Part 1

_Field Book_ PENNSYLVANIAN PLANT FOSSILS OF ILLINOIS

Charles Collinson Romayne Skartvedt

_Illinois State Geological Survey Educational Series 6_

STATE OF ILLINOIS DEPARTMENT OF REGISTRATION AND EDUCATION

First edition 1960 Reprinted 1966

ILLINOIS STATE GEOLOGICAL SURVEY URBANA ILLINOIS John C. Frye, Chief

Printed by Authority of State of Illinois, Ch. 127, IRS, Par. 58.25.

_Field Book_ PENNSYLVANIAN PLANT FOSSILS OF ILLINOIS

Charles Collinson Romayne Skartvedt

_Illustrations by Marie E. Litterer_

FOREWORD

This field book is intended to guide beginners in their collection and general classification of plant fossils. It illustrates the plant fossils most commonly found in Illinois and relates them to the plants of which they were a part. A list of publications that will furnish more detailed identification of specimens is included. The book has been prepared in response to numerous inquiries to the Illinois State Geological Survey from amateur collectors.

Information has been drawn from numerous sources. The works of Hirmer, Janssen, Lesquereux, Noé, and Langford have been particularly useful.

We are especially indebted to Dr. Robert M. Kosanke, paleobotanist at the Illinois State Geological Survey, and Dr. Wilson N. Stewart, professor of botany of the University of Illinois, for helpful suggestions and use of their libraries.

KEY TO PLANTS ILLUSTRATED ON TIME CHART

1. _Foerstia._ These fossils may be the earliest known occurrence of bryophytes, although some authors have referred them to the brown algae. After Dawson. 2. _Psilophyton._ A primitive vascular plant. After Dawson. 3. _Lepidodendron._ After Hirmer. 4. _Sigillaria._ After Hirmer. 5. _Calamites._ After Hirmer. 6. _Sphenophyllum._ After Fuller and Tippo. 7. _Equisetum._ The only living genus of scouring rushes. After Fuller and Tippo. 8. _Megaphyton._ An ancient true fern. After Hirmer. 9. Modern tropical tree fern. After Fuller and Tippo. 10. _Medullosa._ An ancient seed fern. After Stewart. 11. _Williamsonia._ An extinct cycad-like tree. After Sahni. 12. _Cycas._ A modern cycad. After Chamberlain. 13. _Baiera._ A fossil leaf genus of ginkgo, whose only living representative is the species _Ginkgo biloba_, saved from extinction by careful cultivation in China. Several specimens of this “living fossil” were presented to this country by the Chinese and are now flourishing on many college campuses, including that of the University of Illinois. After Mägdefrau. 14. _Cordaites._ After Grand Eury. 15. _Lebachia._ A “transition conifer,” forerunner of present day conifers. After Mägdefrau. 16. _Pinus._ Modern pine. After Mägdefrau. 17. _Acer._ Common maple, an angiosperm whose leaves are also found among Tertiary fossils. After Mägdefrau. 18. _Rosa._ The prairie rose, an angiosperm. 19. _Campsis._ Trumpet vine, an angiosperm.

_Time Units_ _Era and Years_ CENOZOIC “Recent Life” Pleistocene Pliocene Miocene Oligocene Eocene Paleocene MESOZOIC “Middle Life” Cretaceous 70 million herbaceous lycopods Jurassic 25 million yellow-green algae selaginellids Triassic 30 million cycad-like plants Equisetum PALEOZOIC “Ancient Life” Permian 25 million transition conifers pines, spruces, firs, etc. Equisetites Pennsylvanian 25 million ferns related to modern families bryophytes Mississippian 30 million seed ferns calamites cycads Devonian 55 million cordaites sphenophyllids ancient ferns Silurian 40 million psitopsids Foerstia Ordovician 80 million red algae green algae Cambrian 80 million PROTEROZOIC _and_ ARCHAEOZOIC ERAS 4½ billion years blue-green algae chemosynthetic bacteria? NO CERTAIN FOSSILS KNOWN _Plants_ Algae yellow-green algae brown algae red algae green algae blue-green algae Mosses bryophytes Foerstia Vascular Plants “Whisk Ferns” psitopsids Club-mosses scale and seal trees quillworts herbaceous lycopods Scouring Rushes, Horsetails Equisetites Equisetum calamites selaginellids Ferns and Seed Plants Ferns ancient ferns ferns related to modern families Gymnosperms Cycads seed ferns cycads cycad-like plants Ginkgo Conifers cordaites transition conifers pines, spruces, firs, etc. Flowering Plants

