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Chapter XIX — THE DEVONIAN PERIOD | Index | Chapter XXI — THE PENNSYLVANIAN (UPPER CARBONIFEROUS) PERIOD |
[p. 596]
The time from the close of the Devonian period to the end of the Paleozoic era was formerly regarded as the Carboniferous period. But this interval is now commonly divided into two or three divisions, each of which is given the rank of a period. If three divisions are made (as here), the first is the Mississippian period, also known as the Subcarboniferous, or Lower Carboniferous. It represents a time of wide-spread submergence of the North American continent, and was brought to a close by wide-spread emergence of the area where marine sedimentation had been in progress. The second, the Pennsylvanian period, (also known as the Carboniferous, Coal Measures, and Upper Carboniferous), represents a time when the area between the Appalachian Mountains on the east and the 100th meridian on the west maintained a halting attitude, being now slightly above sea-level and now slightly below it. West of the Great Plains, submergence was rather general, as during the preceding period. The third division of the old Carboniferous period is the Permian, a time of notable crustal deformation, general aridity, and, during part of the period at least, low temperature.
The Mississippian was a period of widespread submergence, a submergence which began in the late Devonian; but its close was marked by the emergence of large portions of the continent. In keeping with this definition of the period, the earlier beds, deposited while the advance of the sea was in progress, are less wide-spread than those of later stages, when submergence had become more general.
[p. 597]
The following subdivisions of the Mississippian system are recognized in the regions indicated:
Mississippi River States | Pennsylvania |
---|---|
4. Chester (or Kaskaskia) series (including the Cypress sandstone below, and the Chester beds above). | 2. Mauch Chunk |
3. St. Louis series (including Salem limestone below and the St. Louis and Ste. Genevieve limestones above). | |
2. Osage or Augusta (including the Burlington limestone, Keokuk limestone, and Warsaw shale) | 1. Pocono |
1. Kinderhook (or Chouteau) |
East of the Great Plains
The Kinderhook stage. In the early part of the Mississippian period, coarse sediments (sands and gravels, now a part of the Pocono formation) were gathering along the western border of Appalachia. At the same time, the area of Southern Michigan was a sort of bay or enclosed sea into which sediment was washed rom the surrounding lands. In the central part of the Mississippi basin, the sediments of this stage (Kinderhook) were partly clastic and partly calcareous, and the variations were not only from place to place at the same time, but from time to time in the same place. Of these formations most are marine, but the Pocono has yielded many fossils of land and plants.
The Osage stage. In the second (Osage or Augusta) stage of the period, the sea of the interior became clearer, and the deposition of purer limestone was in progress. Submergence extended westward, probably to New Mexico on the one hand and to Montana on the other. The similarity between the fossils of the Mississippi basin at this stage, and those of the corresponding strata of Europe, is thought to indicate some available route of travel for marine species (especially crinoids) between these widely separated regions. Shallow water, and the absence of great variations of temperature are probably the only conditions necessary for such migration as the faunas of these widely separated regions imply.
The rich deposits of zinc ore (with some lead) in southwestern Missouri and eastern Kansas are chiefly in the Osage beds, though the metallic compounds were concentrated into ores at a much later time.
[p. 598]
East of the Cincinnati arch, which was probably an island in the Osage epoch, the deposition of clastic sediments continued. Those of eastern Ohio constitute a part of the Waverly series. Farther east, the accumulation of the sand and gravel of the first stage (Pocono) either continued, or had been succeeded by the deposition of the mud which now constitutes the Mauch Chunk formation. The sediments of at least a part of this formation seem to have accumulated on land, rather than in the sea. In Mai viand and elsewhere farther south, a formation of limestone (Greenbrier) lies between the Pocono below and the Mauch Chunk above, and the Newman limestone of other regions[1] is perhaps its equivalent.
The St. Louis stage. This stage (including the Salem or Spergen, St. Louis, and St. Genevieve limestones) marks the time of maximum Mississippian submergence, so far as the western interior is concerned (Fig. 426). Limestone deposition continued in the Mississippi basin, but the fauna which it contains is so unlike that of the Osage stage as to indicate that geographic changes of consequence to the marine life of the interior had taken place. One of these changes seems to have involved the removal of a barrier somewhere in the west, permitting the fauna of the Great Basin, heretofore shut off from the interior, to migrate eastward, and mingle with that of the Mississippi basin.
