A Forest of the Coal Age

B. E. DAHLGREN
Acting Curatok, Dbi-aktmbnt of Botany

Geology
Leaflet 14

HELD MUSEUM OF NATURAL HISTORY

CHICAGO, U. S. A.
1933

^

LIST OF GEOLOGICAL LEAFLETS ISSUED TO DATE

No. I. Model of an Arizona Gold Mine $ .10

No. 2. Models of Blast Furnaces for Smelting Iron . .10

No. 3. Amber — Its Physical Properties and Geological

Occurrence 10

No. 4. Meteorites 10

No. 5. Soils 10

No. 6. The Moon 10

No. 7. Early Geological History of Chicago 25

No. 8. Agate — Physical Properties and Origin ... .50

No. 9. How Old Are Fossils? 15

No. 10. Famous Diamonds 25

No. 11. Neanderthal (Mousterian) Man .15

No. 12. Cement 15

No. 13. The Geological History and Evolution of the

Horse 40

No. 14. A Forest of the Coal Age 25

STEPHEN C. SIMMS, Director

FIELD MUSEUM OF NATURAL HISTORY
CHICAGO, U.S.A.

UNIVERSITY Or

ILLINOIS LIBRARY

M URBANA-CHAMPAiOr

BOOKSTACiv^

Leaflet 14

Plate I

DETAIL OF THE CARBONIFEROUS FOREST
RESTORATION IN FIELD MUSEUM

The large tree in the center background is a tree-like clubmoss,
Sigillaria, partly hidden by the trunks of a Lepidophloios and a
tree-fern with large leafscars on the stem and concealed above by
fronds of this fern (Psaronius) and foliage of Lepidodendron. Ascend-
ing the Lepidophloios trunk is a seed-fern (Lyginopteris) with delicate
forking fronds, one of which is shown bearing seed (Lagenostoma).
The strap-shaped foliage in the upper left corner is that of the
gymnosperm Cordaites and below on the left are two seed-ferns
{Neuropteris decipiens above, N. heterophylla below) each presenting a
seed-bearing frond. The small plant at the bottom center and right
is Sphenophyllum. The large fallen trunk is a Sigillaria with a small
herbaceous clubmoss, Selaginellites, spreading over its surface. The
slender log on the ground is a Lepidodendron with rhombic leaf
cushions. On this trunk is seen one of the large roaches of the period
and beyond is one of the four-legged inhabitants of the swamp forests,
a Carboniferous amphibian.

UNIVERSITY OF

ILLINOIS LIBRARY

AT URBANA-CHAMPAIGN

BOOKSTACKS

Field Museum of Natural History

DEPARTMENT OF GEOLOGY
Chicago, 1933

Leaflet Number 14
Copyright 1933 by Field Museum of Natural History

Fig. 1. Unger's landscape of Carboniferous time.

PREFACE

Various attempts have been made by paleontologists
to picture the character of the Carboniferous flora. One
of the first was that of linger in the series of plates
published in Vienna in 1847 under the title "Die Urwelt
in ihren Verschiedenen Bildungsperioden." His land-
scape, reproduced above, will be recognized by many,
for it has been widely copied in texts on geology. In
the middle of the nineteenth century, however, the
Carboniferous vegetation was still very imperfectly known.
As paleobotanical science advanced other attempts were
made on the basis of a better understanding of the plants

Field Museum of Natural History

Fig. 2. Landscape of Carboniferous time. Restoration by Potonie.

Fig. 3. Carboniferous vegetation. Restoration by Potonie and Gothan.

A Forest of the Coal Age 3

composing the coal flora. The latest and most satisfactory
of these are the paintings made under the direction of
the famous paleobotanist Potoni^. One published in
1898 in the form of a chart, issued under the auspices
of the Prussian Geological Survey and School of Mines
of Berlin, is reproduced here (Fig. 2). A still later repre-
sentation of a Carboniferous landscape has since been
sponsored jointly by Potoni^ and Gothan and has been
republished within a year or two in revised form (Fig. 3).
An extensive landscape can be represented on paper
or on canvas with great freedom with details unknown
or imperfectly understood treated in a noncommittal
manner. In a three-dimensional restoration, such as
that in Field Museum, intended to be seen at close range,
this large amount of freedom from troublesome questions
does not exist. On the contrary it becomes necessary
to commit oneself at every point and to determine, cor-
rectly or incorrectly, every particular in sight. The most
that can be left to a painted background is a suggestion
of general landscape features with the continuation and
extension in space of that which is definitely shown,
natural size, in the foreground.

The Museum "group" to which this leaflet refers is
composed of an assemblage of three-dimensional recon-
structions, contained within the space of 28 x 15 x 20 feet
that it occupies. These reconstructions embody as far
as possible the best available present-day knowledge of
Carboniferous plants, as in the case of Lyginopteris and
of Kidston's Neuropteris heterophylla. All essential details
of the plants restored were obtained from fossil specimens
existing in Field Museum collections or loaned by other
institutions. In many cases the surface pattern of the
tree trunks, form and surface markings of leaves, were
produced directly from impressions of the fossils and
then mechanically multiplied to the extent required.
The technique developed in the laboratories for plant
reproductions maintained in the Museum by Mr. Stanley

4 Field Museum of Natural History

Field for the purpose of providing botanical exhibits, has
made possible the execution of the restorations seen
in this group. Throughout the progress of the work
the writer, under whose direction this group has been
produced, has enjoyed the inestimable advantage of
the advice and cooperation of Professor A. C. No6 of the
University of Chicago, Research Associate in Paleobotany
in Field Museum, whose large and intimate acquaintance
with the Carboniferous flora has made him an authority
in this field. The Museum is indebted to Dr. R. S.
Bassler of the United States National Museum and to
Dr. A. S. Romer of the Walker Museum, the University
of Chicago, for the loan of important material. No other
special acknowledgments are considered necessary except
as made in connection with text figures.