_Field Book_ PENNSYLVANIAN PLANT FOSSILS OF ILLINOIS

Charles Collinson and Romayne Skartvedt

Plants that flourished 200 million years ago have made Illinois one of the best known fossil collecting sites in the world. The unusual abundance and preservation of these fossils in the northern part of the state have brought collectors to Illinois from many countries, and prized specimens from that area may be seen in science museums throughout the world.

The remarkable fossils represent plants that lived during the geologic period called the Pennsylvanian or Coal Age and are the result of special geologic conditions that occurred repeatedly during the period.

At the beginning of the Pennsylvanian Period, Illinois was part of a vast lowland that stretched for hundreds of miles to the north, south, and west, and was bordered on the east by highlands. At times much of the plain was swampy and, because the climate was relatively warm and moist, great jungles of fast growing trees, shrubs, and vines covered the landscape. As successive generations of plants lived and died, plant material fell into the swamp waters and, protected there from decay, accumulated.

Frequently during the period, seas spread over the swampy lowlands, submerging the forests and covering them with mud. Each submergence lasted only a short time, geologically speaking. When the seas withdrew, the deposits of sand and mud left behind were cut by streams that carried fresh sand and mud from the eastern highlands. The streams eventually became clogged with sediments and when the lowland was again depressed swamp conditions returned and forests grew afresh. Such a cycle of deposition was repeated again and again during Pennsylvanian time, and after burial each layer of plant material gradually lost most of its liquids and gases and was slowly converted into one of the numerous coal beds presently found in Illinois.

In some places in the state conditions existed that were especially favorable for preservation of plants, and there delicately preserved fossils are found in great numbers. In the most favorable areas, such as in northern Illinois, the plants are preserved in stony nodules called concretions, but they also may be found separately as molds, casts, or petrifactions.

Molds (concave surfaces) and casts (convex surfaces) are fossilization phenomena in which the actual plant, embedded in the surrounding background rock, was dissolved, leaving a hollow space (mold) that subsequently filled with other material. A cast was thus formed that preserved the plant’s external features.

Most petrifactions are fossils in which silica, carbonate, or other material permeated or replaced the internal structures of the plant and preserved them so well that in most specimens the finest cellular details can be observed. Compressions, another kind of petrifaction, are the pressed carbonized remains of the plant itself.

PENNSYLVANIAN FLORA

The far-reaching Pennsylvanian swamplands had abundant species of trees and other plants that long since have become extinct. Today’s common deciduous trees were not present; flowering plants had not yet evolved. Instead, the tangled forests were dominated by giant ancestors of presently existing club-mosses, horsetails, ferns, conifers, and cycads. The undergrowth also was well developed, consisting mainly of ferns, fernlike plants, _Sphenophyllum_, and small club-mosses. The plant fossils give no indication of seasonal variations. The forests, evidently always green, grew rapidly and abundantly, with foliage of unprecedented size and luxuriance. Land animals were just beginning to develop and included sluggish, salamander-like amphibians, large primitive insects, and a few small reptiles. The insects flourished as never before or since in the damp forests and attained remarkable size. Insects more than four inches long were common and some are known to have been more than a foot long with a wingspread proportionately broad. Ancestors of the modern spiders, scorpions, centipedes (one fossil found in Illinois was twelve inches long), cockroaches, and dragonflies are represented by several hundred species.

