THE DEVONIAN
In America the Silurian is not separated from the Devonian by any
mountain-making deformation or continental uplift. The one period
passed quietly into the other. Their conformable systems are so
closely related, and the change in their faunas is so gradual,
that geologists are not agreed as to the precise horizon which
divides them.
SUBDIVISIONS AND PHYSICAL GEOGRAPHY. The Devonian is represented
in New York and southward by the following five series. We add the
rocks of which they are chiefly composed.
5 Chemung . . . . . . sandstones and sandy shales
4 Hamilton . . . . . . shales and sandstones
3 Corniferous . . . . . . limestones
2 Oriskany . . . . . . sandstones
1 Helderberg . . . . . . limestones
The Helderberg is a transition epoch referred by some geologists
to the Silurian. The thin sandstones of the Oriskany mark an epoch
when waves worked over the deposits of former coastal plains. The
limestones of the Corniferous testify to a warm and clear wide sea
which extended from the Hudson to beyond the Mississippi. Corals
throve luxuriantly, and their remains, with those of mollusks and
other lime-secreting animals, built up great beds of limestone.
The bordering continents, as during the later Silurian, must now
have been monotonous lowlands which sent down little of even the
finest waste to the sea.
In the Hamilton the clear seas of the previous epoch became
clouded with mud. The immense deposits of coarse sandstones and
sandy shales of the Chemung, which are found off what was at the
time the west coast of Appalachia, prove an uplift of that ancient
continent.
The Chemung series extends from the Catskill Mountains to
northeastern Ohio and south to northeastern Tennessee, covering an
area of not less than a hundred thousand square miles. In eastern
New York it attains three thousand feet in thickness; in
Pennsylvania it reaches the enormous thickness of two miles; but
it rapidly thins to the west. Everywhere the Chemung is made of
thin beds of rapidly alternating coarse and fine sands and clays,
with an occasional pebble layer, and hence is a shallow-water
deposit. The fine material has not been thoroughly winnowed from
the coarse by the long action of strong waves and tides. The sands
and clays have undergone little more sorting than is done by
rivers. We must regard the Chemung sandstones as deposits made at
the mouths of swift, turbid rivers in such great amount that they
could be little sorted and distributed by waves.
Over considerable areas the Chemung sandstones bear little or no
trace of the action of the sea. The Catskill Mountains, for
example, have as their summit layers some three thousand feet of
coarse red sandstones of this series, whose structure is that of
river deposits, and whose few fossils are chiefly of fresh-water
types. The Chemung is therefore composed of delta deposits, more
or less worked over by the sea. The bulk of the Chemung equals
that of the Sierra Nevada Mountains. To furnish this immense
volume of sediment a great mountain range, or highland, must have
been upheaved where the Appalachian lowland long had been. To what
height the Devonian mountains of Appalachia attained cannot be
told from the volume of the sediments wasted from them, for they
may have risen but little faster than they were worn down by
denudation. We may infer from the character of the waste which
they furnished to the Chemung shores that they did not reach an
Alpine height. The grains of the Chemung sandstones are not those
which would result from mechanical disintegration, as by frost on
high mountain peaks, but are rather those which would be left from
the long chemical decay of siliceous crystalline rocks; for the
more soluble minerals are largely wanting. The red color of much
of the deposits points to the same conclusion. Red residual clays
accumulated on the mountain sides and upland summits, and were
washed as ocherous silt to mingle with the delta sands. The iron-
bearing igneous rocks of the oldland also contributed by their
decay iron in solution to the rivers, to be deposited in films of
iron oxide about the quartz grains of the Chemung sandstones,
giving them their reddish tints.
LIFE OF THE DEVONIAN
PLANTS. The lands were probably clad with verdure during Silurian
times, if not still earlier; for some rare remains of ferns and
other lowly types of vegetation have been found in the strata of
that system. But it is in the Devonian that we discover for the
first time the remains of extensive and luxuriant forests. This
rich flora reached its climax in the Carboniferous, and it will be
more convenient to describe its varied types in the next chapter.
