embraced a portion of the Mackenzie basin and extended southward through Manitoba to Iowa and Missouri. For a time this northwest sea was not in communication with the sea in which the Southern Hamilton fauna lived (Fig. 409), but it finally crossed the intervening barrier and its fauna overran the territory already occupied by the Southern [p. 586] Hamilton fauna (Fig. 410). The northwestern fauna was so closely allied to the Devonian fauna of eastern and central Europe that free intercommunication between the two regions at the north is inferred. The sea arm that reached down into the American interior was perhaps no more than an eastern extension of the Eurasian Middle Devonian province.[22] The southward extension of this great arm of the sea took place late in the Devonian period, for the strata bearing its peculiar life lie on pre-Devonian formations in Missouri, Iowa, and Minnesota, and overlie the Hamilton in the more eastern region.
Evidence of geographic connections. In Europe there is a horizon so well marked by one species of brachiopod (Hijpothyris (Rhynchonella) cuboides) that it is known as the “cuboides zone.” [23] This species also occurs in eastern Asia (China), in northwestern America (Mackenzie valley), in western America (Great Basin), and in the American interior as far east as New York (Tully limestone). Several other species have a similar range. None of them had close allies or progenitors in the Southern Hamilton. On the other hand, allied forms have been found in England, Belgium, France, Germany, Russia, Persia, and China. This seems to make it clear that the derivation was from that quarter. A second wave of European immigration is suggested by the fauna of the Portage beds of western New York, characterized by an abundance of goniatites.[24] A still later wave brought in some of the most characteristic members of the Chemung fauna. The corals of the Northwestern Hamilton fauna were of the Onondaga type, which seems to indicate that at an earlier stage, the Onondaga fauna and the ancestors of the Northwestern Hamilton fauna were in communication, as might well have happened from the northern habitat of both.
The later Devonian (Chemung) fauna. The commingling ami conflict which attended the invasion of the eastern and southern [p. 587] interior sea by the European and Eurasian faunas may be regarded as the controlling event in the evolution of the Upper Devonian fauna. As in the case of the Onondaga invasion, the northern fauna proved the more virile, and gave character to the composite fauna that later arose from the extinction of the weaker species and the adaptation of the survivors to one another. There were three dominant factors in this development, (1) the resident Southern Hamilton species, (2) the invading European and Eurasian species, and (3) the shallow and rather turbid waters in which these species met and merged. The last of these factors expressed itself in a notable rarity of corals. Though the turbid waters would hardly have been congenial to the crinoids, they were represented, as well as a few other echinoderms. The brachiopods best express the outcome of the commingling of resident and immigrant species. As in the case of the total fauna, there was an indigenous set of species developed from the preceding residents, and an exotic set derived from the emigrants and bearing North-European characters. The foreign group was more conspicuous than the native. Among the mollusks, however, the case was the reverse, and the majority seem to have been descendants of the resident bivalves. The record of the fishes was a decline in the arthrodirans, an increase in the sharks, and a continuation ofthe ganoids without great change. The record of the minor forms was not especially characteristic.
The Devonian fauna in the Great Basin area. In the region which constitutes the Great Basin of the west, a large area seems to have been occupied continuously by the sea from about the beginning of the Middle Devonian time to the later portion of the Carboniferous period. It seems to have been measurably free from both the physical and the biological changes which gave such diversity to the eastern provinces. The fauna had a slow, continuous evolution, favored, from time to time, it would appear, by accessions from the north and perhaps from other sources as well. None of the distinctive South American forms appeared in it, nor any of the peculiar Helderberg or Oriskany forms. It is inferred, therefore, that it was disconnected from the eastern and southern interior throughout the whole Devonian period. On the other hand, a notable number of species were common to it and to the [p. 588] northwestern province, as also to the interior province after the latter had been invaded by the Onondagan and Northwestern Hamilton faunas. While its fauna is not yet known well enough to permit final conclusions, it bids fair to offer a fine example of a steady, uninterrupted development of a provincial fauna.
