GEOLOGICAL HISTORY OF ANIMALS (EXCLUDING VERTEBRATES)
A study of the animals of the past is not only of great interest in
itself, but also it furnishes a mainstay of the great doctrine of
organic evolution. At the very outset of our discussion the reader
should have already in mind at least the main subdivisions of the
animal kingdom in order to reasonably well understand where the
important animal types of the different geological ages fit in, and
how those types bear upon the doctrine of evolution. The accompanying,
very brief, general classification includes the usually recognized
subkingdoms with special reference to representatives of those which
are of most geological and evolutionary significance. Reading downward
in this classification, the degree of complexity of organization
steadily increases from single-celled animals to man himself.
I. Protozoans, e.g. foraminifers (with lime carbonate shells)
{Sponges
II. Coelenterates, e.g. {So-called "jellyfishes," graptolites
{Corals
{So-called "sea lilies"
III. Echinoderms, e.g. {So-called "starfishes"
{So-called "sea urchins"
IV. Worms,
V. Molluscoids, e.g. {So-called "sea mosses"
{Brachiopods
{Clams, oysters
{
{Snails {Pearly nautilus,
VI. Mollusks, e.g. { { ammonites
{
{Cephalopods, e.g. {So-called
{ { "cuttle fishes"
{Trilobites
VII. Arthropods, e.g. {Crabs, lobsters
{So-called "sea scorpions"
{Insects
{Ostracoderms
{Fishes
VIII. Vertebrates, e.g. {Amphibians
{Reptiles
{Birds
{Mammals (including man)
Before entering into a brief but rather systematic discussion of some
of the most important types of animals which lived during geological
time, it may be well for the reader to have in mind some of the most
important conclusions which have been reached as a result of the study
of the fossil animal records. These conclusions may be summarized as
follows:
1. Animal life existed many millions of years ago.
2. Not only the animals of to-day, but also those of any given
geological period, directly descended from those of preceding
geological periods.
3. Animal life has undergone continuous change since its introduction
upon the earth, so that each group of strata, representing a particular
geological age, contains a characteristic assemblage of fossil animals.
4. Many of the changes in the history of animals have been progressive
or evolutionary, so that strata of early geological time contain
distinctly more primitive or lower order forms than the strata of late
geological time. But, while the line of evolution has been maintained
without a break, culminating in man, there have been many offshoots of
a retrogressive nature.
5. Even as far along in geological time as the early Paleozoic era,
the highest subkingdom--vertebrates--had no representative whatever.
In other words, all the important subdivisions of animal life from a
little below fishes to man have been evolved since about the close of
the Ordovician period.
6. Any species of animal which ever became extinct has never been known
to reappear, and literally tens of thousands of species are known to
have become extinct.
7. No species like those now living are found in the more ancient
strata, such being confined to the strata of relatively recent
geological dates.
8. While more and more highly organized animals have continuously
been evolved, many of the earlier and simpler types have persisted,
a remarkable case in point being the single-celled animals called
foraminifers which may be traced, without very notable change, through
the tens of millions of years of geological time from the late
Proterozoic era to the present day.
9. Many species have been able to maintain themselves practically
without change through long stretches of geological time, while others
have had only very brief existence.
When did animal life begin on the earth, and what were the first forms
like? We can only partially answer the first question by saying that
animals have existed for tens of millions of years, certainly as early
at least as Proterozoic time. Up to the present time we are utterly
in the dark as to what the earliest animal forms looked like, but we
have positive knowledge that the oldest forms found as fossils in the
rocks represent creatures which were far more primitive and lower in
organization than many animals of to-day, and that since those oldest
known forms lived, the animal kingdom has undergone various profound
alterations. In view of the above statements, and also the fact that
the oldest known plant forms were extremely simple or single-celled,
it is more than likely that the first animal life of the earth was
single-celled. In harmony with this view is the fact that fossil
single-celled animals are found in the very oldest (Proterozoic) rocks
which contain any definitely determinable fossil animals.
Do the most ancient known rocks show that animal life existed during
Archeozoic time? In the preceding chapter we pointed out the fact that
the carbon (in the form of graphite), so commonly present in those most
ancient known strata, proves the existence of life of some kind during
Archeozoic time. But because nothing like definitely determinable
fossil forms have thus far been discovered in those rocks, we cannot be
sure whether the carbon represents plant or animal life or both, though
certainly plants of very low order at least must have existed. Because
of the intense alteration (metamorphism) of those very old strata,
all definite forms have long since been obliterated as such. We may,
however, in the light of the vast evolution which took place through
succeeding geological time, be very sure that any animals which may
have existed during Archeozoic time were in general much simpler forms
than those of even early Paleozoic time.