It was during this epoch that the Bedford limestone[2] of Indiana (Salem or Spergen formation) , famous as a building stone, was deposited. Much of this limestone, long mistaken for oolite is foraminiferal. Many of the great limestone caves- in Kentucky and southern Indiana are in the beds of this epoch. In Michigan, bedfl containing salt (brine) and gypsum were in process of deposition, as also earlier in the period.
In the northern part of the Appalachians the Mauch Chunk shales were doubtless in process of deposition, while other names are applied to the contemporaneous deposits in the mountains [p. 599] farther south.[3] Locally, deposits of this time contain both coal and iron ore.
[p. 600]
The Chester stage. The fourth stage of the Mississippian period (Chester), according to the classification adopted for the Mississippi basin, seems to have been marked by more restricted waters and more varied sedimentation, for sandstone and shale are prominent members of this series. The deposits of this stage resemble in a general way those of the Kinderhook stage. Those were made while the sea was advancing on the land, these while it was retreating. Both are more restricted in their distribution than the beds of the intermediate epochs. In Illinois, the Chester sandstone bears oil locally.[4]
In summation it may be said that the Mississippian beds are predominantly clastic east of the Cincinnati arch, and predominantly calcareous west of it. In many places the limestone of the system carries great quantities of chert.
In Nova Scotia, the Mississippian system rests, locally, on Cambrian and pre-Cambrian terranes, and contains beds of red sandstone and gypsum.
In the Great Plains
In the Great Plains, the Mississippian system is known in Oklahoma and South Dakota, where deformation and erosion have brought the strata to the surface (Fig. 426). In Oklahoma there was some deformation at about the close of the period. The upwarped beds suffered erosion, and an extensive chert-conglomerate, the conditions for which were prepared by the interval of erosion, marks the first stage of Pennsylvanian deposition.
West of the Great Plains
West of the Great Plains the Lower Carboniferous is so widely distributed as to show that the present mountain region, as far west as the 117th meridian, was mostly submerged, though there [p. 601] were perhaps numerous islands,[5] some of which occupied the position of existing mountain cores. North of the United States, also, marine conditions prevailed widely. The several stages of the period, as denned in the Mississippi basin, have not been separately recognized in the west. Much of the system in the west is limestone, though clastic formations are not wanting.[6] The system is exposed about many of the mountains of the west, and over considerable areas in Arizona and perhaps in New Mexico. It rests on the Ordovician in many places, and it sometimes overlaps all earlier Paleozoic systems, lying upon the Proterozoic. It attains a thickness of several thousand feet in places. In some parts of Colorado[7] (Leadville) the Mississippian limestone and dolomite constitute one of the richest ore horizons of the state.
In many parts of the west the Mississippian system is unconformable beneath the Pennsylvanian, and in many places there is an unconformity in the undifferentiated Carboniferous which probably represents the division between the two systems.
Igneous activity. According to present determinations, there was great igneous activity in the west during this period. The area affected by vulcanism at this time, or soon after, extended from Alaska on the north to California on the south.[8] West of the Gold ranges in British Columbia, the early Carboniferous is made up largely of igneous rock, with intercalated beds of clastic sediments. Dikes affect the system of Southern Illinois and adjacent parts of Kentucky, but the date of their intrusion is not known.
General Considerations
Thickness and outcrops. In keeping with the variations in the sediments, the thickness of the Mississippian system varies greatly. In Pennsylvania, there is a thickness of 1,400 feet of sandstone (Pocono), with 3,000 feet of shale (Mauch Chunk) above [p. 602] it; but so rapidly do the formations thin westward, that in the western part of the same state the equivalent formations have a thickness of only 300 to 600 feet. In the region of the Mississippi, where the system is chiefly limestone, it reaches a maximum thickness of about 1,500 feet, being thinner to the north and thicker to the south. In Oklahoma, the thickness is about 1,800 feet, in the Black Hills, 275 to 525 feet, in Colorado (Crested Butte region) 400-525 feet, and in northern Arizona (Grand Canyon of the Colorado), 1,800 feet.