The appended list of popular works and the more
readable and accessible technical ones on plants of the
past will be found to contain ample references to the
voluminous paleobotanical literature which exists on
the subject of the ancient coal plants.

A FOREST OF THE COAL AGE

To most people coal is but a black mineral dug out
of the earth, of interest only because it is useful to bum
in the stove or furnace or under the boilers of ships and
factories. To the geologist it is the remains of a vegeta-
tion that flourished ages ago, that may be studied by
the paleobotanist, and even reconstructed in all its detail.

Ever since land plants gained possession of the surface
of the earth, plant material has accumulated at various
times in different places, and has been preserved from
decay and gradually converted into peat or coal. Many
coal beds have thus originated in relatively recent times,
but none of these compares in importance or extent with
those dating from that vastly remote age known as the
Carboniferous, especially from its latter part, the so-
called Pennsylvanian period, some 250,000,000 years ago,
toward the end of the Paleozoic era.

The world's first great land flora was then reaching
its culmination in the swampy rain forests that covered
much or most of the land exposed in the northern hemi-
sphere. In Europe and in northern Asia as in North
America these forests existed for millions of years, giving
rise to the vast accumulations of plant material that in
the course of time have been compressed and changed
into coal and now constitute the world's chief bituminous
and anthracite deposits. In North America these Carbon-
iferous forests extended from Nova Scotia more or less
interruptedly through Rhode Island along the Appalachian
coal field southward to northern Alabama and to Texas
and westward through Nebraska along the present Rocky
Mountain region even to Alaska. Throughout much of
this great area the formation has become completely
covered up in the course of subsequent geological events,
and in places removed, but various large and distinct
coal-bearing areas remain. The most widely known of

6

Field Museum of Natural History

Fig. 4. Map showing the areas where the Pennsylvania!! system appears at the
surface in North America. The map also shows the areas where the Pennsylvanian
system is thought to exist though buried (lined areas); the areas where it is thought
once to have existed, but to have been removed (dotted areas); and by inference
the relations of land and sea during the Pennsylvanian period. After Chamberlain
and Salisbury.

A Forest of the Coal Age 7

these is the Appalachian, including the Pennsylvanian and
West Virginian coal fields. Another, known as the Middle
Central coal field, includes Illinois, Indiana, and part of
Kentucky. A third, known as the Western field, com-
prises parts of Iowa, Nebraska, Kansas, Missouri, and
Texas. In Europe the Carboniferous forests were some-
what less extensive but covered large parts of the British
Isles, Belgium, parts of FYance, Germany, Poland, and
Russia. In northern Asia, in Siberia, and especially in
China, they were also extensive. Their remains testify
to the general prevalence of the same kind of vegetation
over an immense area under practically uniform conditions
for an enormous length of time.

Essentially a flora of the northern continents, some of
it in the course of time extended southward, crossing the
equator and even reaching remote places in the southern
hemisphere. Though this, at the time or soon afterwards,
was overrun by a similar but much less luxuriant flora of
its own, fossils of Carboniferous plants of the northern
flora have been recovered as far south as Sumatra in the
Old World and Peru and southern Brazil in the New.

It is certain that the face of the world at that distant
period presented an appearance very different from that
of the present day. There were no grass-covered prairies,
no hillsides or mountain slopes with flowering plants in
summer, no woods of broad-leaved flowering trees and
shrubs such as constitute our modern forests in the
temperate as in the tropical zone. None of these had
yet made their appearance. The abundant and even
rank vegetation which existed was made up of lower,
flowerless orders, such as our horsetails, ferns, and club-
mosses, now mostly insignificant plants but then forming
a great and luxuriant flora. The badly drained land was
dotted with bogs and swamps in which flourished stands
of great horsetail rushes. Spreading endlessly, like giant
canebrakes, over the marshy ground of a flat continent
they disputed the possession of the land with forest-like

8 Field Museum of Natural History

formations of huge clubmosses and early gymnosperms
that then attained the dimensions of forest trees. Ferns
and fern-Hke seed plants formed the undergrowth and
as climbers or clambering plants took the place of lianas.
In these rain forests, which were constantly green, growth
was continuous and probably rapid.

The trunks, even of the forest giants of the time,
produced less of solid substance than might be expected
from their size, for they consisted chiefly of a thin cylinder
of wood filled with soft pith. All, however, eventually
littered the ground with their debris and added to the
accumulation of dead plant material.

Naturally, not all of the forest litter was preserved.
A large part of it decomposed to form humus but the
fact that so much was converted into coal indicates not
only a very luxuriant growth during a period lasting
millions of years, but also the prevalence of extremely
favorable circumstances for its preservation, i.e., a mini-
mum of decay. This points to the existence of swamp
conditions, an abundance of rainfall and of standing water,
and an acid or sour subsoil such as is generally associated
with a lack of ready surface drainage. Such conditions
are also indicated by peculiarities of the plants them-
selves, the structure and the horizontal and superficial
disposition of their spreading root system, or perhaps,
more strictly speaking, the rhizomes of the large trees,
which, though interlacing with those of their neighbors,
must have offered a relatively poor support for the tall
trunks. So peculiar is the appearance of the system of
roots that form the support of the trees of the clubmoss
order, that it was once seriously argued that the branching
roots, which are full of air spaces, must have acted as a
raft and served to support a floating forest. One may,
however, feel confident that most of the forest was well,
though superficially, anchored in the mud.