The fossilized plants of Pennsylvanian time belonged to only a few main categories: scale and seal trees, ancient scouring rushes (horsetails), herbaceous _Sphenophyllum_, ferns, seed ferns, and cordaitean trees.

SCALE AND SEAL TREES (Plate 1)

Scale and seal trees were abundant during the Pennsylvanian Period and were important contributors to coal beds. Although distantly related to the diminutive club-mosses and ground pines of the present, the trees grew on straight, slender trunks to heights of more than a hundred feet.

Scale trees were so called because their numerous, closely set, spirally arranged leaves left scarred “cushions” on the branches and trunk, making them appear scaly. Seal trees derived their name from the signetlike appearance of their leaf cushions. The two best known types belong to the genera _Lepidodendron_ (scale tree) and _Sigillaria_ (seal tree), and fossils of both are common in Illinois.

_Lepidodendron_ had long, slender, somewhat tapering trunks. Some of the trees reached heights of more than 100 feet and measured more than two feet in basal diameter. The trunk ended in a spreading crown formed by repeated dichotomous branching. The leaves were awl-shaped or linear, ranging from one to 30 inches long.

The leaf cushions of _Lepidodendron_ are diamond-shaped, longer than broad, and arranged in spiral rows around the trunk and branches. A different name, _Lepidophyllum_, is used for fossils of the long, bladelike leaf when it is found detached.

Spores were borne in long cylindrical cones at the tips of the branches. Those cones referred, or assigned, to the genus _Lepidostrobus_ bore both small spores (microspores) and large spores (megaspores) in the same cones. Those in which only a large single spore, a somewhat seedlike structure, was developed in a spore sac (sporangium) are referred to the genus _Lepidocarpon_.

The rather commonly found genus _Stigmaria_ comprises so-called “appendages” which, although stemlike in structure, apparently served as roots for the scale and seal trees. These appendages are identified by irregular spirals of circular scars (pits) that mark the attachment points of former rootlets.

_Sigillaria_, although less common than _Lepidodendron_, was widely distributed during the Pennsylvanian Period. It differed in growth habit from _Lepidodendron_ in that it generally had fewer branches and not uncommonly was unbranched. Some species also possessed a thicker trunk, with hexagonal to elongate leaf cushions separated by vertical ribs. The trunk was crowned, in the manner of the modern palm tree, by a cluster of large, grasslike leaves.

The detached leaves of _Sigillaria_, extremely difficult to distinguish from _Lepidophyllum_ (leaves of _Lepidodendron_), are referred to the genus _Sigillariophyllum_ if preserved as compressions and to _Sigillariopsis_ if preserved as petrifactions. Unbranched _Sigillaria_ trunks have been found that are more than 100 feet long and six feet in diameter near the base, but the average height probably was closer to 50 feet.

Not all Pennsylvanian trees were large, however. Small forms are known, including the important undergrowth genera _Lycopodites_ and _Selaginellites_. In woody types the trunk consisted of an inner region of conducting and supporting tissues, surrounding concentric cortical layers, and an outer layer of corklike bark. Although the fossil impressions of the various bark layers have been given separate generic names, these are not commonly used.

SCOURING RUSHES (Plate 2)

Although related to the small, inconspicuous horsetails of today, the ancient scouring rushes of the Pennsylvanian Period grew to the size of trees and were among the most widely distributed plant groups.

Some of these plants attained heights of 40 feet or more, but the average was closer to 20 feet. The trunks were jointed and bore a whorl of branches at the joints (nodes). Their small leaves also grew in whorls at nodes along the smaller branches. Internodal regions were ribbed in the same manner as present day horsetails. Fossils of the trunks are assigned to the genus _Calamites_ and quite commonly are preserved in sandstone and shale.

The leaf whorls are placed in the genus _Annularia_. One form commonly found in Illinois has long, pointed, needlelike leaves and is given the name _Asterophyllites_. _Calamostachys_, shown on plate 5, is one of the most common calamite cones.