RHIZOCARPS. In the shales of the Devonian are found microscopic
spores of rhizocarps in such countless numbers that their weight
must be reckoned in hundreds of millions of tons. It would seem
that these aquatic plants culminated in this period, and in widely
distant portions of the earth swampy flats and shallow lagoons
were filled with vegetation of this humble type, either growing
from the bottom or floating free upon the surface. It is to the
resinous spores of the rhizocarps that the petroleum and natural
gas from Devonian rocks are largely due. The decomposition of the
spores has made the shales highly bituminous, and the oil and gas
have accumulated in the reservoirs of overlying porous sandstones.
INVERTEBRATES. We must pass over the ever-changing groups of the
invertebrates with the briefest notice. Chain corals became
extinct at the close of the Silurian, but other corals were
extremely common in the Devonian seas. At many places corals
formed thin reefs, as at Louisville, Kentucky, where the hardness
of the reef rock is one of the causes of the Falls of the Ohio.
Sponges, echinoderms, brachiopods, and mollusks were abundant. The
cephalopods take a new departure. So far in all their various
forms, whether straight, as the Orthoceras, or curved, or close-
coiled as in the nautilus, the septum, or partition dividing the
chambers, met the inner shell along a simple line, like that of
the rim of a saucer. There now begins a growth of the septum by
which its edges become sharply corrugated, and the suture, or line
of juncture of the septum and the shell, is thus angled. The group
in which this growth of the septum takes place is called the
GONIATITE (Greek GONIA, angle).
VERTEBRATES. It is with the greatest interest that we turn now to
study the backboned animals of the Devonian; for they are believed
to be the ancestors of the hosts of vertebrates which have since
dominated the earth. Their rudimentary structures foreshadowed
what their descendants were to be, and give some clue to the
earliest vertebrates from which they sprang. Like those whose
remains are found in the lower Paleozoic systems, all of these
Devonian vertebrates were aquatic and go under the general
designation of fishes.
The lowest in grade and nearest, perhaps, to the ancestral type of
vertebrates, was the problematic creature, an inch or so long, of
Figure 297. Note the circular mouth not supplied with jaws, the
lack of paired fins, and the symmetric tail fin, with the column
of cartilaginous, ringlike vertebrae running through it to the
end. The animal is probably to be placed with the jawless lampreys
and hags,--a group too low to be included among true fishes.
OSTRACODERMS. This archaic group, long since extinct, is also too
lowly to rank among the true fishes, for its members have neither
jaws nor paired fins. These small, fishlike forms were cased in
front with bony plates developed in the skin and covered in the
rear with scales. The vertebrae were not ossified, for no trace of
them has been found.
DEVONIAN FISHES. The TRUE FISHES of the Devonian can best be
understood by reference to their descendants now living. Modern
fishes are divided into several groups: SHARKS and their allies;
DIPNOANS; GANOIDS, such as the sturgeon and gar; and TELEOSTS,--
most common fishes, such as the perch and cod.
SHARKS. Of all groups of living fishes the sharks are the oldest
and still retain most fully the embryonic characters of their
Paleozoic ancestors. Such characters are the cartilaginous
skeleton, and the separate gill slits with which the throat wall
is pierced and which are arranged in line like the gill openings
of the lamprey. The sharks of the Silurian and Devonian are known
to us chiefly by their teeth and fin spines, for they were
unprotected by scales or plates, and were devoid of a bony
skeleton. Figure 299 is a restoration of an archaic shark from a
somewhat higher horizon. Note the seven gill slits and the
lappetlike paired fins. These fins seem to be remnants of the
continuous fold of skin which, as embryology teaches, passed from
fore to aft down each side of the primitive vertebrate.
Devonian sharks were comparatively small. They had not evolved
into the ferocious monsters which were later to be masters of the
seas.