Individual coal seams ordinarily range in thickness
from a few inches to two feet or more, at times to as much

A Forest of the Coal Age 9

as fifty feet. They are usually underlaid by clay and
separated by intervening layers of sandstone or shale,
for the growth of the vegetation in any one place was
apt to be discontinuous, being interrupted from time to
time by more or less extensive subsidence and submergence
of the forest during which the deposition of sand or mud
took place, covering up the accumulated debris. On
renewed elevation the vegetation would again spread over
the land. A mass of forest litter which has given rise to
two feet of coal must have had from twenty to thirty
times that thickness when it was laid down. One pro-
ducing fifty feet of coal must have been prodigious,
probably a quarter of a mile thick.

One theory of coal formation would explain all of the
coal as due to the accumulation of drift deposited in river
estuaries. This is the so-called "drift theory" which has
been upheld as against that of the in situ formation
of coal. Much evidence has been presented for both views
and there can be little doubt that coal was formed in
both ways, but by far the greater part of it where it grew.
The present consensus of opinion seems to be that the
coal forests were formed under conditions precisely such as
exist in the estuaries of some great river system, and that
their nearest present-day counterparts are to be sought
in places such as the Dismal Swamp in Virginia, the
brackish cypress swamps of southern Florida, the swampy
rain forests at the mouth of the Amazon, or the Sundar-
bans in the delta of the Ganges and the Brahmaputra.

It may be asked how such conditions could prevail
over large areas in the interior of the continents. The
presence of brackish water shells in association with coal
seams indicates the invasion of the coal-forming areas by
arms of the sea. These extended far into the interior of
the present continental areas, receiving the excess water
from the rivers and exercising a profound influence on
the climate. In Illinois there is evidence, according
to Professor No6, of more than sixty such invasions by

10

Field Museum of Natural History

gulfs of the sea. Much has been written on the subject
of the climatic conditions of the Carboniferous age, the
great humidity prevailing, the generally murky atmos-
phere, the large amount of precipitation, and the large
quantity of carbon dioxide in the air. It is generally
accepted that the climate was warm and moist, without
changes sufficiently marked to interfere with continuous
growth or large enough to impress on the structure of
the vegetation any perceptible indication of a seasonal
periodicity.

Fig. 5. An impression in Pennsylvanian sandstone of the surface of the trunk of
one of the Sigillarias or ribbed-stem clubmoss trees of Carboniferous forests. Above
each of the five leaf cushions included in this fossil impression is seen the leafscar
proper with its two distinct smaller scars, those of the vascular bundles that entered
the leaf.

A Forest of the Coal Age

11

For our knowledge of the botanical character of the
Carboniferous flora we are dependent on its fossil remains,
especially the casts and impressions preserved in the layers
of shale or sandstone that separate and cover the coal
beds. The plant remains in the coal itself have for the
most part been so greatly modified and compressed that
individual parts are not readily distinguishable. The
fossil remains are very numerous. They abound on the
mine dumps in all coal areas. Unfortunately they are
always fragmentary, but, thanks to the painstaking work
of many paleobotanists in many parts of the world, there
is available at present a large body of knowledge concern-
ing the plants of that time. One is thus able to obtain
a very definite idea of the character of many of the species
which formed part of the Carboniferous vegetation and,
on the whole, to arrive at a very fair conception of the
general constitution of the flora which covered the earth

Fig. 6. Lepidophloios acadianos Dawson. A sandstone impression of another
order of clubmoss trees of the Carboniferous forest. The scale-like leaf cushions are
broader than long and closely crowded.

12

Field Museum of Natural History

at a time when feeble-limbed amphibious vertebrates were
first emerging from the waters of the swamp.

This detailed knowledge of the ancient flora makes it
possible to restore with a considerable degree of confidence
a typical assemblage of plants of Carboniferous time.
This is what has been done in the preparation of the group
which forms a part of the historical geology exhibits in
Ernest R. Graham Hall in Field Museum. A selected
number of the most common and typical plants of the
Pennsylvanian flora, especially as this is represented in
the Middle Pennsylvanian Series of Illinois and adjoining

Fig. 7. A split sandstone nodule from Mazon Creek, Illinois, with an included
fossil leaf ( Neuropteris decipiens), a fragment of a frond of the large seed-fern shown
in the group.

A Forest of the Coal Age 13

states to the east, has been restored to three-dimensional
form from the flattened impressions and casts in the rocks
which constitute the fossil record. So many of the fossils
on which these reconstructions are based are from Mazon
Creek, Illinois, and of the others so many are to be found
there, that the group may be considered a restoration of
the Carboniferous vegetation of that famous locality,
which has yielded so much fossil material.

The exhibit in Field Museum shows a scene in a dense
swamp forest at the margin of a stretch of shallow water
which may be an arm of an inland lake, the quiet back-
water of a large watercourse, or a lagoon at the estuary
of a slow-flowing river, a locality where stagnation prevails
rather than movement — a landscape which may be con-
sidered typical of the conditions under which the flora of
the period reached its climax. Of an upland flora of the
Carboniferous age nothing is known. If it existed, it was
probably much less luxuriant and consisted of species
adapted to a drier environment or able to endure it,
such as those elements of the flora that survived into
the next period. The Pennsylvanian flora of which there
are fossil remains, was of a kind that flourished only in
the presence of abundant moisture.