The distribution of the uncovered portions of the Mississippian beds in the eastern part of the continent is shown in Fig. 426. The beds themselves are of course much more extensive than their outcrops, being extensively concealed by younger beds. Like all preceding systems, the Mississippian doubtless has wide distribution beneath the sea, where it is probably thin.
Close of the period. At the close of the period, the eastern interior sea was contracted to very narrow limits if not completely obliterated. Great changes took place in the western half of the continent too, but they are less fully determined. Evidence of their existence is found in the wide-spread unconformity above the Mississippian. In some parts of the west, however, so far as now known, marine conditions prevailed uninterruptedly from the early Mississippian period to the later part of the Pennsylvanian.
Reasons for regarding the Mississippian a distinct system. The withdrawal of the sea from a large part of the eastern interior at the close of the Mississippian period exposed the newly deposited sediments to erosion. The exposure was long and the erosion considerable. At the opening of the Pennsylvanian period, as will be seen in the sequel, a large part of this area was again the site of deposition, and the new system rests unconformably on the Mississippian over wide areas, from Pennsylvania and Tennessee on the east, to Utah and Montana on the west (Fig. 426).
The wide-spread emergence, erosion, and subsequent submergence recorded by the unconformity between the Mississippian and the Pennsylvanian systems is just the sort of change which is held to separate periods, not epochs. Nowhere else in the whole course of the Paleozoic era are so great physical changes embraced [p. 603] within the limits of one period. It is for this reason primarily that the Mississippian should be separated from the Pennsylvanian as a distinct system. These physical changes were accompanied by great changes in life, as will be seen.
The Lower Carboniferous of Other Continents[9]
Europe. The post-Devonian Paleozoic systems of Europe resemble the corresponding systems of North America in some ways, and are in contrast with them in others. The formations in eastern and western Europe, as in eastern and western America, are notably unlike. In western Europe, two great series, or systems, are included under the Carboniferous, namely, (1) the Lower Carboniferous, chiefly of marine origin, and (2) the Coal Measures or Carboniferous proper, deposited partly in lagoons, marshes, and lakes, and partly in the sea. These two systems correspond, in a general way, to the Mississippian and Pennsylvanian, respectively, of eastern North America.
In southern Europe the Upper and Lower Carboniferous formations are like the Mississippian and Pennsylvanian of western North America, in that they are chiefly marine. In eastern Europe the Lower Carboniferous is partly marine, and partly nonmarine and coal-bearing, while the Upper Carboniferous is largely marine.
Though the rocks of the Lower Carboniferous series rest con [p. 604] formably on the Devonian in many parts of Europe, they nevertheless record considerable geographic changes, for the early Carboniferous formations of western Europe are of marine origin, while much of the underlying Devonian (the Old Red Sandstone) is not. The distribution of the two systems and the character of their formations, indicate a somewhat wide-spread, even if slight, submergence at the opening of the early Carboniferous period. This is in keeping with the course of events in North America.
The Lower Carboniferous system of western Europe, like that of North America, is largely of limestone, sometimes known as the Carboniferous Limestone. In Great Britain, the system early received the name of “mountain limestone,” and this name has been applied frequently in North America. East of the Rhine the Lower Carboniferous limestone is replaced by shale, sandstone, and even conglomerate, collectively known as the Culm. This phase of the system contains coal in some places.
In eastern Europe, the system contains much coal. The coalfield of Moscow covers 13,000 square miles, but the beds of coal are mostly thin and poor. The coal-field of Donetz covers 11,000 square miles, and contains 44 workable beds (some of them Upper Carboniferous) which have an aggregate thickness of 114 feet. Workable coal beds also occur in the upturned Lower Carboniferous strata on the flanks of the Urals.
The Lower Carboniferous of some parts of Great Britain and western Europe contains much volcanic rock. Some of the eruptions were probably submarine, and some subaerial in origin.