DIPNOANS, OR LUNG FISHES. These are represented to-day by a
few peculiar fishes and are distinguished by some high structures
which ally them with amphibians. An air sac with cellular spaces
is connected with the gullet and serves as a rudimentary lung. It
corresponds with the swim bladder of most modern fishes, and
appears to have had a common origin with it. We may conceive that
the primordial fishes not only had gills used in breathing air
dissolved in water, but also developed a saclike pouch off the
gullet. This sac evolved along two distinct lines. On the line of
the ancestry of most modern fishes its duct was closed and it
became the swim bladder used in flotation and balancing. On
another line of descent it was left open, air was swallowed into
it, and it developed into the rudimentary lung of the dipnoans and
into the more perfect lungs of the amphibians and other air-
breathing vertebrates.
One of the ancient dipnoans is illustrated in Figure 300. Some of
the members of this order were, like the ostracoderms, cased in
armor, but their higher rank is shown by their powerful jaws and
by other structures. Some of these armored fishes reached twenty-
five feet in length and six feet across the head. They were the
tyrants of the Devonian seas.
GANOIDS. These take their name from their enameled plates or
scales of bone. The few genera now surviving are the descendants
of the tribes which swarmed in the Devonian seas. A restoration of
one of a leading order, the FRINGE-FINNED ganoids, is given in
Figure 301. The side fins, which correspond to the limbs of the
higher vertebrates, are quite unlike those of most modern fishes.
Their rays, instead of radiating from a common base, fringe a
central lobe which contains a cartilaginous axis. The teeth of the
Devonian ganoids show a complicated folded structure.
GENERAL CHARACTERISTICS OF DEVONIAN FISHES. THE NOTOCHORD IS
PERSISTENT. The notochord is a continuous rod of cartilage, or
gristle, which in the embryological growth of vertebrate animals
supports the spinal nerve cord before the formation of the
vertebrae. In most modern fishes and in all higher vertebrates the
notochord is gradually removed as the bodies of the vertebrae are
formed about it; but in the Devonian fishes it persists through
maturity and the vertebrae remain incomplete.
THE SKELETON IS CARTILAGINOUS. This also is an embryological
characteristic. In the Devonian fishes the vertebrae, as well as
the other parts of the skeleton, have not ossified, or changed to
bone, but remain in their primitive cartilaginous condition.
THE TAIL FIN IS VERTEBRATED. The backbone runs through the fin and
is fringed above and below with its vertical rays. In some fishes
with vertebrated tail fins the fin is symmetric, and this seems to
be the primitive type. In others the tail fin is unsymmetric: the
backbone runs into the upper lobe, leaving the two lobes of
unequal size. In most modern fishes (the teleosts) the tail fin is
not vertebrated: the spinal column ends in a broad plate, to which
the diverging fin rays are attached.
But along with these embryonic characters, which were common to
all Devonian fishes, there were other structures in certain groups
which foreshadowed the higher structures of the land vertebrates
which were yet to come: air sacs which were to develop into lungs,
and cartilaginous axes in the side fins which were a prophecy of
limbs. The vertebrates had already advanced far enough to prove
the superiority of their type of structure to all others. Their
internal skeleton afforded the best attachment for muscles and
enabled them to become the largest and most powerful creatures of
the time. The central nervous system, with the predominance given
to the ganglia at the fore end of the nerve cord,--the brain,--
already endowed them with greater energy than the invertebrates;
and, still more important, these structures contained the
possibility of development into the more highly organized land
vertebrates which were to rule the earth.
TELEOSTS. The great group of fishes called the teleosts, or those
with complete bony skeletons, to which most modern fishes belong,
may be mentioned here, although in the Devonian they had not yet
appeared. The teleosts are a highly specialized type, adapted most
perfectly to their aquatic environment. Heavy armor has been
discarded, and reliance is placed instead on swiftness. The
skeleton is completely ossified and the notochord removed. The
vertebrae have been economically withdrawn from the tail, and the
cartilaginous axis of the side fins has been fotfoid unnecessary.
The air sac has become a swim bladder. In this complete
specialization they have long since lost the possibility of
evolving into higher types.
It is interesting to note that the modern teleosts in their
embryological growth pass through the stages which characterized
the maturity of their Devonian ancestors; their skeleton is
cartilaginous and their tail fin vertebrated.