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Popular Science Monthly/Volume 73/August 1908/Florissant; a Miocene Pompeii

< Popular Science Monthly‎ | Volume 73‎ | August 1908

By Professor T. D. A. COCKERELL


IN attempting to trace the evolution of plants and animals, the naturalist finds himself continually regretting what is called "the imperfection of the geological record." Of all the creatures which have lived and died upon the earth, only a very small proportion have left any record in the rocks; and since the remains are widely scattered and belong to very diverse periods of time, anything like a complete consecutive series is usually unattainable. It is somewhat as though the student of languages of some future age might be obliged to depend for his knowledge of the English tongue upon small fragments of the pages of Webster's Dictionary, perhaps about an inch square for each page. He would gather his precious scraps together, and by diligently comparing them, would readily deduce a number of things about the construction of the language. He would feel able to restore, in some measure, a certain proportion of the missing words, forming derivatives according to the rules he had been able to ascertain. But how he would long for a single complete page!—for a single series actually presenting to him the different modifications and amplifications of some root in all their richness and variety.

Each year witnesses an increasing number of paleontological discoveries, so that the incomplete series in our museums are gradually becoming more complete and more representative of the actual course of evolution. In some well-known instances, such as those of the horse and elephant groups, the successive stages are now so well known that it is not very difficult for imagination to supply the connecting links; but in others the record is either a total blank or a miserable scrap merely sufficient to awaken curiosity. Take, for instance, the butterflies. According to Dr. D. Sharp, the living species of butterflies known to science number about 13,000, while it is not impossible that 30,000 or even 40,000 actually exist. Butterflies form such a large and varied group, spread over nearly every part of the world where vegetation grows, that it is certain that they have a long history behind them, and that the total number of forms which have existed must run into the hundreds of thousands. Yet the actual number of fossil butterflies so far discovered is only twenty-two! Even this meager figure is in a sense an exaggeration, inasmuch as many—indeed most—of the PSM V73 D117 Fossil calyx of porana tenuis.pngFossil Calyx of Porana tenuis.
(The genus is now Asiatic.)
species are only partially known from very incomplete fragments. The paleontologist and the historian not only desire to know how successive events are related, but are keenly alive to the necessity for detailed information concerning the contemporaneous events and objects of any one period. Hence it is that the uncovering of Pompeii and Herculaneum stirs the blood of the most lethargic, for there is presented to our gaze the actual life of nearly two thousand years ago in all its detail and variety. We know, perhaps, that the ancients had certain customs, used certain tools, enjoyed particular kinds of art and literature; but to accurately restore their daily life, even with the aid of many brilliant descriptive passages left by their writers, was a difficult feat for the imagination. PSM V73 D117 Fossil land snail vitrea fagalis.pngFossil Land Snail (Vitrea fagalis). (Enlarged.) To find two cities buried just as they stood at the beginning of the Christian era is not merely to gain an incalculably precious insight into the life of, that period, but to obtain a landmark of the utmost service for comparisons, both with earlier and later times.

For like reasons, the naturalist may well look for earlier deposits in which living animals and plants are preserved somewhat as at Pompeii; and may feel thankful if in all the world one or two such places exist.

The most famous of such localities is Œningen, on the north or Baden side of a narrow arm of Lake Constance leading to the Rhine. The products of this wonderful fossil-bed—or rather, succession of fossil-beds—have been principally elucidated by Heer, who was professor at the University of Zurich, and for many years the leading authority on fossil plants and insects. There were found more than 450 different kinds of plants, over 470 species of insects, and many fishes, reptiles and other animals. In America the corresponding locality, much more recently discovered, and much less extensively worked, is Florissant, in Colorado.