Thickness. The thickness of the European Lower Carboniferous is very great, considering the fact that it is so largely of limestone. In England and Ireland, the limestone attains thicknesses ranging up to 2,000 feet, and perhaps even to 2,500 feet. In the northern part of England and in Scotland, where the beds are clastic, they have a thickness which, at the maximum, is much greater. In Belgium, also, the limestone is very thick, bespeaking the great duration of the period. At the rate at which limestone is supposed to accumulate, 2,500 feet of limestone would call for at least some hundreds of thousands of years.
The close of the early Carboniferous period was marked, in [p. 605] Europe, by wide-spread crustal disturbances. It was at this time that a great system of mountains, sometimes called the Paleozoic Alps, began its development. These mountains crossed central Europe from east to west. Their remnants are seen in the Vosges, Black Forest, Harz, Sudetes, etc., mountains of the present time.
The development of the Ural Mountains appears to have begun at the same time. Geographic changes which were not deformative were also in progress, shifting somewhat the areas of sedimentation. In Europe as in America, therefore, there is a notable break between the Lower and Upper Carboniferous, as shown by unconformities at many points. The distinctness of the two systems is further emphasized by their unlike distribution, and by the physical unlikeness of their formations.
[p. 606]
Other continents. In other continents, where geological work is less advanced, the Lower and Upper Carboniferous have not always been carefully separated, but the system is known in all of them. In Australia and New Zealand, the Lower Carboniferous is much disturbed and metamorphosed, and associated with more or less igneous rock. In Western Australia, as in some parts of North America, gypsum and salt are found in the system.
Climate and Duration
Most of the data at hand indicate the absence of great diversity of climate during the Mississippian (Lower Carboniferous) period, and suggest at the same time that it was genial. The salt and gypsum of the Mississippian series in Montana, Michigan, Nova Scotia, and western Australia, suggest aridity, but it is not now clear that this aridity was general. Certain conglomerate formations (the Culm) of western Europe have been thought to suggest glaciation, but the evidence does not seem to warrant this conclusion. Recently, phenomena which have been interpreted to imply floating ice, have been found in Oklahoma.[10]
The period was probably somewhat longer than most of the Paleozoic periods which had preceded it.
1. The Marine Faunas
Just as there was no great stratigraphic break between the Devonian and Mississippian systems in the American continent, so there was no radical break in the succession of life. It will be recalled that the life history of the Devonian in North America included a series of great invasions from different quarters, and that the invaders and the invaded mingled with one another until at the close there was an approach to a single continental fauna. The life of the Great Basin, however, was still partly isolated, and there were dependencies of the main fauna that still retained provincial features.
[p. 607]
The Kinderhook fauna. The Kinderhook fauna of the great Mississippian sea varied from region to region, apparently in response to different physical conditions. But few of its features need be noted here.
In this fauna are found the beginnings of the great deployment of the crinoids, which reached its climax later in the period, but other forms of echinoderms were not abundant. The brachiopods were transitional between those of Devonian and Later Mississippian types. Among them, the genus Productus was conspicuous (Fig. 429). Not a few Devonian species may be named as the probable direct ancestors of Kinderhook species, while some Devonian species still lived. Mollusks were prominent, the pelecypods (i, j,Fig. 429) alone having a larger number of species than the echinoderms or the brachiopods. Among them are species indistinguishable from those of the Waverly fauna that lived east of the Cincinnati island. The gastropods were less abundant, and among them the capulids, conspicuous among the gastropods of the Devonian, were common. The chief representatives of the cephalopods were the goniatites and although not abundant, they show a notable advance over any of their known ancestors, in the more highly complicated lobing of the suture. Trilobites were few and small. Their high stage of ornamentation had passed, and the day of their disappearance was drawing near. Among corals cup-shaped forms were most common. Fishes, especially sharks, were abundant.
The Osage fauna. The physical conditions of the Osage epoch present the key to the character of its fauna. Its extended shallow sea, relatively free from silt, afforded a favorable field for the evolution of the varied assemblage of forms that had come together in the preceding epochs under less favorable conditions. There is evidence also of rather free migratory communication with the Eurasian continent, since many identical and allied species were common to America and Europe.