II. The Life of the Land Waters
Principle of interpretation. For geological purposes, all fishes that lived in streams and lakes, and those of the freshened waters of the estuaries, bays, and inlets on the land border, may be classed as fishes of the land waters. But there is difficulty in some cases in determining which sedimentary formations were made on the land or in land waters, for terrestrial, marine, fluviatile and estuarine types of life may be mingled by the carriage of land forms to the sea. Fossils do not always tell us, therefore, whether a formation was made in the sea or on land. The deposits forming to-day in the great valley of California embrace pluvial, fluvial, lacustrine, and marine deposits. At a geologically distant day, only small parts of this aggregate would probably be fossiliferous, and the evidence from various points might be quite opposed. This appears to be very much the state of things recorded in the "Old Red Sandstone/’ in the Catskill formation, and elsewhere. As all these deposits were laid down in more or less local basins, probably, their exact correlation is impossible, and their faunas may be considered together. The general faunal conception is that in the Appalachian tract and in the Canadian provinces lying to the northeast of it, as well as in Great Britain and Russia, there were many lodgment basins that were progressively filled by land-wash and fresh-water sediments, and that these basins were the home of a fresh- or brackish-water fauna, of which ostracoderms, fishes, and crustaceans were the most conspicuous members. Perhaps the geological record presents no more suggestive combination of ancient life. The type of the fauna was foreshadowed by the eurypterids and fishes, or fish-like forms of the late Silurian; but the record of that time is more imperfect than that of the late Devonian.
The ostracoderms. The center of interest in this fauna is found in the ostracoderms (Figs. 420 and 421), which were first [p. 589] interpreted as placoderm fishes, later as jawless fishes, and now as a distinct class between the arthropods and the vertebrates.[25] Their chief interest lies in their suggestion that the vertebrates sprang from the arthropods. The ostracoderms bear external resemblances, in the head and trunk, to trilobites and king-crabs, while some of them have caudal fins and fish-like bodies. Because of their fish-like features and the presence of bone cells in the dermal armor of some of them, and because of their intimate association with true fishes, they were formerly classed as fishes. But do vertebrae (or notochord) has been found, or appendages or jaws of the vertebrate type. On the contrary, each jaw consists of two separate parts which act upon one another, not vertically, in vertebrate fashion, but laterally, as in arthropods. This is especially true of the upper jaw. The ostracoderms probably formed the climax and almost the end of their own strange race, for they practically disappeared with this period. Their disappearance is not surprising in view of the development of powerful fishes, for the ostracoderms were obviously not a masterful race. Besides being small, they were clumsy, their locomotive organs lacked flexibility and efficiency, and their mouth-parts were weak. They probably plowed the soft bottoms of the sluggish waters, half buried in the mud, above which little beside their peculiarly placed eyes and the backs of the plated bucklers were habitually exposed. While the ostracoderms are sometimes reported as occurring in beds with marine fossils, there is little evidence that they were dwellers in the open sea.
[p. 590]
In view of the arthropodan relations of the ostracoderns, it is suggestive that in the Devonian time, as in the closing stages of the Salina, there were gigantic eurypterids associated intimately with the ostracoderms and the fishes. The largest of the arthropods (Pterygotus) reached the extraordinary length of two meters. It has already been suggested that this association of arthopods and vertebrates ran back to their origin, and it may be added that it has run on until the present day, for the fish and the arthropods (crayfish and smaller crustaceans) dominate the fresh waters.
[p. 591]
Another strange class of organisms related to the fishes, but not true fish, was represented by the singular little lamprey-like cyclostome Palccospondylus (Fig. 422), which, it has been conjectured, was really an ancestral lamprey. In any case, this animal represents the vertebrate idea in great simplicity; a slender column of vertebrse, modified at one end into a head and finned at the other for a tail, without ribs or paired fins, or any suggestion of limbs, make up the known structure.
The true fishes found in the supposed fresh-water deposits of the Devonian exceed in number and variety those found in the contemporaneous marine formations. Perhaps the strangest of them were the arthrodirans (Fig. 423), whose relations to other fishes are puzzling, but most paleontologists regard them as a specialized and rather divergent branch related to the ancestors of the lung-fishes (Dipnoi) which reached their climax at about this time. Crossopterygians (fringe-finned ganoids) were present, [p. 592] and showed in their teeth and skull-bones, many resemblances to the amphibians, of which they were perhaps the ancestors. Like the lung-fishes, they appear to have been at or near the climax of their evolution at this time, though they lived on in large numbers to the Cretaceous. Sharks (elasmobranchs) are now chiefly marine, and (unless the Paleozoic era be excepted) have been so throughout known geological time, though a few live in fresh water, as in Lakes Nicaragua and Baikal. In the Devonian period they seem to have lived in the open sea, but their remains are also found in the Old Red Sadstone and equivalent formations, so that they probably lived in fresh and brackish waters, as well as in the ocean.