The first notice of Florissant as a locality for fossils was given by Mr. A. C. Peale in the "Annual Report of the U. S. Geological Survey of the Territories," in 1874. It was remarked that in the upper part of the valley on the South Platte River, a few miles from Pikes Peak, there was an ancient lake basin, marked by extensive deposits in which were found remains of leaves. In the years following, the place was visited by various naturalists, several of whom made collections. In 1877, Dr. S. H. Scudder, accompanied by Messrs. Arthur Lakes, of Golden, Colorado, and F. C. Bowditch, of Boston, spent the summer there, and made an enormous collection, especially of fossil insects. Mrs. Charlotte Hill, a resident of Florissant, also became interested, and with the aid of the neighboring children gathered together many valuable specimens, which are now to be found in various museums. An expedition from Princeton University, including the well-known paleontologists, W. B. Scott and H. F. Osborn, also went to Florissant, and the collections obtained were in part sent to the British Museum, and probably to other institutions. Another large collection was made by Dr. G. Hambach, of St. Louis, Mo., forming the basis of a paper on the fossil flora by Mr. W. C. G. Kirchner.

After a period of activity lasting a number of years, interest in Florissant died down, and not only were the fossil beds neglected, but hundreds of precious specimens already gathered were allowed to remain hidden away in various museums and colleges unstudied. When the material first came in, all the fossil plants were referred to Leo Lesquereux, who was at that time the one great authority on paleobotany in this country. Lesquereux published many descriptions of Florissant plants in his great works on the "Tertiary Flora" and "Cretaceous and Tertiary Floras," issued in sumptuous form by the U. S. Geological Survey in 1878 and 1883, respectively; while Dr. Scudder took charge of the insects, and wrote several very important monographs, the largest being that on "Tertiary Insects," published by the Geological Survey in 1890. When Lesquereux died, and later Dr. Scudder was incapacitated by paralysis from doing any further work, it was natural that Florissant should be neglected, for these two men were almost the sole authorities upon the subject. Furthermore, although the Geological Survey had in former years used funds for the Florissant work, and in particular had published the results at great expense, the fact that the latter were of no obvious value to the mining or kindred interests led to a withdrawal of financial support. That the restoration of the past—the discovery of the conditions which obtained in Colorado perhaps a million years ago—is of no value to mankind is a proposition which would scarcely be endorsed by any scientific man; but the fact remains that our present-day public makes a stronger demand for cash than for philosophic illumination, and the latter must often wait.

In the summer of 1905 the Florissant work was taken up anew by the University of Colorado, and Judge J. Henderson and Dr. F. Ramaley, of that institution, went there and collected a quantity of material. In 1906 a new expedition, consisting of Dr. W. M. Wheeler, of the American Museum of Natural History, and Mr. and Mrs. Cockerell and Mr. S. A. Rohwer, of Boulder, Colorado, spent a considerable time excavating fossils; and in 1907 the work was carried on still more extensively, with the financial cooperation of the American Museum of Natural History, Yale University, the British Museum and the University of Colorado. As a result, there has accumulated an almost embarrassing amount of material, and many remarkable things have been discovered.

Florissant is to-day a small town on the Midland Railroad of Colorado, about as far west of Pikes Peak as Colorado Springs is east of it. It is situated in an open valley or "park," surrounded by low hills, and consisting mainly of rather barren grass-land, with moister and even swamp areas along the creeks. The altitude is about 8,000 feet, and is far above the zone of oak bushes which forms such a conspicuous feature in the vicinity of Manitou. The hills are clothed with an open forest of conifers, mostly pines, and owing to the dry and sunny character of the locality, some of the southern plants, such as the Spanish Bayonet or Yucca, grow in abundance, The town itself is small, and exists principally for purposes connected with the railroad; it is a shipping point for a certain amount of lumber, but in no sense a mining center.