The margins of the watercourses, the shallow lakes
and estuaries, were bordered with dense formations of
calamites, huge horsetails resembling in character the
present-day scouring rushes (Equisetum), but then tree-
like forms with wood and bark. They approached bamboos
in height and probably in rapidity of growth and rivaled
or exceeded the giants among present-day bamboos in
diameter. The term calamites (from calamus, a reed)
was originally applied only to the remains of stems, the
cylindrical casts of the hollow, ribbed interior from which
the pith had disappeared, or flattened impressions showing
the jointed exterior surface. Many kinds have been
distinguished and described according to the external
characters of the stem. Some have numerous branches

14

Field Museum of Natural History

at all or at most of their nodes, others at certain regular
intervals only, others again appear to have had few or
no branches. Some had foliage branches of one order
only directly on the main stem, others branched repeatedly

and profusely, producing a
bushy or graceful tree-like
top. The leaves were al-
ways borne in whorls. Two
distinct types of foliage
were common. One con-
sisted of filiform, jointed
leaves in dense whorls at
the nodes of the slender
ultimate branches (Astero-
phyllites), the other of flat-
tened leaves in whorls of a
stellate shape (Annularia).
The latter are particularly
abundant at Mazon Creek
and are to be seen on the
calamites in the group.
Like their smaller modern
relatives, the horsetails and
scouring rushes, the cala-
mites had extensive under-
ground rootstocks. They
were reproduced by spores
formed in cones or catkin-
like spikes, usually situated
at the tips of the foliage-
bearing branches.

The great horsetail
rushes were greatly ex-
ceeded in size by the trees

FIG. 8. A fragment of a branch of a cala- that fOHned the bulk Of

mite in a Carboniferous sandstone nodule, i-Un tTarrai-n-i-\n-n TTrtyamnai-

showing the type of calamite foliage (Annu- l^^e VegCiailOn . r OI eillUb L

laria radiata) most common at Mazon orvir^nn- fViooo axtovo fV»A plnVk-

Creek, Illinois, and represented in the group. among LUetse VVCl tJ Llie UlUU

A Forest of the Coal Age

15

Fig. 9. A fossilized fragment of the armor of leaf cushions that invested the trunks
of scaly-stemmed clubmosses of the Carboniferous age {Lepidodendron clypeatum
Lesquereux).

mosses of many species that attained the size of large
forest trees. They sometimes reached six feet in diameter
and a hundred feet in height. With their columnar trunks
fluted or ornamented as if by a sculptured pattern, with
their unbranched or sparsely branched tops terminating
in a single tuft or in a canopy of equally and repeatedly
forking branches bearing grass-like foliage, and with their
horizontally extended and regularly forking roots, these
big clubmosses must have set their somber stamp on the
entire landscape. Those in the background of the group
in the Museum may be seen with their rootlets exposed
at the edge of the water.

The two principal types of these characteristically
Carboniferous trees are Lepidodendron and Sigillaria. The
former take their name from the armor of rhombic leaf
cushions (Figs. 9, 10) that covered the trunks and indicate
the spiral arrangement of the leaves. The latter are so-

16

Field Museum of Natural History

Fig. 10. An impression in sandstone of the Pennsylvanian period of the surface
pattern on the trunk of an ancient clubmoss {Lepidodendron obovatum Sternberg).
Mazon Creek, Illinois.

A Forest op the Coal Age

17

l^-'iP^/ ?i^

t \l

Fig. 11. A slab of Carboniferous limestone from Mazon Creek, Illinois, showing
fossil fern fragments and a large fossil clubmoss cone (Lepidostrobus ovatifoliua
Lesquereux) .

18

Field Museum of Natural History

called because of the seal-like impressions of their persistent
leaf scars, generally in vertical rows on the ribbed or smooth

stem (Figs. 5, 12).
There were other forms
intermediate between
these, and still others
distinguished by vari-
ous kinds of character-
istic sculpturing on the
surface of their stems,
such as the forking
Lepidophloios (Fig. 6),
near the center of
the group. High on
the trunk and large
branches of this are
clumps of short, leafy
shoots, terminating in
a spore-bearing cone.
Such masses of cones
were probably pro-
duced periodically,
were shed and replaced
by a new crop of shoots.
The Sigillarias also
produced their cones
on the trunk, differing
in this respect from the
freely branching Lepi-
dodendrons that bore
their foliage and cones
(Fig. 11) at the tips of slender terminal branches.
Clubmoss cones must have appeared in great profusion
and produced their pollen-like spores in enormous quan-
tities, for masses of spores constitute an important element
of the plant material that was converted into coal. Indeed,
certain coals seem to be made up entirely of spores.

Fig. 12. Sigillaria saulli. A cast from an im-
pression of the stem of another ribbed-stem Penn-
sylvanian clubmoss. The leafscars are closer set
than in Fig. 5. The small scars of the pairs of
vascular bundles that entered each leaf are plainly
visible.

Leaflet 14

CARBONIFEROUS SWAMP FOREST GROUP

A restoration of plant and animal life of the Carboniferous period
especially as this is represented in the fossil remains of the Upper
Pennsylvanian in the Middle Central coal field at Mazon Creek,
Illinois. The forest formation of giant clubmosses, ferns and fern-

Plate II

[N FIELD MUSEUM OF NATURAL HISTORY

like plants on the left is repeated in the distance. On the right is seen
a solid formation of the large Paleozoic horsetails known as calamites.
The group occupies a space 28 by 15 by 20 feet, and forms a part of
the historical geology exhibits in Ernest R. Graham Hall.

A Forest of the Coal Age

19

The SigiUarias with their single or few terminal tufts
of long, slender leaves must have had a general appearance
suggestive of the so-called grass trees of Australia.