Shells believed to have belonged to fresh-water mollusks, and closely resembling living genera, have been found, in association with land plants and fishes. Little is known of the fresh-water vegetation.
III. The Land Life
The known land life of the Devonian period consisted of plants, snails, insects, myriapods, scorpions, and traces of amphibians; but the record of land life is very imperfect. In the early stages of its evolution, vegetation was doubtless more perishable than later; but even now plant tissues are less well fitted for fossilization than the shells and skeletons of animals. The normal fate of u pland plants is to perish where they grow, and to disappear by decay, consumption, combustion, or some other form of destruction. The chances of the prompt burial and preservation of lowland vegetation are better; but it is only when the low land is being rapidly aggraded, that the conditions for the fossilization of its vegetation are even fairly favorable. Even when preserved, it is rare that leaves, fruit, twigs, limbs, trunk, and roots are all preserved together. So true is this that in the case of ancient plants. especially if not closely analogous to modern types, it is more or less hazardous to attempt to restore the original plant by combining the dissevered parts of different individuals.
The Devonian period covers much of the early development, though probably not the actual beginning of terrestrial plant life. It saw the origin of ferns, scouring rushes, lycopods, the [p. 593] seed-bearing relatives of the conifers, and probably the “seed-bearing” ferns.[26] The Devonian plants were, on the whole, but sparsely foliate, their leaves being spinoid and small. They perhaps descended from amphibious ancestors, which, in turn, were derived from some of the various types of plants which lived in the sea. If so, the expansion of leafage and the development of an aerial system of transpiration were probably evolved slowly as the plants were weaned from their aqueous habitat. The occurrence of most of the fossil remains in fresh or brackish water or lowland deposits gives a suggestion of the habitats of the flora. Of the upland vegetation nothing is known.
The early Devonian plants appear to have had a strengthened cuticle which helped to support their weight, a function which was discharged later by the woody axes developed by their descendants. It is inferred from the fossils that some of the plants were unable to stand alone, but sprawled about on the ground or clambered over other plants.
The Middle Devonian flora of Maine (Chapman sandstone) is so like a flora of Scotland, Belgium, and the Rhine provinces, as to indicate the probability of the migration of land plants between our continent and Europe, perhaps by way of a land bridge between high latitudes of America and Europe. The Portage flora of New York is found also in Bohemia. In general, the Upper Devonian flora was very similar from Pennsylvania to southern Europe, and this wide-spread flora has something in common with the Devonian flora of Australia. The Devonian fossil woods show no rings indicative of seasonal changes or long periods of drought.
The types of Devonian plants were similar to those of the next period. The dominant forms were fern-like plants, some of which were seed-bearing, and the lower gymnosperms. The forerunners of the lepidodendrons were present in the Middle Devonian, and before the close of the period the forerunners of the sigillarias appeared.[27] Angiosperms had not yet come into existence, so far as known.
[p. 594]
The forests were made up chiefly of (1) the calamites (Equisetales) the gigantic ancestors of the horsetails, (2) the lepidodendrons, gigantic ancestors of the club-mosses, and (3) the cordaites. All of these were better developed in the flora of a later period.
The record of the lower land plants is almost negative, except that, singularly enough, bacteria have been reported.[28] The identification of such simple forms in fossilized woody tissue at so ancient a period is remarkable, though the presence of bacteria is altogether probable in itself, for the record of plant life should have been more perfect than it is, had decay not been promoted by bacteria.
The general aspect of the fern-like, seed-bearing plants was very like that of existing ferns, but some families were archaic and peculiar. The larger number were herbaceous, but there were arboreous forms not unlike existing tree-ferns in general appearance. These plants were already so far advanced in their evolution that little is certainly known relative to their ancestral relations. They are generally thought to have been the progenitors of the Bennettitales, and through them of the cycads and of most or all other gymnosperms. In numbers, the fern-like forms appear to have surpassed all others.
Numerous wings and other fragments of insects have been found, chiefly near St. Johns, New Brunswick.[29] They show that [p. 595] the insects were of archaic and synthetic types most nearly allied to modern neuropters and orthopters; but they combined characters’ now found in different orders. Myriapods, arachnoids, and a scorpion are reported from plant-beds in New Brunswick, and myriapods have been found elsewhere. Terrestrial mollusks are also reported from New Brunswick.