In ancient times—say about a million years ago—the valley was the site of a beautiful lake—Lake Florissant. This body of water was perhaps about nine miles long, but very narrow, and strongly indented by wooded headlands at every point. Here and there were small islands, upon which grew tall redwood trees and other vegetation. It was just such a place as would have delighted the heart of Fenimore Cooper and his hero of the Leatherstocking tales. The climate was very different from that of modern Colorado; mild, warm and damp, not unlike that of the uplands of our southern states. Yet the hills were probably rather dry. for we find remains of plants suited to diverse ecological conditions, from semi-arid uplands to swamps. Not far away vapor and fumes arose from several volcanic vents, and at frequent intervals there were showers of fine ash, while more rarely molten lava escaped and flowed down the slopes into the lake. During the larger eruptions, as has been recently witnessed at Martinique, there were violent gusts of wind, and blazing cinders of various sizes fell on all sides. Green branches were torn from the trees, and are found with the leaves still attached in the shales; while many leaves are burned and torn, and an abundance of charcoal testifies to the existence of forest fires. The clouds of fine ash bore to the ground all winged insects and, when falling in the shallow water of the lake, gave rise to layers of soft mud and sand, which, under the pressure of subsequent deposits, solidified into rock. In this way the shales were formed, the best being under heavy flows of mud or lava, which compressed them and permitted the preservation of the remains they contained, before decomposition had gone too far. Within a limited area hot waters strongly charged with silica surrounded and bathed the remains of the redwood trees, with the result that these are now wonderfully preserved as fossil stumps, one of them of great size.

In the course of ages, after the lake had disappeared, the streams flowing through the valley cut out the soft shales and carried them in fine particles to the Platte River, whence they found their way toward the plains. It is sad to think of the thousands of magnificent fossils which must have been thus destroyed—the infinitesimal fragments of which are now scattered far and wide over the Colorado plains, or have traveled perhaps to the Mississippi and the sea. The result of all this destruction, however, has been favorable to the paleontologist in this sense, that it makes the shales readily accessible at many points. Along the sides of the valley, close to the old shore lines, the fossil-bearing layers are plainly visible, and no doubt those which are preserved are far richer than were those which occupied the middle of the lake. The amount of the deposit still remaining is not known, but it must be very great, so that its possibilities could not be more easily exhausted than those of Herculaneum. Only a few places have been worked for any length of time, while small outcrops, inviting investigation, are very numerous. The work of uncovering the fossils is necessarily very slow. First of all, the heavy cap of solid rock has to be removed—and the farther one goes into the hillside, the greater it is—and then the shale has to be split with a knife into fine layers, often with great difficulty. With the utmost care, it is certain that many things will be lost, either from not being observed when uncovered, or from the shale not splitting in the right places. It would not be practicable to have the work done by untrained laborers, with the exception of the preliminary digging and shoveling, for they would destroy and lose far more than they found. Frequently the most precious and perfect insect remains are

Glossina oligocenus Scudder 1892 PSM V73 D121.png

Fossil Tsetse Fly (Glossina oligocena). The tsetse flies are no longer found in America. (Much enlarged.)

scarcely visible at all, especially on the wet shale as it is dug out; but under a strong lens they show every detail of the structure of the wings. On the other hand, some specimens which superficially appear excellent prove upon minute examination to be of small scientific value.

When the fossils have been obtained, it is no easy matter to determine and describe them. In the case of the plants, many species are easily recognized and can be classified with much certainty; but there are living species of oaks and maples, for example, which possess foliage wholly unlike that which we usually associate with those names. A maple from Japan, judged by its leaves, would be taken for a hornbeam; some oaks resemble willows. The commonest leaf in the shales was considered by Lesquereux to be allied to the water elm of the southern states; but we have found some pieces with the fruit attached, and it seems to be a beech. Calyces, once supposed to belong to persimmon or some allied plants, prove to be those of a poplar; while the giant redwood itself was first introduced as a moss, from a fragment of a twig!

The insects, when well preserved, offer much better characters than most of the plants. Unfortunately, however, they are frequently indistinct or fragmentary, and the accurate determination of the remains becomes extremely difficult. Only those who have worked on fossil insects can appreciate at their proper value the tremendous labors of Scudder, resulting in the description and classification of many hundreds of species. Sometimes very striking structural characters may be observed; but when assistance is sought from the literature on living forms, the student finds that characters of this class have been ignored, and it is necessary to make a fresh study of the modern genera before proceeding with the fossils. In order to do this, however, large collections are needed, and it is no easy matter to secure sufficient specimens. Thus, in one way and another, the opportunities for error in the study of fossil insects and plants are very many; so many, that it is easy to become discouraged, and yield to the temptation to confine oneself to the comparatively easy problems presented by modern types. In such times of discouragement, however, the student may be cheered by the discovery of some splendid thing, telling a tale beyond dispute; and so he returns to his labors, determined to unravel the secrets of the past, and to accept as philosophically as may be the inevitable results of his inability to avoid a certain percentage of error.