No single group so well characterizes the Osage fauna and expresses its dependence of physical conditions as the crinoids, whose abundance and diversity were climacteric (Fig. 430). The rapid decline after this epoch is one of the most remarkable incidents in the life history of the invertebrates. In the day of their glory, the crinoids were most prolific, as indicated by the fact that a single genus (Batocrinus,) had more than a hundred species. The ornamentation of the crinoids at this time was notable, and as in the case of the trilobites, preceded the decline of the group. The repetition of this singular phenomenon at different times, and in quite different groups of organisms, is worthy of notice, though its meaning is not altogether clear. The crinoids made large contributions to the limestone of the period, the “encrinital limestone” taking its name from the numerous plates and stems which make up much of its substance. Other echinoderms were not very abundant.
[p. 608]
[p. 609]
It is a matter of surprise that the corals had so small a place in this fauna, in view of the favorable physical conditions. It is [p. 610] probable that the explanation of their paucity lies in organic conditions or relations, — perhaps in the corals themselves, perhaps in unrecorded enemies, or perhaps in the preoccupation and rivalry of the crinoids. Brachiopods (Fig. 431), as usual, held a leading place in the fauna, and some of their species ranged to the eastern continents. Mollusks were subordinate, a few winged pelecypods, a few capulid gastropods, an occasional pteropod, and an even rarer cephalopod, making up the poor representation of this class. There were a few lingering trilobites and some other crustaceans, an ample growth of bryozoans, some supposed sponges and doubtless many forms not readily fossilized. Marine plants left but an obscure record.
[p. 611]
The Waverly fauna. Contemporaneous with the evolution of life in the Kinderhook and Osage seas, there was a rather more provincial development east of the Cincinnati axis. There was no impenetrable barrier between this tract and the sea farther west, but, owing partly to the partial separation and more perhaps to other physical conditions, the fauna east of the Cincinnati arch was somewhat distinct from that west of it.
The Waverly fauna was the direct descendant of the Devonian faunas that had occupied the same ground, and had changed but slowly, as the environment remained nearly constant. It was modified by some immigration of the Kinderhook and Osage types, and took on slowly a Mississippian aspect, while retaining many Devonian characteristics. Its most prominent members were the pelecypods, as might have been anticipated from the silty conditions. Many of them were the same as the late Devonian of the same region. Brachiopods were numerous and similar to those of the Kinderhook and Osage faunas. The other forms were not very different from those of the more open sea to the westward.
The Waverly fauna was characterized negatively by the rarity of both corals and crinoids, the apparent reason being their dependence on clear seas. Locally, however, crinoids abounded.
The Great Basin fauna. It will be recalled that the Great Basin was a province by itself in the Devonian period, where a slow faunal development of provincial aspect took place, modified slightly by foreign contributions. The transition from the Devonian to the Mississippian fauna seems to have been gradual, and effected through the progressive evolution of some forms, the elimination of others, and the immigration of a few from westerly sources, and after the Osage epoch, perhaps from easterly sources as well. At the close of the Osage epoch, the Basin fauna united with the Osage fauna to form the Genevieve (St. Louis-Kaskaskia) fauna of the interior.
Previous to this union, one of the most salient distinctions between the Great Basin fauna and the Osage fauna was the rarity of crinoids in the former… Although the Osage sea stretched westward to the mountains at least, and was prolific in crinoids, they do not seem to have invaded the Great Basin sea, a fact which [p. 612] points to the distinctness of the two provinces. Brachiopods were abundant in both provinces, but the species were different. The large spirifers so characteristic of the Osage fauna and of the “Mountain Limestone ” of Europe were absent. On the other hand, there had arisen, under the genus Productus, probably by parallel evolution, species closely allied to some found in the Osage fauna, implying that the Great Basin fauna was taking part, in a conservative way, in evolutionary progress. Were it not for such forms of Mississippian aspect, and the evidence that developed when the two faunas commingled, the Great Basin fauna of this time might be thought to be Devonian, for many species of marked Devonian aspect remained.
