This is Mazon Monday post #189. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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You might remember Raymond Janssen from his "Leaves and Stems from Fossil Forests" book (Mazon Monday #51), which was published in 1939. It was part of a series of books distributed by the Illinois State Museum. Until George Langford wrote his books in the late 1950's and early 1960's, "Leaves and Stems" was the go to guide for Mazon Creek fossil collectors. Janssen worked extensively with the collection of George Langford, a large portion of which was donated to the Illinois State Museum in the late 1930's.
Janssen wrote multiple geology books from the 1930's to the 1950's. Most of those books were popular science, with a few aimed at kids. Additionally, he wrote articles for The Scientific Weekly. Some of those articles became the basis of later books like "Geological Aspects of Our National Parks". He wrote a few books about coal - "Buried Sunlight - Story of Coal" and "The Beginnings of Coal"... likely due to his study of the coal fields of Illinois. Before writing Leaves, Janssen prepared paleobotany exhibits for the Field Museum and the Chicago Museum of Science and Industry, among others.
Janssen wrote "Fossil Plant Miniatures of Mazon Creek" for the March 1945 edition of "The Scientific Weekly". There were some nice photos included in the article, unfortunately they are black and white. It's interesting to see how Janssen described the fossils. Asterotheca is what more recently was called Pecopteris. Asterotheca is now generally classified as one of the Crenulopteris species.
FOSSIL PLANT MINIATURES OF MAZON CREEK
By RAYMOND E. JANSSENIN THE northeastern part of Illinois, about sixty miles southwest of Chicago, an extensive coal seam lies close to the surface. Today this seam is being mined by the strip method of coal recovery. Immediately overlying the coal seam proper are layers of shale that have supplied science with a superabundance of fossil plant remains. This area, known as the Mazon Creek Region of Illinois, a name derived from a small stream flowing through that section of the state, has become world famous for the excellent coal plant fossils which have been found there. As a general rule, the plant remains are so well preserved that surface details, such as tiny veinlets, hairs, and other structures, can be studied almost as easily as those of living plants, even though they have been buried for a quarter of a billion years.
These fossil plants owe their remarkable preservation to an unusual form of entombment in the shales. Each individual plant fossil, whether it be a single leaflet, a branch of leaves, or a stem, is here found enclosed in a specially constructed stone crypt made by Nature to fit the enclosed plant body. Each is "hermetically sealed," but is readily split open when given a sharp blow.
These stone crypts, termed nodules, or concretions, by geologists, vary in size and shape according to that of the enclosed fossil. The smallest are round or oval in shape and are about the size of a marble or walnut. The larger ones are usually elliptical, being several times longer than wide, with maximum lengths approaching twenty inches or more and widths up to six inches. The larger ones generally contain complete plant parts, such as a fern frond, a leafy tree branch, or an entire section of stem or trunk (Fig. 1). The smaller ones contain single units, such as seeds, tiny individual leaflets, buds, or immature portions of plants. The uninitiated observer invariably inquires how the fossil plants became entombed within the concretions which, to all outward appearances, are nothing more than ordinary stones or pebbles. Actually each stone nodule grew around a buried plant fossil, effectively enclosing it in a stony coffin.
In order to visualize the manner in which the fossils were formed, we must first picture this region of Illinois as it appeared during the Great Coal Age when these plants flourished. At the outset, the existence of a vast tree-covered swampland is indicated by the coal seam itself which consists of the closely compressed plant remains of the former forest. This swampland was later inundated by the sea, possibly forming a great bay or estuary in this particular locality. A former river of that age apparently entered the sea at this point, depositing delta sediments of mud and silt on top of the drowned swampland. Interbedded with the layers of mud and silt were numerous fragments of trees, ferns, and other plants which had drifted down the stream from highlands farther away. In this way the plant fragments became imbedded in the delta deposits as isolated, scattered units rather than as thick masses of plant debris such as comprises the underlying coal seam proper. At a later time, the layers of mud and silt hardened into strata of shale, and the entire region was dried by uplift of the land.
In the ages which followed, ground waters percolating through the shales brought dissolved mineral matter into contact with the plant remains which had been buried in the layers of shale. This dissolved mineral matter, when coming in contact with a buried plant, reacted with the organic compounds to form new minerals in the immediate vicinity of the buried body. In this way each isolated plant fragment acted as a nucleus around which a concretion was formed, enclosing the plant in its own individual stony tomb. The mineralization which thus took place in the shale surrounding the plant nucleus usually extended from a small fraction of an inch to an inch or more in all directions round about the plant. Thus a concretion roughly approximates, in shape and relative size, the enclosed plant part. The chemical reactions which occurred in the production of the concretions produced iron oxides, thereby causing the concretions to be much harder than the shales in which they were formed. For this reason the concretions are frequently called ironstones.
Since the larger concretions contain the larger fossils which are more readily examined, more attention has heretofore been devoted to them in studies of the fossil plants. Recent strip mining operations on a great scale, however, have uncovered increasing numbers of the miniature concretions. When placed on display, as in museum exhibits, these miniature concretions have attracted unusual attention. Perfectly preserved in minutest detail, these fossil miniatures may be likened to exquisitely carved cameos, wherein the tiny veins and nervules of a leaflet stand out in bold relief against the stony matrix.