In an effort to reconstruct the landscape of the Miocene period in Colorado, we may well begin with the plants. The number of fossil plants described from Florissant is not nearly so great as that from (Eningen; but the deposits of the latter locality are, apparently, not so nearly contemporaneous; while, on the other hand, not nearly all the Florissant species that have been found have yet been published. The collections of the 1907 expedition, rich in new materials, have only

Zootermopsis coloradensis PSM V73 D122.png

Fossil White Ant (Hodotermes coloradensis of Scudder).

recently been unpacked, and their study has scarcely more than begun. The shores of the lake, while doubtless steep in places, must have sloped gradually at many points, and there was much shallow water. In this grew the broad-leafed cat-tail of those days, the Typha lesquereuxi. The remains of its leaves are found in the greatest abundance and upon some of them it has even been possible to detect a fungus, which has been called Didymosphæria betheli. In the water near the bases of the cat-tails was a duckweed. Water lilies have not been found; a round water-lily-like leaf proves to belong to a semiaquatic plant, a kind of frog's-bit. A small rush, a species of Juncus, scattered its fruits everywhere. Small fresh-water molluscs, similar to those of the present day, abounded. There were bivalved forms (Sphærium), and representatives of the pond snails Lymnæa and Planorbis. Dragon-fly and May-fly nymphs were excessively numerous, especially the latter; while the adult insects flew along the shore. Minute bivalved Crustacea were in myriads, but no crayfishes have been detected, and there is no reason for supposing that any existed. In slightly deeper water, there were innumerable fishes, and well-preserved specimens of several species have been obtained. From the fish specimens actually secured, one might suppose that these animals were comparatively rare; but this idea is contradicted by the abundance of their excrement, often containing ants and other insects which may have been killed by the volcanic fumes or ash. It would seem, indeed, that at the beginning of an eruption, the fishes gorged themselves with the falling insects; but when things got too hot for them, they mostly retreated in safety to deeper waters, where they escaped entombment. No frogs or turtles have been obtained, much to our disappointment; it is hardly to be supposed that there were none—we may rather anticipate that they will be unearthed by some happy collector of the future. Near the shores, the principal trees were the narrow-leafed cottonwood—differing little from the one common in Colorado to-day—a kind of beech, Fagus longifolia, and a Myrica with slender twigs, which may not have been more than a shrub. A little more distant from the water, perhaps, were the redwoods, Sequoia haydeni, very like those growing in California at the present time. Under or near the redwood grew the incense cedar, a tree now confined to the Pacific coast (where it still grows with Sequoia) and China, though a closely allied genus occurs in the southern hemisphere. There were no firs or spruces, but two or three species of pine trees were plentiful, probably upon the tops of the little hills; and a shrubby or tree-like juniper—like the so-called cedar of modern Colorado—was a conspicuous object. The warmth and dampness of the climate are indicated by an abundance of ferns, such as may be seen in the forests along the Hudson at the present time. Indeed, the whole aspect of the country must have been much more like that of the forest region of the eastern states than any part of Colorado to-day. After working at Florissant in the summer of 1907, I had occasion to spend some time at Garrison-on-Hudson, and as I daily went through the splendid woods of that region, it seemed to me as if the flora of the shale had come to life. There was the sweet fern, Comptonia, to-day confined to a single species of the eastern United States; in Miocene times wide-spread, and represented by two kinds at Florissant. There was the large-toothed aspen; leaves with even larger teeth, but very similar, were found in the shales. Then the chestnut—great chestnut leaves, of an undescribed species, were among the best of our finds. So also the basswood, the walnut, the ironwood, elm, hickory, holly and many other trees, now wholly

PSM V73 D124 Fossil fern phegopteris guyottii.png

Fossil Fern (Phegopteris guyottii).

absent from Colorado, but common to the Miocene shales and the eastern states. One of the most interesting of the fossils is the sweet-gum, or Liquidambar; a genus widely dispersed in the Miocene, from America to Europe, while now its few scattered remnants are found in our Atlantic coast region, in Mexico and Central America and in Asia. On the other hand, the existing Rocky Mountain flora is represented in the Florissant shales by species of oak, alder, birch, hackberry, barberry, mock-orange or Philadelphia, maple, gooseberry, grape, rose, hawthorn, mountain ash, willow, Virginia creeper, sumach and a number of others, showing that with all the change of climate which has occurred, the flora has not been totally transformed.