The early and middle Proterozoic strata throw no more light upon the
early history of animal life than do the Archeozoic strata. The upper
or later Proterozoic rocks, however, contain the oldest recognizable
animal fossils. Very recently fossil remains of single-celled,
shell-bearing Protozoans have been found in northern France, while
the upper Proterozoic strata of the Rocky Mountains in Montana, and
the Grand Canyon of Arizona have yielded worm tracks, a Molluscoid
(brachiopod) and fragments of lower forms of Arthropods. This record,
although very meager, clearly proves that animal life was so well
advanced by late Proterozoic time, that next to the highest subkingdom
was actually represented (see above classification), and that there
must have been a long line of simpler and simpler ancestors, probably
extending far back into the Archeozoic era. When we stop to consider
that Archeozoic and Proterozoic time was fully as long as all
succeeding geological time, it is not so surprising that fairly highly
developed animals (except Vertebrates) had been evolved before the
close of the Proterozoic era.
In regard to abundance of fossil animals the oldest (Cambrian)
Paleozoic strata stand out in marked contrast to the Proterozoic. Many
hundreds of species of animal fossils have been described from Cambrian
strata, and a great many others yet remain to be discovered. Cambrian
fossils are remarkably numerous, varied in species, and complex
in organization (Plate 13). All subkingdoms of animals except the
Vertebrates were represented, though usually only by the simpler types
in each subkingdom. It is quite generally agreed that no less than 50
per cent of animal evolution had taken place before the beginning of
the Cambrian period. The reader should, however, clearly bear in mind
that tremendous advances in evolution have taken place since early
Cambrian time when not only all forms from lower scale Arthropods to
the highest mammals (including man) have evolved, but also when many
thousands of species of lower subkingdom animals developed.
Why are the very early Paleozoic strata so rich in fossils, while the
immediately preceding Proterozoic rocks show so few? The seemingly
sudden appearance of so many highly developed animals in earliest
Paleozoic (Cambrian) time is one of the most important considerations
in the history of animal life, and it is by no means definitely
understood. The following statements bear directly upon the problem:
The early animal forms were probably soft or gelatinous without shells
and lived mostly in the open sea where food (seaweeds, etc.) was
abundant. Such animals were very unfavorable for preservation in fossil
form. Then, late in Proterozoic time or very early in the Paleozoic,
a severe struggle for existence set in, probably due to crowding
along shores, and hard parts began to develop both for support and
defensive purposes. Such hard parts or shells were commonly favorable
for fossilization. This view is strongly supported by the fact that
very thin shells only are known from late Proterozoic rocks, and mostly
very thin shells from the earliest Cambrian, the heavier shells having
been evolved later. A fact of importance to bear in mind in this
connection is that just at the critical time (late Proterozoic) in
shell development, the lands of the earth were undergoing widespread
and deep erosion as pointed out early in the chapter on "Ancient Earth
History." The earliest Cambrian strata, therefore, nearly everywhere
rest upon the deeply eroded surface of the Proterozoic rocks so that
the transition strata--the very ones which would contain most fossils
of the early shell development stage--are nearly everywhere missing.
Finally, mention should be made of the fact, that all Archeozoic
strata are profoundly altered (metamorphosed), and so are nearly
all Proterozoic strata, except the later. Fossils once present in
those rocks would of course have been obliterated by the process of
metamorphism, but the fact remains that very considerable thicknesses
of practically unaltered Proterozoic strata show few if any animal
fossils.
We shall now proceed to a rather systematic consideration of the most
interesting and significant types of creatures which have inhabited
the earth since the beginning of Paleozoic time at least twenty-five
million years ago. It is our purpose to bring out the salient features
in the history of each subkingdom of animals, beginning with the lowest
or simplest, and taking up in turn the higher and higher subkingdoms.
By this method the reader may easily follow the main thread of organic
evolution or progressive change which runs through most of the known
history of animal life of our planet, and which is so important in the
science of geology.