Chapter XVIII — THE SILURIAN (UPPER SILURIAN) PERIOD | Index | Chapter XX — THE MISSISSIPPIAN (EARLY CARBONIFEROUS) PERIOD |
See Reports of Iowa, Missouri, Arkansas, Indiana, and Michigan. ↩︎
Weller, Jour. Geol., Vol. X, pp. 423-432. ↩︎
Ulrich has recently proposed grouping the Upper Devonian with the lower part of the Mississippian, as a new system, under the name of Tennesseean. Geol. Soc. Am., Dec., '08. ↩︎
Prosser, Jour. Geol., Vol. IX, pp. 415-442. This article is a concise summary of the Paleozoic systems of Maryland. ↩︎
Geol. Surv. of Ohio. ↩︎
Taff, Atoka (I. T.) folio, U. S. Geol. Surv. ↩︎
Hill, Physical Geography of the Texas Region, folio 3, topographic atlas, U. S. Geol. Surv., p. 4. ↩︎
Ransome, Professional Paper, No. 12, and Bisbee folio, U. S. Geol. Surv., pp. 39-46; also Reagan, Am. Geol., Vol. XXXII, p. 278. ↩︎
Walcott, Am. Jour. Sci., Vol. XXVI, p. 437, 1883. ↩︎
Geol. Expl. of the 40th Parallel, Vol. I; see also Weller, Jour. Geol., Vol. X, pp. 423-432. ↩︎
Weed, Yellowstone Nat. Park folio. U. S. Geol. Surv. ↩︎
Brooks, Bull. Geol. Soc. Am., Vol. XIII, pp. 256-261, and Professional Paper, No. 1, U. S. Geol. Surv., p. 211; Schrader, Bull. Geol. Soc. Am., Vol. XIII, p. 241; and Professional Paper, No. 20, U. S. Geol. Surv., pp. 62-67. ↩︎
Whiteaves, Am. Geol., Vol. XXIV, 1898, pp. 210-240. ↩︎
Carll, Rept. I, 5, Penn. Geol. Surv., 1890. For statistics on the production of oil, gas, etc., see Mineral Resources of the United States, an annual publication of the U. S. Geol. Surv. ↩︎
White, West Virginia Geol. Surv., Vol. I, Oil and Gas, pp. 208 and 212. This volume is an important contribution to the literature of oil and gas. ↩︎
Columbia (Tenn.) folio, U. S. Geol. Surv. ↩︎
See notes concerning such thicknesses, p. 461. ↩︎
Geikie, Text-book of Geology, 4th ed., Vol. II, p. 999. ↩︎
Schwarz, Jour. Geol., Vol. XIV, p. 683, and I tevid, Q. J. G. S., Vol. XLIII, p. 195. ↩︎
This conclusion is not universally accepted. ↩︎
Eastman, Mem. N. Y. State Mus., Vol. X. ↩︎
Weller, Jour. Geol., Vol. VI, p. 306. ↩︎
Williams, The Cuboides Zone and its Fauna, Bull. G. S. A., Vol. T. p. 481, 1890. ↩︎
Clarke, J. M., 16th Ann. Rept. N. Y. State Geol. and Mem. N. Y. State Mus., Vol. VI. ↩︎
W. Patten, On the Origin of Vertebrates, with special reference to the structure of the Ostracoderms. International Zool. Cong., Berlin, 1901. On the Appendages of Tremataspis, Am. Nat., XXXVII, 1903, p. 223. On the Structure of the Pteraspidce and Cephalaspidoe, Am. Nat., XXXVII, 1903, p. 827. New Facts concerning Bothriolepis, Biol. Bull. No. 2, 1904, p. 113. ↩︎
David White, Jour. Geol., Vol. XVII, 1909. Many of the statements of the following paragraphs are from this article. ↩︎
For classification, see p. 944. ↩︎
Renault. Ann. 8ci. Notes, Vol. II, 1896. See also Seward, Fossil Plant*, 1898, pp. 133 138. ↩︎
Scudder; a lisi is given in Bull. No. 71, U.S.Geol.Surv., 1891. Question has been raised as to the Devonian age of these. ↩︎