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).
[Illustration: Fig. 296. A Goniatite]
=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 vertebræ 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.
[Illustration: Fig. 297. Palæospondylus]
=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 vertebræ were not ossified, for no trace of them has been found.
[Illustration: Fig. 298. An Ostracoderm]
=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.
[Illustration: Fig. 299. A Paleozoic Shark]
=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.
[Illustration: Fig. 300. A Devonian Dipnoan]
=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.
[Illustration: Fig. 301. A Devonian Fringe-Finned Ganoid]
=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 vertebræ.
In most modern fishes and in all higher vertebrates the notochord is
gradually removed as the bodies of the vertebræ are formed about it;
but in the Devonian fishes it persists through maturity and the
vertebræ remain incomplete.
=The skeleton is cartilaginous.= This also is an embryological
characteristic. In the Devonian fishes the vertebræ, as well as the
other parts of the skeleton, have not ossified, or changed to bone,
but remain in their primitive cartilaginous condition.
[Illustration: Fig. 302. Vertebræ of Sturgeon in side view _A_;
and vertical transverse section _B_, showing Notochord _ch_, and
Neural Canal _m_]
=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 (Fig. 300), 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 vertebræ have been
economically withdrawn from the tail, and the cartilaginous axis of
the side fins has been found 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.