In order to evaluate the variety of flora represented in the miniature concretions, a study was made of an assemblage totalling 440 miniatures ranging in size from a half inch to about one and three quarters inches in diameter. The types of plant fragments found in them were as follows:
Fern leaflets ................................... 307
Fern seeds ...................................... 9
Tree leaves ..................................... 28
Tree seeds ...................................... 8
Rushes (stems and leaves) ....................... 83
Herbs (stems and leaves) ........................ 5Total ........................................... 440
Fern foliage has always predominated in any representative collection of coal plant fossils, and for this reason the great coal age has often been called the Age of Ferns. Among the miniature concretions this predominance is even greater, and can be accounted for by the nature of the fern foliage itself. A single fern leaf is called a frond, and each frond is subdivided into one or more series of branchlets, termed pinnae. Each pinna, in turn, consists of many individual leaflets, or pinnules. As the fallen fronds drifted down the watercourses of their time, they readily separated into their ultimate parts, each tiny pinna or pinnule then becoming isolated and eventually being preserved in a miniature concretion.
Modern research has shown that many of these so-called ferns were not true ferns as we know them today. True ferns reproduce by means of spores borne on the undersides of the leaflets. The majority of the coal age ferns did not bear spores, but reproduced by means of seeds. These seeds resembled modern nuts in size and appearance and were borne at the tips of the pinnae (Fig. 2, D). In order to designate these plants from true ferns, they are termed seed-ferns. The proportions, as represented in the miniature concretions, are about two-thirds seed-ferns to one-third true ferns.
The majority of the seed-ferns are represented by a single genus, Neuropteris (Fig. 2), which accounted for 168 of the 307 fern leaf specimens in this collection. Neuropteris has pinnules that are more or less oval, or tongue-shaped, with cordate, or heart shaped, bases. Entering the base, at the point of attachment of the pinnule, is a single vein that thereafter subdivides into numerous secondary veinlets which arch to the margins. Each secondary vein may fork from one to many times before reaching the leaflet margin.
Among the true ferns, the majority are represented also by a single genus, Asterotheca (Fig. 3, A. D), which accounted for 80 of the 307 fern leaf specimens. Asterotheca differs from Neuropteris in having its pinnules attached to the stems by their entire breadths. Each pinnule has a midvein extending the full length from base to apex. Numerous lateral veinlets branch off the midvein at almost right angles to it, and these, in turn, may be simple or may fork from one to several times before reaching the margins.
Occupying a position more or less midway between Neuropteris and Asterotheca is the genus Alethopteris, a seed-fern (Fig. 4). Its leaflets more closely resemble those of Asterotheca than any other form. It differs from the latter, however, in having the leaflets either more linear or more obtuse in shape; also their bases are frequently decurrent, that is, joined to one another at their points of attachment along the stem. Since Alethopteris is a seed-fern, actually it is more closely related to Neuropteris than to Asterotheca. Among the miniature concretions, this genus accounted for only eighteen of the 307 specimens of fern leaflets.
Next in order of abundance among the fossil plant miniatures are those representing leaves and stems of gigantic rushes, known as Calamites (Fig. 5, C-F). These forms grew along the watercourses of their time just as bullrushes and bamboos do today. Many of the individuals attained the dimensions of trees, reaching heights of fifty feet and diameters of two feet. Their trunks were jointed at intervals, some species giving off leafy branches at every joint, and others only at distant joints. Because of the ordinarily large sizes of these plants, they usually are represented by large fossils. Their relatively small leaves and portions of tiny branches, however, accounted for 83 of the 440 miniature specimens studied. The leaves of Calamites were borne in rings, or whorls, completely around the slender branches. Because of their characteristic appearance, they have frequently been called fossil asters by coal miners. They are classified under two fossil genera, Annularia and Asterophyllites, depending upon their relative proportions of width and length. Reproductive spores were borne in cones carried at the tips of the branches (Fig. 5, A).
The trees of the coal age are represented among the miniature concretions almost entirely by leaves, seeds, and bracts. Trees of that age were comparable in size to the average forest trees of today, reaching heights in excess of a hundred feet and trunk diameters up to six feet. Their leaves, however, were markedly different from those of most modern trees, being long and grass-like in appearance. Only the very small and undeveloped leaves are represented in the miniature concretions (Fig. 6, C). More numerous than the leaves, as miniatures, are the bracts, or scales, which protected the spore cases of the reproductive organs (Fig. 6, B). These were borne on cones, ranging in size from three inches to a foot or more in length, somewhat similar in general appearance to the cones of modern pine trees. Hence, we find that most of the coal age trees reproduced by means of spores, whereas most modern trees produce seeds or nuts. This is the exact opposite of the ferns, wherein all modern ones reproduce by spores, but most of the coal age forms produced nut-like seeds.