Most of the Paleozoic clubmosses became extinct at
the end of the Carboniferous age but some of the smaller
ones persisted much longer. The group is represented
today by the ground pines and Selaginellas, the former
mostly of the temperate zone, the latter in the tropics.

The other great group of tall forest trees of the period
were early gymnosperms, called Cordaites in honor of the
paleontologist Corda. They have long since disappeared
but belonged to a line which probably gave origin to
such conifers as Araucaria and Agathis, the Australian
kaun tree, essentially Tertiary trees still in existence today
in the southern hemisphere. The Cordaites were truly
large-leaved (Fig. 13). The species of this order included
in the group has a sparsely branching trunk and broadly

Fig. 13. A slab of Pennsylvanian limestone with fossil leaves of several species
of the Carboniferous gymnosperra Cordaites. That in the rectangular area at the apex
is of Cordaites borassifolius, which was reconstructed for the group.

20 Field Museum of Natural History

strap-shaped, leathery leaves closely massed on the
branches, as in some trees of the lily family, such as
Dracaena or Cordyline. The Cordaites bore true seeds,
which were not produced in cones but on small shoots
in the leaf axils much as in the Ginkgo, a surviving, chiefly
Mesozoic relative of the Cordaites line.

The shade of the upper canopy of branches was not
too dense to prevent the existence in the Carboniferous
forest of a luxuriant vegetation of lesser size and the
fossil remains include an extraordinary quantity and
variety of stems and foliage of fern-like aspect. This
element formed so large a part of the Carboniferous
vegetation that the age has been called the Age of Ferns.
It is now known that these fern-like fossils are derived
from at least two superficially similar but very distinct
kinds of plants, true spore-bearing ferns, many on the
order of present-day tree-ferns, and a large variety of
seed-bearing plants with cycad-like seeds but with fern-
like foliage that at first led to their confusion with the
ferns. The usual and convenient form of classification
of fern-like fossils and its terminology current among
geologists is based on the appearance of the fronds alone,
and fails to make the important botanical distinction. In
most cases it is in fact not yet known whether certain
well-known fronds or parts of fronds are those of a fern
or of one of these seed-bearing plants. In other cases,
now sufficiently numerous, it has been possible for
paleontologists to associate definitely the fern-like foliage
with stems, fruit, and even with special pollen-bearing
fronds, so that all essential characters of certain seed-
ferns (Cycadofilices or Pteridosperms) are well under-
stood. Parts formerly described as widely separate species
are thus now known to belong to the same plant.

The most famous case of this kind is the seed-bearing
Lyginopteris, a climber or semi-climber shown supported
by the trunk of one of the larger trees near the center of
the group (Plate I), where it is easily recognized by its

A Forest of the Coal Age

21

Fic, I 1. Fruijinenis I ; iiLi^j of the seed-fern Lyginopteris. These fronds were
long known as Sphenopteris hoeninghauai before their connection with the other parts
of this plant was even suspected.

22

Field Museum of Natural History

"1^

Fig. 15. A block of sandstone from Mazon Creek, Illinois, with imbedded fossil
seeds of the large seed-fern shown in the group.

A Forest of the Coal Age

23

proproots, its delicate fern-like foliage (Fig. 14) the slender
forked rachis of its fronds and its special fruiting frond
among its upper leaves. The seeds of this with their
calyx-like envelope have long been known as Lagenostoma.
The forking of the main rachis of the frond appears to
be a constant characteristic of these seed-bearing plants.
It is conspicuous in one of the heavier, large-leaved species
with large seeds to the left in the group, a restoration of
one of the most abundant of Carboniferous fossils of the
state of Illinois (Fig. 7). Another peculiar feature which
the forked leaves have in common is the presence on the
lower part of the leaf stalk, of large, simple leaflets very
different in outline from the rest of the foliage. This is
particularly noticeable in the bracken-like seed-fern in
the immediate foreground of the group and is especially
well seen on the still coiled young leaves. Many of these
plants had stems like the tree-ferns. The fossil stems
sometimes reach two feet in diameter.

The true botanical status of the characteristically
Carboniferous climber, Mariopteris, with stiff, twice-
forked fern leaves (Fig. 16), ascending the small Lepido-
dendron tree on the right (Plate I), is not determined.

Fig. 16. A part of the frond of the climbing fern or seed-fern Mariopteris seen near
the large dragon fly in the group.

24 Field Museum of Natural History

Stems of tree-ferns are conspicuous and frequent among
Carboniferous fossils. Several types are known, two of
which are represented in the group. No Paleozoic fern
can be included in any family of living ferns, but one of
those shown in the group does not differ greatly in general
appearance from its modem relatives found today in the
southern hemisphere. Its leaves and the leafscars on the
stem are in straight rows, but stems of tree-ferns with
the leafscars in the now usual spiral arrangement were
also found fossil in the coal measures. The other is
distinctly peculiar in appearance due to the disposition

Fig. 17. A portion of the stem of a tree-fern (Psaronius) . The rock is Carbon-
iferous sandstone. The large leafscars are seen to be placed in straight rows. Compare
with the tree-fern trunk in the restoration.

A Forest of the Coal Age

25

of its leaves in only two vertical rows, which produces
a fan-shaped top as in some banana plants or in the
traveler's tree of Madagascar. It is called Megaphyton,
"large plant," not a very well-chosen name for such a
distinctive fern.