No recognizable mammal has been obtained at Florissant; only a

PSM V73 D125 Fossil soapberry tree sapindus stellariaefolius.png

Fossil Soapberry Tree (Sapindus stellariæfolius).

few fragmentary remains, without a skull. There is no doubt that mammals of many kinds abounded in the vicinity of the lake, and it is very likely that some of them were entombed, their bones waiting to be exhumed by some fortunate paleontologist of the future.

Feathers are occasionally found, and two fairly complete birds have been discovered, one a plover, the other apparently a finch. The fishes already mentioned number eight species. Of molluscs, we know two terrestrial species and four or five fresh-water ones. Thirty different spiders have been described by Scudder, and we have found others. The harvest spiders or phalangids, and the millipedes, are each represented by a single kind. It is for the insects, of course, that Florissant is most famous, surpassing even Œningen.

Mr. Scudder wrote:

The insects preserved in the Florissant basin are wonderfully numerous, this one locality having yielded in a single summer more than double the number of specimens which the famous localities at Œningen furnished Heer in thirty years. Having visited both places, I can testify to the greater prolificness of the Florissant beds. As a rule the Œningen specimens are better preserved, but in the same amount of shale we still find at Florissant a much larger number of satisfactory specimens than at Œningen, and the quarries are fifty times as extensive and far more easily worked.

The total number of species of insects so far described from Florissant is about seven hundred, but several hundreds have been found, belonging to groups not yet worked up. The list, as it stands, contains only a single ant, but, as a matter of fact, ants are much more abundant than any other insects, at least in individuals. Those obtained by Scudder, and also the materials secured by the recent expeditions, are all in the hands of Dr. W. M. Wheeler, of the American Museum of Natural History. His account of them will be of extraordinary interest, based as it will be on the examination of thousands of specimens by a naturalist fully competent to make them contribute, not

PSM V73 D126 Fossil incense cedar heyderia coloradensis.png

Fossil Incense Cedar (Heyderia coloradensis) with small piece of redwood (Sequoia haydeni). (Enlarged.)

merely to the science of myrmecology, but to wider biological theories as well. It is, indeed, through the examination of myriads of specimens representing particular periods in the history of the world, that we may expect to solve some of the most difficult questions of evolution. Scudder noticed that among certain of the groups of fossil insects there were particular tendencies observable throughout, notwithstanding the 'fact that the species belonged to different groups. Legs had grown longer, or wing-cells had shortened, since the Miocene, and different series, already at that time quite separate and free from crossing, had been affected in the same manner. The same sort of thing was later remarked by Professor Osborn in the teeth of extinct animals, and he became convinced that there were fundamental predispositions to vary in particular directions. Theories of this sort, if completely verified,

PSM V73 D127 Fossil chestnut leaf castanea dolichophylla.png

Fossil Chestnut Leaf (Castanea dolichophylla). Close to the leaf is also seen a small freshwater shell (Planorbis florissantensis).

would greatly affect our ideas of the process of evolution; but the chief need at present is for more light, derived from more and more diverse groups of animals. Hence the study of the fossil insects, at first seeming of purely entomological interest, is likely to lead to results of the first importance.

While we thus search for trends of evolution, we note also the great conservatism of insect types. It is well known that warm-blooded animals have undergone great changes since the Miocene, one of them being the evolution of man himself. In the case of the insects, however, the modifications have been slight indeed, even where these have been driven far and wide by adverse changes of climate.

We find, it is true, a fair number of extinct genera; types which no longer persist in the modern world; but these appear to be merely those which have died out, not the ancestors of any modern kinds. None of the species, of course, are supposed to agree with living ones; but most of the genera agree well, except in certain groups, like the plant lice, in which there is a similar divergence throughout.