Another point of contrast with the Osage fauna was the abundance of the pelecypods as compared with the brachiopods. The pelecypod species seem to have been peculiar to the Great Basin sea, and doubtless originated there, and finally became extinct without migration. There were also many gastropods, among which were air-breathers, the oldest aquatic pulmonates known, though terrestrial pulmonates have been described from Devonian strata. Cephalopods were not abundant. Trilobites were about as rare as in the eastern fauna, and of the same genera. Corals were present in some abundance, the horn-shaped type predominating. Neither bryozoans nor fishes have been reported. Unless this is due to the imperfection of the record or of present investigation, it adds much to the evidence of the distinctness of the province, for fish abounded in the eastern sea, and they are free-moving forms of migratory habits.
As remarked in the physical discussion, the barrier which enforced the distinctness of the Great Basin and the KinderhookOsage seas appears to have been an elongated insular tract lying between the Rocky Mountains and the Great Basin. The yielding of this barrier about the close of the Osage epoch, by erosion or submergence, permitted this singular semi-Devonian, semi-Mi sippian fauna of the west to invade the greater eastern sea. The commingling of the two faunas gave rise to the Genevieve[11] fauna [p. 613] of the interior. It flourished while the St. Louis and Kaskaskia formations were being deposited.
The Genevieve (St. Louis-Kaskaskia) fauna. As already stated, the St. Louis formation marks the stage of maximum seaextension in the interior of North America, and the Kaskaskia deposits represent a narrowing and shallowing sea. The Genevieve fauna, representing the two stages, may be regarded as including the culmination of the cosmopolitan evolution of the marine life of the Mississippian period on the North American continent, and the initiation of its decline. The commingling of the Great Basin and the Osage faunas was the most distinctive feature of the Genevieve fauna. It introduced into the main Mississippian sea what seemed to be a retrograde change, for species of a Devonian aspect that still lived in the isolated Great Basin province and elsewhere, migrated eastward, and their relics are found with species whose evolution had reached an advanced Mississippian phase.
Crinoids were less plentiful than in the Osage fauna, and notably changed (Fig. 432). Of one group which had upwards of 300 species in the Osage fauna, less than 25 species are known in the Genevieve, and among the 25, no Osage species persisted. Other groups of crinoids, however, did not show so remarkable a decline [p. 614] and new and curious forms appeared. The blastoids had their climax here so far as numbers of individuals are concerned, although there was greater generic diversity in the Osage faunas. The leading genus, Pentremites (Fig. 432), was so prolific in individuals in some beds as to give rise to the name Pentremital limestone. A swift decline seems to have followed this climax, and the beautiful forms disappeared for reasons quite unknown. In the Osage and Genevieve faunas, echinoids for the first time showed signs of the prominent development they were to attain later.
[p. 615]
Polyps seem to have profited by the decline of the rival crinoids, or by other conditions, for they were more numerous than in the Osage fauna. A compound coral (Lithostrotion canadense) became conspicuous in the St. Louis limestone at some localities. Aside from this, the simple horn-shaped forms remained the most common.
Bryozoans made a new departure in their mode of support. The delicate branches of their colonies could not extend themselves indefinitely without special means of support. As one mode of securing this support, the genus Archimedes (Fig. 433) , which made its first appearance in the Osage, secreted an axis with a spiral flange upon which the colony spread itself, producing a unique form whose slight resemblance to Archimedes’ screw gave it name. Archimedes became so abundant in the Kaskaskia epoch that a part of the series is known as the Archimedes limestone, on account of the great abundance of the fossils of this genus.
For the first time there is clear evidence that the protozoans were an important factor in the fauna, though it is not to be understood that they were not really plentiful before; but they are here recorded in certain limestones (p. 598) of the St. Louis series, some layers of which are almost wholly composed of foraminiferal shells of one species (Endothyra baileyi,Fig. 433).