SPHENOPHYLLUM (Plate 2)

The name _Sphenophyllum_ refers to both stems and leaves of this extinct genus, which was related to the scouring rushes—note its resemblance to _Annularia_.

A small herbaceous plant, _Sphenophyllum_ formed much of the swampy undergrowth of the Pennsylvanian Period and is abundant among Illinois fossils. It had a slender, ribbed stem bearing whorls of delicate, wedge-shaped leaves, generally less than three-fourths of an inch long, attached around the stem in multiples of three.

The cones of this group also are slender, delicate structures, bearing a number of sporangia, and are correctly called _Bowmanites_, although they also have been called _Sphenophyllostachys_. These fossil cones frequently are found in Illinois.

_Sphenophyllum_ first appeared during the Devonian Period, some 300 million years ago, but did not become abundant until Pennsylvanian time. The genus continued through the Permian but died out in Triassic time.

FERNS (Plates 1 and 3)

True ferns, like those living in today’s woodlands, were common in the Pennsylvanian forests. Some species attained heights of 30 to 40 feet. Their fronds (compound leaves divided into segments or leaflets) commonly were five to six feet long.

True ferns do not produce cones or seeds, but spores, which develop in cases called sporangia. The sporangia frond showing are attached in clusters (sori) to the lower side or margins of the leaves. In modern ferns the sporangia may also occur on fertile spikes.

The shape and position of the sori are used to identify modern ferns, but because leaves that bear sori (“fertile” leaves) are rare among fossil specimens, the number, shape, and attachment of the leaflets and the pattern of the veins are more commonly used for identification.

Because fossils of complete fern plants have not yet been found, separate names have been adopted for detached leaves, stems, and other parts. For example, the fossil stems of some Pennsylvanian ferns found in Illinois have been referred to two genera, _Megaphyton_, whose leaf attachment scars are arranged in two vertical rows, one on either side of the stem, and _Caulopteris_, whose leaf scars are arranged in a steep spiral that becomes progressively flatter upward until near the top they appear to be whorled. When the stem is a petrifaction, with internal structures preserved, it is called _Psaronius_. The fronds are referred to a number of genera, but those most commonly found in Illinois are _Pecopteris_, _Asterotheca_, and _Ptychocarpus_.

_Pecopteris_ _Asterotheca_ _Ptychocarpus_

SEED FERNS (Plate 4)

Seed ferns resembled true ferns in general, but they produced seeds, borne on modified leaves. Where spore sacs and seeds are absent, the leaves of seed ferns are difficult to distinguish from those of spore ferns, although individual seed fern leaflets, called pinnae, are somewhat larger.

Seed ferns included vinelike plants in the undergrowth and trees such as _Medullosa_. Some tree genera were very tall, with trunks more than two feet in diameter. Unlike the true ferns, still living today, seed ferns declined steadily after the close of the Pennsylvanian Period and finally became extinct during Jurassic time. During Pennsylvanian time, however, they were much more numerous and varied than true ferns.

_Alethopteris_ _Odontopteris_ _Mariopteris_ _Neuropteris_ _Linopteris_

Most of the common seed ferns found as fossils in Illinois can be referred to the following leaf genera: _Alethopteris_, _Neuropteris_, _Odontopteris_, _Linopteris_, _Mariopteris_ (which may be a true fern), _Cyclopteris_, and _Spiropteris_. _Cyclopteris_ includes circular leaves that occurred at the base of leaves referable to _Neuropteris_. _Spiropteris_ includes young leaves that had not yet uncoiled and may belong to either true ferns or seed ferns.

CORDAITES (Plates 1 and 2)

Cordaitean trees, forerunners of modern conifers such as pine and spruce, were important during the Pennsylvanian Period for they were distributed throughout the world. These trees, among the tallest plants of the time, sometimes grew more than 100 feet high.