Protozoans, which include all the tiny single-celled animals, are
known in fossil form even in late Proterozoic rocks and, as proved by
the fossil records, they have been more or less abundant ever since,
even now swarming in large portions of the surface sea waters. One of
the most remarkable facts in the history of animal life is, that such
exceedingly simple creatures persisted almost without change through
the tens of millions of years when such profound and even revolutionary
changes took place in the animal kingdom in general. The only fossil
Protozoans are those which developed delicate shells either of
carbonate of lime (the foraminifers) or silica. Special mention should
be made of the Cretaceous period when foraminifers must have been
exceedingly profuse in clear sea waters which spread over the Gulf
Coastal Plain of the United States, parts of southern England, much of
France, and other areas, as proved by their accumulated shells which
make up formations of chalk hundreds of feet in thickness and many
miles in extent.
[Illustration: Fig. 49.--A compound colony of fossil graptolites
characteristic of late Ordovician time, fully 20,000,000 years ago.
Each little prong once held a tiny individual living graptolite which
was a very simple type of animal belonging to the subkingdom called
"Coelenterates." (Modified after Ruedemann.)]
The Coelenterates, which comprise the simplest of the many-celled
animals, are saclike forms with mouth openings, but with few other
differentiations of parts. All are marine animals. Of these the sponges
are porous, and the other types (including corals) have tentacles
around their mouths. Sponges have been more or less common from early
Paleozoic time to the present, and they have undergone relatively
little change. "Jellyfishes," which are in truth not fishes at all,
are wholly soft or gelatinous Coelenterates which have left some very
remarkable impressions and casts in strata of very early Paleozoic age,
those very ancient forms evidently having been almost exactly like
those of to-day. Graptolites were slender, plumelike, delicate forms
consisting of colonies of tiny individuals, in many cases in branching
or radiating combinations. They existed only during the first half of
the Paleozoic era. Both because they floated in the open sea, thus
permitting widespread distribution, and because they underwent many
distinct species changes during short geologic intervals, they are
among the most useful fossils for separating the various subdivisions
of strata of the earlier Paleozoic.
[Illustration: Fig. 50.--Corals, representing the very simple
subkingdom of animals called "Coelenterates": a, fossil shell of an
individual "cup coral" found only in Paleozoic strata; b, a compound or
"chain coral" skeleton found only in relatively old Paleozoic strata;
and c, part of a modern coral colony showing living corals.]
Corals comprise another important branch of the Coelenterates. During
the Cambrian period there were corallike sponges and possibly simple
corals, but from the early Ordovician to the present true corals
have been common, especially in the clearer, warmer seas. Their
carbonate of lime skeletons have accumulated to help build up great
limestone formations representing almost every geologic age from early
Paleozoic time to the present. Paleozoic corals were in general
notably different from those of later time. There were three main
types including the compound "honeycomb" and "chain" types, and the
solitary or compound "cup" type. They all had four, or multiples of
four, radiating partitions; were rarely branched; and were generally
large, some individual cup corals ranging in length from half an inch
to a foot or more. Modern corals (beginning with the Mesozoic) have six
or eight partitions; are nearly all profusely branched; and are mostly
tiny individuals.
Echinoderms are all marine animals, including the so-called
"starfishes," which are not really fishes. They have body cavity, with
digestive canal, low order nervous system, and a water circulatory
system. Most of them have radially segmented shells or skeletons. The
oldest fossil forms are found in Cambrian strata, these being very
simple or primitive types, with a bladderlike head set on the end of a
segmented stem, both head and stem having been supported by carbonate
of lime. Such forms lived only to middle Paleozoic time. Ordovician
strata contain representatives of all the main types of Echinoderms in
well-fossilized forms.
[Illustration: Fig. 51.--Fossil Echinoderms or so-called "starfishes":
a, simple type known as the "stone lily" with head, stem, and roots
intact from Silurian strata; b and c, irregular and regular higher type
Echinoderms called "sea urchins" from Cretaceous strata.]
A stemmed Echinoderm of special interest, first known from the
Ordovician, has persisted to the present day. It is the so-called
"sea lily" or "stone lily," consisting of a complex, headlike portion
attached to the sea bottom by a long segmented stem, the whole being
supported by lime carbonate. They were very numerous during the
Silurian, but they seem to have culminated in variety of species and
numbers of individuals during the Mississippian period when they were
exceedingly profuse. Hundreds of species of "stone lilies" are known
from Mississippian strata alone, and in certain localities, as at
Crawfordsville, Ind., and Burlington, Ia., the "stone lily" remains are
so numerous that when living they must have literally forested parts of
the sea bottom. From Mississippian time to middle Mesozoic time they
occupied a relatively subordinate position when they again developed in
great profusion. The Mesozoic forms were distinctly more like those
of to-day, and it scarcely seems credible that any creature could have
contained such a multiplicity of hard parts, more than 600,000 segments
having been counted in a single fossil from Jurassic strata. The "sea
lilies" of to-day are relatively unimportant.
