[Illustration: _Aphthoroblattina_]
[Illustration: _Teneopteron_]
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.
[Illustration: Reconstruction of _Lepidodendron_
(after Hirmer)]
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.
[Illustration: Reconstruction of _Sigillaria_
(after Hirmer)]
_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.
[Illustration: Reconstruction of _Calamites_
(After Hirmer)]
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.
[Illustration: Portion of fern frond showing sori on lower side of
leaflets]
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.
[Illustration: Reconstruction of _Megaphyton_
(after Hirmer)]
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_.
[Illustration: Venation of seed fern leaflets]
_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.
[Illustration: _Medullosa_ Reconstruction and original drawing by
Wilson N. Stewart]
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.
[Illustration: Venation of seed fern leaflets]
_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)
[Illustration: Reconstruction of _Cordaites_
(after Hirmer)]
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.
[Illustration: Mazon Creek Strip Mine Area Showing Distribution of
Spoil Heaps. The small circular areas represent waste from
underground mines.]
_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.