The cordaitean trunk was unbranched for three-fourths of the height of the tree and was topped by dense branches bearing large, simple, straplike leaves spirally arranged. The leaves had closely set parallel veins and measured from half an inch to three feet or more long.

Internally, the structure of the trunks was similar to that of modern pine trunks. Casts of the pith are referred to the genus _Artisia_. The seeds were borne in clusters on branches in leaf axils.

The _Cordaites_ were major contributors to some coal beds.

FRUITING BODIES (Plate 5)

Fossils representing many kinds of plant reproductive structures are found in Pennsylvanian rocks, but unfortunately most of them are not attached to any identifiable part of the parent plant and they cannot be assigned definitely to a particular plant. Such fossils are referred to genera and species solely on the basis of their own characteristics, although, as in other fossil classifications, such “form genera” are presumed to be parts of, or related to, the plants with which they are found in habitual association.

A few such fossils, fairly common in Illinois, are illustrated on plate 5 to show their general shape and size. When attached to an identifiable leaf or leaflet, the seed is referred to as the seed of that leaf genus.

For example, _Holcospermum_, a radially symmetrical seed with ribs and grooves, _Codonotheca_, a stalked, spore-bearing, lobed “cup,” and _Neuropterocarpus_, a flask-shaped seed with longitudinal ribs and grooves, all have been associated with _Neuropteris_, a leaf genus.

_Trigonocarpus_, commonly found as a cast of the internal part of a seed, is a trimerously symmetrical body frequently associated with _Alethopteris_. _Pachytesta_ includes preserved structures and outer layers of a seed. _Carpolithes_ is a catch-all “genus” functioning as a general term for seeds and seedlike forms whose plant group affinities cannot be determined.

COLLECTING AREAS FOR PENNSYLVANIAN PLANTS Northern Illinois

Plant fossils can be found in almost any northern Illinois area where Pennsylvanian rocks are exposed (see back cover), but in some places they are much better preserved and more numerous than in others. Most of the well known collecting areas and a few of the lesser known ones are discussed below. Even though some of the localities were discovered many years ago, they may indicate areas that are still favorable for collecting.

Mazon Creek Area

Of all the fossils that have been found in Illinois, the most famous are the plant remains from the world-renowned Mazon Creek area in the northeastern part of the state. In this area in Grundy and Will Counties, plant fossils are found in ironstone concretions in the lower part of the Francis Creek Shale directly overlying the Colchester (No. 2) Coal.

Fossils were discovered in outcrops along Mazon Creek more than a century ago and collections later were made from scores of conical spoil heaps at underground mines. After coal stripping began in the 1920’s, great numbers of specimens were collected.

In the stripping operations, the concretion-bearing beds are commonly the last to be placed on the spoil heap. Weathering softens and removes the shales and leaves the nodules concentrated on the surface. Each season brings a new crop of concretions to the surface.

_Calamites_ ⅓× _Stigmaria_ ⅖× _Lepidodendron_ ⅖× _Calamites_ ¹/₁₀× Fern Stem ⅔× _Caulopteris_ ¼× _Sigillaria_ ⅖× _Megaphyton_ ⅗× _Calamites_ ⅗× _Artisia_ ⅗× _Sigillaria_ sub-bark ⅗×

The concretions generally are oval to elongate and range from less than an inch to a foot or more in maximum dimension. Only about one nodule in ten contains plant remains.

Approximately 25 to 30 species have been found in this region. The productivity of the area was shown by George Langford, Sr., a well known midwestern fossil collector. He and his son split about 250 thousand concretions during a 140-day period and obtained some 25 thousand plant specimens. Fine specimens still can be collected in a few hours.

The plant collecting localities in Will and Grundy Counties along Mazon Creek, four to six miles southeast of the town of Morris, were the first to be well known. Ferns are especially abundant. Fossils of insects, crustaceans, worms, and salamanders also have been found. Collecting conditions vary considerably from season to season, and fossils are not as easily obtained there as from the strip-mine spoil heaps.