The familiar five-pointed "starfishes," so common along our seacoasts,
are first known from the Ordovician, and they persisted through the
many millions of years to the present time with remarkably little
change. The so-called "sea urchins" live in rounded, segmented
lime-carbonate shells bristling with movable spines. "Sea urchins"
are first known from the Ordovician, but they did not become abundant
and diversified until Mesozoic time, when many of them took on a very
modern aspect.
Worms are known to have existed ever since late Proterozoic time,
as proved by the occurrence of tracks, borings and more rarely
delicate impressions on rock surfaces. Because of their softness they
have rarely been well fossilized and are, therefore, of no great
evolutionary or geological importance.
[Illustration: Fig. 52.--Fossil brachiopods belonging to the subkingdom
of animals known as "Molluscoids": a, b, c, forms characteristic of the
Ordovician, Devonian, and Triassic periods, respectively.]
The subkingdom Molluscoids has been richly represented by both the
so-called "sea mosses" and brachiopods. The "sea mosses" form colonies
of tiny mosslike tufts, resembling corals outwardly, though they are
much more highly organized. They have been common from Ordovician
time to the present, their carbonate of lime skeletons often having
contributed to the building of limestone formations. Brachiopods always
have two external shells or valves, in most cases working on a hinge,
and also a pair of long, spiral-fringed arms associated with the soft
part of the animal inside the shells. They differ from the other type
of bivalve (e.g., clam, oyster) in that they are symmetrical with
reference to a plane passed through the middle of the shells at right
angles to the hinge line. They have rarely grown to be more than a few
inches long. A few scant brachiopod remains are known from the late
Proterozoic, but throughout known geologic time they reached their
greatest development in the Paleozoic era, more especially in the
Devonian period. Combining number of species and number of individuals,
the brachiopods probably hold the record of all important groups of
fossil animals, more than 7,000 species being known. Many layers of
rock are filled with their shells (Plate 14). Since the close of the
Paleozoic they have fallen off notably, and are now represented by
relatively few small forms. From the standpoint of evolution it is
interesting to note that in very early Paleozoic time the brachiopods
were mostly small, of relatively simple organization, and their thin
shells were not joined by hinges. Later they became larger and more
complex and their thicker shells worked on hinges. Nearly all the
Paleozoic forms had long, straight hinge lines, which made it difficult
for their enemies to open them. Along with the change to narrower,
curved hinge lines came the decline of the tribe. They have been of
great value to the geologist in subdividing the geological column of
strata into its many formations.
The Mollusks, which are more highly organized than the Molluscoids,
have more or less distinctly developed heads and locomotive organs.
Many thousands of species are now extinct, the classes of most
geological importance being represented by clams, snails, and the
pearly nautilus. Most of them have shells and gills for breathing. The
members of the simplest group, well represented by the clam tribe,
possess two similar shells working on hinges, so that in this regard
they are much like brachiopods, but, unlike the latter, they are not
symmetrical with reference to a plane at right angles to the hinge
line. Cambrian strata contain the oldest known of the fossil forms
where they are small, relatively thin-shelled, and rare. In marked
contrast to the brachiopods these bivalves have rather steadily
increased in numbers of species and individuals to the present time,
now being represented by thousands of forms. During the Mesozoic era
they greatly out-numbered the brachiopod bivalves and took on a more
distinctly modern aspect, when the oyster tribe and closely related
types were prominently developed. Culmination in size and thickness of
shell seem to have been reached in early Cenozoic time, strata of that
age in certain places, for example in Georgia and southern California,
being filled with oyster shells 10 to 20 inches long and 4 to 6 inches
thick! In addition to their gigantic size and thickness, many of the
shells were fluted or ribbed, and so they represented an extreme type
of defensive armor among the lower animals.
Snails have existed from the earliest Paleozoic era to the present
time, and the outstanding fact of interest concerning them is that
they furnish one of the finest illustrations of an important class
of animals which has undergone practically no conspicuous change or
evolution during all those millions of years of time.
[Illustration: Fig. 53.--Sketches of chambered cephalopods showing
the main steps in the evolution of the shell forms and compartment
partitions: a, b, the only kinds in Cambrian time; c, d, forms added
in the Ordovician; e, added in the Devonian; f, added in the late
Paleozoic; g, h, characteristic of the Mesozoic era; and i, a living
form (pearly nautilus) cut through. (Drawn by the author.)]