A notable modification took place in the brachiopods (Fig. 433), though Productus (g and h) continued to be abundant and characteristic. Many large spirifers disappeared, though small ones remained. An odd feature was the diminutive size of the brachiopods in the Bedford limestone of Indiana at Spergen Hill and elsewhere. The associated fossils of other kinds were also dwarfed, implying pauperizing conditions of some sort, for the sp’ecies seem to be identical with those that grew larger elsewhere. It is not improbable that this limestone was deposited in a partially isolated body of water that was so highly charged with lime and other salts as to be somewhat unfavorable to life. A similar dwarfed fauna is recorded from Idaho.
[p. 616]
Among mollusks, pelecypods (Fig. 433) were rather abundant, and in some of the sandy and silty beds predominated. Some of them still retained a Devonian aspect, and those in the Indiana foraminiferal limestone were diminutive, like the brachiopods. Gastropods were more diversified than in the Osage fauna, and some Devonian genera which had apparently been absent from the Osage reappeared. The cephalopods (Fig. 433) also were more abundant than they had been in the Osage. Trilobites were almost unknown, and the other crustaceans left an unimportant record. Sharks were important and other fish were present.
The most striking peculiarity of the fauna, it may be repeated, resulted from the invasion of the more conservative fauna of Devonian aspect from the sea of the Great Basin, and perhaps from a similar incursion of lingering forms from the Waverly gulf on the east. The remarkable thing is that these should have succeeded, so far as they did, in impressing themselves on the composite result, and in giving tone to the whole. It is more natural to expect an antiquated fauna to be overwhelmed by a younger and more progressive one. It would be interesting to know what happened from [p. 617] the counter-migration of the Osage fauna into the Great Basin region; but this has not been determined.
This persistence of Devonian types through to the end of the Mississippian period, taken with the close continuity of the life of the last Devonian epoch with that of the first Mississippian epoch, and the absence of any notable physical break at that point, raises the question whether the Mississippian might not better have been regarded as the later portion of the Devonian period. This would have given to the united period a cosmopolitan climax in the life of the Osage-St. Louis limestones, and a fitting close in the decline of many forms, and the unconformity at the top of the Mississippian. The connection of the Mississippian with the Devonian, whether physically or faunally, is closer than its connection with the Pennsylvanian. But no divisions of a history, which is in reality continuous, can be altogether without infelicities. The pulsations of the history, which alone are the true basis of natural divisions, are rarely the same everywhere at the same time.
With the close of the Mississippian period, the chief center of life interest passes from the sea to the land, first to the vegetation of the Coal period, and then to the land vertebrates. The history of the marine invertebrates will hereafter be followed with less fullness. With the introduction of fishes it had reached its great adjustments, and its further history bears a close likeness to the struggles and adaptations of the history already sketched.
The Evolution of the Fishes in the Mississippian Period [12]
Many of the ancient invertebrates were as fixed as plants, and their migrations were confined to their early stages; but the fishes were constant rovers of free and rapid movement. While restrained by conditions of food, temperature, etc., they were relatively independent of local conditions. They appear to have effectually invaded the open sea for the first time in the Devonian period, but at that time true marine fishes seem to have been inferior, in number and variety, to those of the inland waters. But by the middle of the Mississippian period, the marine fishes had made such relative [p. 618] progress that they were in unquestioned supremacy, while the fresh-water forms had declined notably, so far as the record shows. The extension of the epicontinental seas, and the consequent reduction of the land-areas, and doubtless of the land-waters, favored the former and restricted the latter. In the seas the supremacy of the sharks was almost uncontested. They were more abundant, apparently, than in any later period. Up to 1889, about 400 species had been reported from the Mississippian formations of America, and about 200 additional species from Europe.[13] The fossils arc chiefly teeth, spines, and dermal ossicles. Three-fourths of the species had crushing or pavement teeth, adapted to breaking the shells of mollusks and crustaceans, and the trituration of seaweeds! The tooth-pavement was formed of large plates of thicknes ranging up to one and one-half inches, composed of solid dentine below and a thick sheet of enamel above, which was pitted, ridged, or otherwise roughened. The number of spines preserved is exceptionally great when compared with the teeth and dermal ossicles, and implies that the spines were more numerous than in later times. The subsequent loss of spines, like the loss of the plates and other clumsy defensive devices, was compensated for by a gain in agility, intelligence, and more effective weapons of attack. Bui the great development of species at this time doubtless had its special reason in the fact that most of the sharks were provided with pavement teeth, which were ineffectual weapons against other sharks armed with piercing and cutting teeth. The spines of the one group were therefore a defense against their aggressive kin. The arthrodirans and lung fishes had declined, as compared with the Devonian period.