On the ground are small clumps of a very common
and characteristic plant of the Carboniferous swamp
vegetation, Sphenophyllum, a term referring to the wedge-
shaped leaf. It is further characterized by numerous,
slender, little-branched, jointed stems, whorls of delicate,
notched or slit leaves and long, fruiting, catkin-shaped

spikes, which resemble closely
the spore-bearing spikes of the
large calamites. Because of its
sometimes finely divided leaves,
suggesting those of some modem
water plants, Sphenophyllum
was formerly thought to have
been an aquatic. It has also
been considered a climber, be-
cause of its long stems, too
slender to stand erect alone, but
it was probably neither. It has
been represented in the group
as of the general habit of a
straggling herb such as the well-
known bedstraw Galium, to
which it is of course not in the
least related. Sphenophyllum is
now entirely extinct. With the
calamites, to which it is un-
doubtedly related, it is some-
times classed as one of the
Articulates, or jointed-stem
plants. It may represent an early
Fig. 18. A fragment of Spheno- stock from which the calamltcs

phyllum in a split sandstone nodule l-„ J ^-rAm-nr^i-t^A
from Mazon Creek, Illinois. i^aa Onginaiea.

26

Field Museum of Natural History

Fig. 19. A clump of Sphenophyllum showing the slender, jointed stems, whorled
leaves, and the catkin-like strobile.

A Forest of the Coal Age 27

In swamp forests where the larger vegetation is so
dense as that shown in this group, a noticeable poverty
of small herbaceous plants usually prevails and beyond
the Sphenophyllum little has been indicated. There is,
however, evidence of the presence in the Carboniferous
flora of smaller clubmosses resembling the ground pines
and Selaginellas of today, of small ferns, mosses, and
liverworts that all together must have constituted the
ground cover in places favorable for their development.
Fungi parasitic on old tree trunks and on the fallen stems
have been found and they, as well as bacteria, undoubtedly
brought about the disintegration of some portion of the
enormous mass of the vegetation litter of the forests.

Most of the characteristic flora of the Carboniferous
age became extinct with the elevation of the continental
land masses and the onset of arid conditions in the northern
hemisphere in the next period, the Permian, which closes
the Paleozoic age. The explanation of the extinction of
this first great land flora of the world lies in the manner
and conditions of fertilization of spore-bearing plants.
The giant clubmosses and most of their contemporaries
were unable to maintain their numbers or to propagate
their kind except in the presence of abundant moisture.

Not the least interesting feature of the Carboniferous
forest is its animal life. Insects were very numerous,
though they were then only at the beginning of their
long course of evolution, which has led to the myriads
of species of the present time. They were still in large
part primitive and much less varied in form and in habits
than they have subsequently become. Most modem
groups were entirely unrepresented, not yet having
appeared, but, together with the simplest forms embody-
ing all primitive characters of winged insects, and transi-
tional forms intermediate between these primitive insects
and various groups existing today, a few well-defined
types, such as mantids, stone flies and roaches had already
appeared.

28

Field Museum of Natural History

Compared with most recent insects the primitive ones
were large and rather clumsy, with two pairs of straight,
equal wings and in front of these a pair of wing-like expan-
sions on the first segment of the thorax. The abdomen
was regularly segmented, often with a pair of lateral
expansions on every segment and with simple terminal
appendages. The head bore compound eyes as well as
simple ones, simple, jointed antennae and chewing mouth
parts. The legs were simply jointed for crawling. On
the whole the insects were apparently adapted more for
short flits and glides than for flight. With the end of the
period these primitive forms disappeared.

Among the hordes of other insects of the period were
forms pointing clearly to dragon flies, locusts, termites, and
bugs. Some of the early dragon flies reached an enormous
size and attained dimensions that never since have been
equaled in the insect world. One of the giant Paleozoic

Fig. 20. Primitive winged insect of the Carboniferous age {Stenodictya lobata
Brongniart) from Handlirsch (order Palaeodictyoptera).

A Forest of the Coal Age 29

"dragon flies" had a spread of wings of almost thirty
inches. '' ^^ '' .■'*- ' > . «.

The roaches were especially well established and almost
incredibly numerous, with hundreds of species. Among
these were some of large size. All the unmistakable
peculiarities which characterize their tribe today had
already been fixed in their ancestors of the Carboniferous
forests. In point of ancient and well-established lineage
few present-day inhabitants of the earth can vie with
the cockroach.

The small types of insects so common today were
relatively few if not entirely lacking. The evolution and
specialization of the insects, in contrast to that of other
groups of animals, have been accompanied by a general
diminution in size. Ever since their advent in a great
burst of forms in the Carboniferous age, the insects have
been the chief contestants of the land vertebrates for the
possession of the earth. It is an interesting coincidence
that these two fundamentally different groups, that have
since progressed in almost diametrically opposite direc-
tions, made their appearance simultaneously in the
Carboniferous forest.

An impression in the rocks of the Upper Devonian,
the preceding period, is interpreted as a footprint of a
four-legged animal, and a Greenland find of large numbers
of skeletons has recently been reported. Otherwise the

,,,,,, I . i.»..t.t>U>'Ai.iJ

r

gmm.

Fig. 21. Restoration of the skeleton of Diplovertebron Watson, one of the early
amphibia (Stegocephalia) of the Carboniferous age. From a model prepared under
the direction of W. K. Gregory.