Owing to this singular constancy of fundamental structure, we are able to ascertain that some striking types now confined to particular parts of the world, were once very widely spread.

Perhaps the most remarkable instance of this sort is afforded by the tsetse fly, Glossina. Scudder obtained from Florissant an imperfect specimen of a large fly, which he regarded as representing a new genus and species. We were so fortunate as to secure a much better example, showing the long proboscis, and it was not difficult to recognize it as a veritable tsetse fly. The species, of course, is extinct; but the genus is the same as that now confined to Africa, where it is dreaded as the disseminator of some of the most terrible diseases known. What part the existence of such flies may have played in the destruction of the Tertiary mammalia we can only surmise; but it is not impossible that their influence was great. How it happened that they disappeared entirely from America and survived only on the Ethiopian continent is, of course, unknown.

Another discovery, hardly less interesting, was a species of Neuroptera belonging to the family Nemopteridæ. These insects are very fragile and delicate, somewhat dragon-fly-like in form, but with the most extraordinary hind wings—consisting of a long narrow stalk, with a dark-fiddle-shaped expansion at the end. One species of this family has been found in Chile, while others are known from the warmer parts of the old world. The whole group has become extinct in North America, but the fossil proves that it once existed there. Such a fossil as this not merely throws a flood of light on the past migrations of a peculiar group, but is the first and only indication we have of the past history of its race.

Florissant is famous for its fossil butterflies, having nearly half of the number known in that condition. My wife was particularly anxious to find a fossil butterfly, and often as we went out to work, we asked, would this be the day to yield the coveted treasure? Yet all the first season passed, and no butterfly was obtained. Toward the close of the second season, however, my wife sat down one day at a new place, to see what it might be worth. She had scarcely begun to turn ever the shale when, behold, a truly magnificent specimen! It showed the upper wings, the body and one antenna, the spotting still plainly visible upon the wings. It proved to be an undescribed species, but of a genus still existing in Colorado, though more common southward. When compared with the Scudder collection, it was seen to be the second finest of the butterflies, yielding place only to Scudder's incomparable Prodryas persephone. During the same season we secured a second butterfly, much larger, but very poorly preserved.

When the fine butterfly was discovered, it was naturally expected that the new locality would yield other like treasures. Alas! it was worked all one day, with practically no result. Such are the fortunes of fossil hunting. While the first season yielded no butterfly, it did produce a wonderfully preserved caterpillar, still showing the bristles it bore in life. It is not the usual custom to describe a lepidopterous insect from the caterpillar alone; but in this case we had no option, since it could not be ignored, and it certainly could not be raised to maturity! Its characters were peculiar, so that it did not fit comfortably into any modern family, so far as we were able to judge.

Marquettia americana (Cockerell 1907) PSM V73 D129.png

Fossil Neuropterous Insect (Halter americana) of a Family not now found in North America. (Much enlarged.)

Among the plants, one great treasure was a branch of the narrow leafed cottonwood, with about ten leaves upon it. Although the leaves of this tree are exceedingly common in the shale, such a magnificent specimen is very rarely obtained. During the second season another nearly as good was found; we packed it up with the greatest care, and sent it by express to Yale University Museum, where it arrived in safety.

The large specimens, however, are not necessarily the most valuable. One small but unique object was a tuft of moss, with the fruiting bodies upon it. This was the first really recognizable moss ever found fossil in America, and it was appropriately transmitted for study to Mrs. Britton of the New York Botanical Garden, the best authority on American mosses. It may now be seen in the great Botanical Museum in Bronx Park.

Flowers and fruits are quite numerous in the shale, though, unfortunately, nearly always detached from the plants, so that they can not be correlated with the leaves. Some of them are very well preserved and easily recognizable; others, even though reasonably perfect, have puzzled all the botanists who have examined them. One curious specimen is a bean pod, with most of the beans still in it, but one just in the act of falling out. Grass-seeds are often found, and we have one which had just begun to sprout when it was overwhelmed by a fall of ash.