Of the fishes frequenting the inland and coastal waters, probably the culminating type was of the order to which the modern garpike belongs. The curious tribe of ostracoderms (p. 590) had nearly or quite disappeared.
The fishes probably made up the whole, or nearly the whole, of the vertebrate fauna of the seas. The ostracoderms may nave entered the sea, but they were probably fresh-water forms in the [p. 619] main. There are reasons for thinking that amphibians frequented the fresh waters and the adjacent lands, but probably not the seas.
II. The Land Life of the Mississippian
Since the period was one of sea extension and its known deposits mainly marine, the record of land life is poor. There was doubtless some notable restriction of the terrestrial life by reason of the encroachment of the sea. Enough fossil vegetation has been recovered to show that the plant life of the early Mississippian land was little more than an expansion of that of the preceding period. There were, however, notable changes in detail. The geographic diversity of the Mississippian floras was somewhat greater than that of the Devonian floras. The mid-Mississippian flora is thought by White[14] to have had its origin on the islands of western Europe, and to have spread thence to Siberia and southward, even to South Africa and Australia; but by what route is not known with certainty. Seventy-five per cent of the species of a Mississippian flora of Argentina are identical with European species, a fact which suggests strongly a land bridge between South America and the continents just named.
The floras of the early and middle parts of the period are better known than those of its closing stages. The flora of the latter indicates adverse conditions of life, and prepares the way for the great floral changes, largely exterminative, which followed. From this stage comes the only American pre-Permian wood (Dadoxijlon pennsylvanicum) which shows rings.
The most interesting suggestion of advance in land life is found in the footprints of a supposed amphibian (Peleosauropus [Saaropus] primcevus) from the Mauch Chunk shale near Pottsville, Penn. They imply a stride of about thirteen inches, and a breadth between outer toes of eight inches. Nearly complete specimens of amphibia (labyrinthodonts) have been found in the Lower Carboniferous of Scotland.
Probably the insects and their allies found in the preceding system were represented, but their fossils are not known to have been found.
Chapter XIX — THE DEVONIAN PERIOD | Index | Chapter XXI — THE PENNSYLVANIAN (UPPER CARBONIFEROUS) PERIOD |
See Tennessee folios. ↩︎
The name Bedford, as applied to their limestone, is really a trade name. As a geological term, Bedford is applied to a member of the Waverly aeriei farther east. ↩︎
The name Mauch Chunk is applied as far south as Maryland; see Jour, Geol., Vol. IX, pp. 422-424. ↩︎
Bain, Econ. Geol. Vol. Ill, and Bull. 2, 111. Geol. Surv. ↩︎
Emmons, S. F., Bull. Geol. Soc. Am., Vol. I, pp. 263-267. ↩︎
The Mississippian is not differentiated from the Pennsylvanian on the maps of most of the western folios of the U. S. Geol. Surv., though the two are sometimes differentiated in the text, especially in the later folios. ↩︎
Eldridge, Anthracite-Crested Butte folio, U. S. Geol. Surv.; and Girty, Prof. Paper No. 16, pp. 162, 163, and 217. ↩︎
Dawson, Can. Geol. Surv., 1886, p. 85. ↩︎
The term Lower Carboniferous is here used, instead of Mississippian, because it is the term in common use in Europe. ↩︎
Taff. Geol. Soc. Am. Dec. 1908. ↩︎
The Genevieve fauna is not restricted to the St. Genevieve limestone. ↩︎
References: Newberry, Paleontology of Ohio, Vols. I and II; Woodward, Vertebrate Paleontology; Dean, Fossil Fishes. ↩︎
Newberry, Paleozoic Fishes of North America, Monogr. XVI, U. S. Geol. Surv., 1889. ↩︎
Jour, of Geol., Vol. XVII, 1909. ↩︎