30 Field Museum of Natural History

Fig. 22. Diplovertebron, one of the early tetrapods as restored in Field Museum
on the basis of the skeleton in Fig. 21.

earliest known tetrapod remains are from the Lower
Carboniferous rocks of Europe. They are mostly of
salamander-like forms, decidedly amphibian, though with
abundant indication of their aquatic origin. Their nearest
relatives are to be sought among the lungfishes and lobe-
finned ganoids of the preceding period, with whom they
probably share a common ancestry. In Upper Carbonif-
erous time they became more numerous and many species
are known from Europe as well as from North America.
The American ones are derived from a single coal mine in
Ohio, They range in size and shape from tiny newts
to mud puppy and eel-like forms eight or nine feet in
length. Many of them perhaps seldom or never emerged
from the water of the lakes and pools in the forest. Most
of them, judging by their structure, were amphibious
rather than purely terrestrial in their habits, retaining
the elongated body form adapted to swimming, with at
least the ventral portion of the armor of the original
aquatic stock from which they sprang. This protection
for the lower surface of the body was no doubt valuable
on land, for the newly acquired legs appear in many
cases still too weak to support much weight and some of
these earliest four-legged adventurers in the woods must
have moved over the ground with the aid of fish-like
twists and wrigglings of the body.

A Forest of the Coal Age 31

This first group of amphibia, known as Stegocephalia,
persisted throughout the next period and for some time
after the close of the Paleozoic era. As is usual in the
course of evolution of vertebrates, they increased in size
until they became very large and unwieldy and finally
disappeared. One of the last of their kind had a skull
four feet in length. The early terrestrial vertebrate stock
gave rise to the reptiles, and by way of these in the course
of time to all the higher vertebrates.

Of the present-day practical importance of the ancient
coal beds there can be no question. They are today our
main source of readily available energy for power and
light. To realize how exceedingly important they are, it
is only necessary to note on the map how largely the
centers of the world's present industrial activity parallel
in location the remains of forests of the Carboniferous age.

To those who have had the patience to read the
preceding brief account of the swamp forests of the past,
it will be of interest to pursue a step farther the natural
history of the ancient plant material.

It is well known that coal consists chiefly of carbon.
The presence of coal in the rocks, even to the graphite in
the pre-Cambrian or the diamond is always attributed to
plant remains, for the carbon of coal is derived directly
from organic carbon compounds such as starch, sugar
and cellulose that with water formed the bulk and
substance of the vegetation in ancient times as well as now.

It is perhaps less often realized that the carbon of the
plants is not obtained from the soil by way of their roots
but from the air by way of their leaves and that conse-
quently the original source of the vast amount of carbon
locked up in the great coal fields was the carbon dioxide
that formed part of the atmosphere in Carboniferous time.

Plants are the only natural agency known capable of
abstracting carbon from the air. They employ it in the
construction of their own tissues and store it up for future

32 Field Museum OF Natural History

use. The mechanism by which this is accompHshed is the
plant cell with its green coloring matter. The energy-
utilized by plants in breaking down the stable carbon
dioxide, separating its two elements, carbon and oxygen,
is that portion of the sunlight passed by the green filter.

It is the energy from sunlight used in the formation
of the carbon compounds of plants and stored by them
in potential form, that constitutes the importance of coal.
Coal has been called "bottled sunshine," which sounds
fanciful but is substantially correct, for it is the stored
energy of the sunlight of a past geological age that is
liberated as heat in burning coal when the carbon
abstracted from the air of several million years ago is
returned to the atmosphere of today.

It has been said that the characteristic peculiarity of
modern civilization consists of the rapid expenditure of
this stored energy of the past. The statement might be
paraphrased into terms of natural history to read that
the ultimate descendants of the Carboniferous tetrapods
have now reached the point of utilizing the long-buried
remains of their ancestral forests. On the other hand,
throughout the ages the cockroach remains unchanged,
a living fossil, its prospects of survival still unsurpassed.

The geography and climate of the Carboniferous age
have been the subject of much speculation. It is difficult
to explain, without assuming a shifting of the poles, how
the now temperate zone or parts of the circumpolar
region of the northern hemisphere could have had a
uniformly warm climate, while in the next period extensive
glaciation took place in corresponding latitudes in the
southern hemisphere. It is equally difficult to explain
on the basis of the present geography of the continents
how the special and different flora of the southern hemi-
sphere, the Gondwana flora, coming up from the south at
the close of Carboniferous time, could have spread simul-
taneously over places as remote from each other as

A Forest of the Coal Age 33

Australia, India, Africa, and South America. It is, there-
fore, usual to assume the existence at the time of a large
land mass (Gondwana land) serving as a link between
the continents to the south.

In this connection great interest attaches to the
theory recently brought forward to the effect that the
continents as we know them today were not always
fixed in their present position but were once part of a
great single land mass which later split up into portions
that have gradually drifted apart. This is Wegener's
celebrated "theory of continental drift." A theory so
radically opposed to the usual conception of the stability
of the continents has naturally not met with a uniformly
favorable reception but no other explains so well the
course of events in the plant world of the time. The
map on page 34 shows the general distribution of the two
coal floras, the northern and the southern, and that on
page 35 the approximate position of the present-day
continents as parts of Wegener's "continental block" of
Carboniferous time.

With the poles located as indicated on this map, the
northern coal forests come to occupy a position along
the equator, accounting for the uniform climate and other
peculiarities and the Gondwana flora of the various
southern continents in a compact area showing evidences
of glaciation in late Carboniferous or Permian time.

