Part 3

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.

Fossiliferous concretions may be found in a number of the strip mines in the area, although probably most have come from the Northern Illinois Coal Corporation mine between the towns of Braidwood and Wilmington.

In earlier years good collections were made from the spoil heaps of underground mines. Especially notable are the mine dumps of the Wilmington Star No. 7 mine, 2¼ miles west of Coal City, and Skinner No. 2 mine, two miles northeast of Braidwood.

In the vicinity of Morris on the northwest edge of the Mazon Creek area, fossil ferns have been found along the north side of the Illinois River and in the banks of the Illinois-Michigan Canal. About a mile north in an area of strip mining, fossil-bearing concretions have been found in shale and irregular sandstone layers.

Sphenophyllum ⅗× Lepidostrobophyllum ⅗× Annularia ⅗× Sphenophyllum 1× Lycopodites ⅗× Cordaites ⅗× Asterophyllites ⅓×

Fossils in concretions also have been collected from a shaly limestone at the south end of the Kankakee River bridge along the Grundy and Will county line.