Relatively rare among the coal age trees were a few which bore true seeds, known as Cordaites. The branches were clothed in very large, stemless, strap-like leaves, and bore catkins which produced the seeds. These seeds were small, rounded, flattened disks which were surrounded by a thin wing, some species of which resembled modern elm seeds (Fig. 6, A). Presumably, as in the ease of modern tree seeds of this general type, the wing was an aid in dispersal by the wind.
Rarest among the miniature concretions, and represented by only five specimens of the total of 440, is a herbaceous plant known as Sphenophyllum. This rather delicate plant was relatively small in size, seldom attaining heights of more than two feet. The leaves of Sphenophyllum were more or less triangular, or wedge-shaped, and were borne in whorls of six to twelve at intervals along the slender stems. One or more veins entered the base of each leaf, forking repeatedly until a final single veinlet extended to the margin of each segment of the leaf (Fig. 5, G). Small upright cones, borne on the tips of the branches, produced the reproductive spores (Fig. 5, B).
The miniature concretions of Mazon Creek contain nearly fifty fossil plant species. This does not mean, necessarily, that an equal number of coal age plants is represented. When dealing with living plants, the scientist can study all of their parts together. But when working with fossils it is unusual to find complete plants, even among This is true more the largest specimens. often of the miniature concretions because most of them contain only the smallest unit of a plant fragment. Since it is not always possible to piece fossil units together and reconstruct a complete plant, it has become necessary to describe various forms under distinct generic and specific names. Hence, units with different names may actually have belonged to the same plant. Occasionally, among very large specimens, fossils will be found which prove that fragments heretofore described as distinct species were actually parts of the same plant. It then becomes possible to reclassify such fragments more accurately. The miniature specimens herein discussed have, so far as possible, been identified by comparing them with larger, more nearly complete fossils of the same corresponding types.
Typical examples of the miniature concretions, which have been split open to reveal their fossil contents, are here reproduced in natural size. Also, all have been photographed in their natural condition without benefit of any coloring or contrasting surficial material, nor have the photographs been retouched in any way. The dark color of many of the plant impressions in contrast to the lighter stone background of the concretions is caused by the plants having been converted to carbon-a form of mummification in which the plant body has become a veritable tissue paper thin layer of coal. The remarkable clarity of their details, in spite of their tremendous age, makes them unique gems of fossil preservation.
Figures (strange, but there wasn't a figure 3).
FIG. 1. A FOSSIL FERN PINNA REPRESENTS AN AVERAGE-SIZED CONCRETION HAVING A COMPLETE PINNA OF Asterotheca miltoni ARTIS.
FIG. 2. NEUROPTERIS, MOST ABUNDANT GENUS IN THE MINIATURE CONCRETIONS
A. TERMINAL LEAFLET OF N. flexuosa STERNBERG.
B. LATERAL LEAFLET OF N. flexuosa.
C. TERMINAL PORTION OF PINNA OF N. rarinervis BUNBURY.
D. NUT-LIKE HUSK OF SEED-FERN, Carpolithus noéi JANSSEN.
E. TERMINAL LEAFLET OF N. scheuchzeri HOFFMANN.
F. PAIR OF BASAL LEAFLETS OF N. scheuchzeri.
G. TERMINAL AND TWO LATERAL LEAFLETS OF N. flexuosa STERNBERG.
ALL ARE ILLUSTRATED IN NATURAL SIZE.
FIG. 4. LEAFLETS OF ALETHOPTERIS 4. IMMATURE LEAFLET OF 4. serli BRONGNIART.
B. MATURE LEAFLET OF A. serli BRONGNIART.
C. IMMATURE PINNA OF A. sullivanti LESQUEREUX.
D. MATURE LEAFLET OF A. sullivanti LESQUEREUX.
FIG. 5. CONES AND LEAVES FROM VARIOUS RUSHES AND HERBS
A. Macrostachya, A SPORE-BEARING CONE OF Calamites.
B. Paleostachya, A SPORE-BEARING CONE OF Sphenophyllum.
C. Annularia stellata MARTIN, A LEAF WHORL OF A CALAMITE RUSH.
D. Annularia galioides LINDLEY & HUTTON, LEAF WHORL OF A CALAMITE RUSH.
E. Asterophyllites equisetiformis SCHLOTHEIM, CALAMITE LEAF WHORLS.
F. Annularia sphenophylloides ZENKER, CALAMITE LEAF WHORLS.
G. Sphenophyllum emarginatum BRONGNIART, LEAF WHORLS OF A SMALL HERB.
ALL ARE PICTURED IN NATURAL SIZE.
FIG. 6. FOSSILS FROM COAL AGE TREES
A. Cordaicarpus, SEED OF THE Cordaites TREES.
B. Lepidostrobophyllum ovatifolium LESQUEREUX.
C. Lepidophyllum longifolium BRONGNIART.
D. Lycopodites meeki LESQUEREUX.
Note: This edition of The Scientific Monthy came are way from Jeremy Zimmerman. It was at one time part of the Mazon Creek Collection at Northeastern Illinois University.