B. E. Dahlgren

34

Field Museum of Natural History

es

•Sn

>.fe

o c
«.2

'b'Z

g o

o o
§2

A Forest of the Coal Age

35

TABLE OF GEOLOGICAL PERIODS

Name and age in
years of eras

Geological periods

Predominant forms
of life

Cenozoic

Pleistocene — Recent
Pliocene, Miocene
Oligocene, Eocene

Flowering plants,
mammals

60,000,000
Mesozoic

Cretaceous
Comanchian
Jurassic
Triassic

Reptiles, early birds

Gymnosperms, cycads
and other seed plants

200,000.000

Permian

Gondwana flora

Pennsylvanian

Carboniferous
swamp forests

Mississippian

Early amphibia, insects

Paleozoic

Devonian

Early land flora

Silurian

Fishes

Ordovician

Higher invertebrates

Cambrian

Algae, probably primi-
tive land plants

550,000,000
Proterozoic

Lower invertebrates
Algae

1,200,000,000

Archean

or
Archeozoic

1,850,000,000

Dawn of life
Unicellular organisms

2,000,000,000

Formation of earliest
rocks

36

PLANTS AND ANIMALS REPRESENTED IN THE
CARBONIFEROUS FOREST GROUP

PLANTS

Clu bmosses — Ly copods

Lepidodendron clypeatum Lesque-
reux

Lepidodendron obovatum Stern-
berg

Lepidodendron modulatum Lx.

Lepidostrobus ovatifolius Lx.

Sigillaria rugosa Brongniart

Sigillaria savlli Bgt..

Sigillaria scutellata Bgt.

Sigillaria lacoei Lx.

Sigillaria laevigata Bgt.

Sigillaria trunk

Stigmaria ficoides Sternb.

Lepidophloios laricimts Sternb.

Selaginellites

Seed-ferns — Cycadofilices
Neuropteris heterophylla Bgt.
Neuropteris decipiens Bgt.
Medullosa stem
Cyclopteris leaves
Trigonocarpus seeds
Lyginopteris oldhamnia William-
son

Sphenopferis hoeninghansileaves
Lagenostoma ovoides seeds

Ferns — Pteridophytes
Caulopteris giffordi Lx.

Psaronius stem

Pecopteris leaves
Megaphyton frondosum Artis

PsaronitLS disfichus stem
Mariopterismuricata (Schlotheim)

Zeiller

Jointed-stem Plants —

Articulates

Sphenophyllum emarginatum

(Bgt.) Koenig
Calamites

Annulariaradiata (Bgt.) Sternb.

Gymnosperms

Cordaites borassifolius (Sternb.)

Unger

INSECTS

Primitive Insects —

Palaeodictyoptera

Stenodictya lobata Bgt.

Early Dragon Flies —

Protoodonata

Meganeura monyi Bgt.

Protoorthoptera

Gerarus danielsi Handlirsch

Roaches— Blattoidea
Archeoblattina beecheri Sellards

VERTEBRATES

Primitive Amphibia —

Stegocephalia

Diplovertebron pundatum Fritsch
Ceraterpeton galvani Huxley
Eojyrinus attheyi. In background

37

BIBLIOGRAPHY

Arber, E. a. N.

The Natural History of Coal. The University Press, Cambridge,
1911.

Berry, E. W.

Paleobotany: A Sketch of the Origin and Evolution of Floras.
Ann. Rept., Smithson. Inst., Washington, 1920.

Crookall, R.

Coal Measure Plants. Edward Arnold and Company, London,
1929.

Handlirsch, Anton

Die fossilen Insekten. Verlag von Wilhelm Engelmann, Leipzig,
1908.

HiRMER, Max

Handbuch der Palaobotanik. Druck und Verlag von R. Olden-
bourg, Munich and Berlin, 1927.

Knowlton, F. H.

Plants of the Past. University Press, Princeton, 1927.

Lesquereux, Leo

Atlas to the Coal Flora of Pennsylvania, and of the Carboniferous
Formation throughout the United States. Published by Board
of Commissioners for Second Geological Survey, Harrisburg,

1879.

MooDiE, Roy Lee

The Coal Measures Amphibia of North America. Carnegie Inst.,
Washington, 1916.

NOE, A. C.

Pennsylvania Flora of Northern Illinois. 111. State Geol. Surv.,

Bull. No. 52, Urbana, Illinois, 1925.
Ferns, Fossils and Fuel. Thomas S. Rockwell Company, Chicago,

1931.

POTONIE, H.

Landschaft der Steinkohlen-Zeit. Verlag von Gebriider Born-
traeger, Leipzig, 1899.

PoTONiE, H. and Gothan, W.

Lehrbuch der Palaobotanik. 2nd ed., Berlin, 1921,

Reeds, Chester A.

How Old Is the Earth? Amer. Mus. Nat. Hist., Natural History,
31, No. 2, New York, 1931.

RoMER, Alfred S.

The Pennsylvanian Tetrapods of Linton, Ohio. Bull. Amer. Mus.
Nat. Hist., 59, New York, 1930.

38

A Forest of the Coal Age 39

Roy, Sharat K.

How Old Are Fossils? Field Mus. Nat. Hist., Geol. Leaf. Ser.,
No. 9, Chicago, 1927.

Scott, D. H.

Studies in Fossil Botany. 3rd ed., Adam and Charles Black,
London, 1900.

The Evolution of Plants. Henry Holt and Company, New York,
1911.

Extinct Plants and Problems of Evolution. Macmillan and Com-
pany, London, 1924.

Seward, A. C.

Fossil Plants. 1, The University Press, Cambridge, 1898.
Plant Life through the Ages. Macmillan and Company, University
Press, New York, Cambridge and England, 1931.

Watson, D. M. S.

The Evolution and Origin of the Amphibia. Philos. Trans. Roy.
Soc. Lond., (B), 214, pp. 189-257, 1925-26.

ZiMMERMANN, WALTER

Die Phylogenie der Pflanzen. Gustav Fischer, Jena, 1930.

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