We shall now turn our attention to the highest order of Mollusks--the
cephalopods. These creatures, whose heads are armed with powerful
tentacles and supplied with complex eyes, propel themselves by forcible
ejection of water. One general type--the chambered cephalopod--has a
shell divided into compartments (e.g., modern pearly nautilus) which
are successively built up and abandoned by the animal as it grows
larger. These chamber-shelled cephalopods constitute one of the most
remarkable and instructive illustrations of evolutionary change within
any important subgroup of invertebrate animals, ranging from early
Paleozoic to the present. Both because of the abundance of fossil
forms in rocks of all these periods of geological times, and because
certain of the evolutionary changes are so clearly expressed in the
well preserved shell portions, they are specially adapted for study. In
the late Cambrian only straight and slightly curved forms with smooth,
nearly straight chamber partitions existed. Notable advance took place
during the next (Ordovician) period when there were straight, curved,
open-coiled, and even close-coiled forms. All had simple partitions,
and the straighter forms predominated. "The size attained by the
Ordovician cephalopods was probably never surpassed by representatives
of the class. Some of the (straight) shells were twelve to fifteen feet
in length, and a foot in diameter. From this great size they ranged
down to or below the size of a pipe stem." (Chamberlin and Salisbury.)
They were more than likely the undisputed masters of the Ordovician
seas. Silurian time marked no important change in their structures,
but the coiled forms predominated for the first time. During the second
half of the Paleozoic era all preceding types with simple partitions
persisted, but in some forms the simple partitions gradually became
angled and finally rather complexly curved. During the Mesozoic era
the partition lines of the close-coiled forms evolved until a most
remarkable degree of complexity was attained, comparable, indeed, to
the sutures of the human skull plates. These remarkable forms called
ammonites, of which more than 2,000 species are known, began with the
Mesozoic, reached their climax, and passed out of existence toward the
close of the same era. Certain strata of Jurassic age are literally
filled with ammonites, some shells being several feet in diameter.
Various eccentric changes took place in the ammonites shortly before
their extinction. Some shells became uncoiled and even straight, thus
outwardly at least showing reversion to the original early Paleozoic
ancestors, but with retention of the complex partitions. Others assumed
spiral shapes and still others became curved or coiled at each end.
While these extraordinary evolutionary changes were going on among
the chambers of cephalopods during Mesozoic time, some of the ancient
close-coiled forms with very simple partitions managed to persist.
In fact this simple type, almost exactly like its early Paleozoic
ancestor, has been the only one out of this whole remarkable class of
animals to persist to the present time, being now barely represented by
the well-known pearly nautilus of the Indian Ocean.
During the Mesozoic era the highest type of cephalopod, represented by
modern squids and so-called "cuttlefishes," branched off and developed
in great profusion. These had slender internal shells, but no external
chambered shells. An inky black liquid secreted in a bag was forced
out to cloud the water when the animal was escaping its enemy, thus
antedating by millions of years the principle of smoke screen so
effectively used by ships during the World War. Some Jurassic species
got to be over two feet long, and a few specimens of that age have
been found in such perfect state of preservation that drawings of the
fossils have actually been made with the ink (after moistening) taken
from their own ink bags.
[Illustration: Fig. 54.--A fossil nonchambered cephalopod of Jurassic
age. It was closely related to the modern squid, and its ink bag is
well shown just to the left of the middle. (Modified after Mantell.)]
Before concluding this chapter we shall take up the salient points
in the geological history of Arthropods which constitute the highest
subkingdom of all animals except the Vertebrates. They are now very
abundant and varied, familiar examples being crabs and insects. A few
scant remains of simpler forms are known from the Proterozoic, but
since very early Paleozoic time they have been very common and have
undergone great evolutionary changes. A few striking examples only will
be dwelt upon. Among the most common and interesting of all Paleozoic
animals were the trilobites, distantly related to modern lobsters and
crabs.
[Illustration: Fig. 55.--Restorations of trilobites based upon actual
fossils characteristic of earlier Paleozoic time: a, Cambrian; b,
Ordovician; c, Devonian; b shows the appendages.]
Some of these grew to be two feet long, but usually they were only
one or two inches long. First known from the earliest Paleozoic, they
reached their culmination relatively early in the era and then dwindled
away to utter extinction before its close. "They were characteristic
of the Paleozoic era, beginning in great variety in the Lower Cambrian
and dominating the seas of the Cambrian (300 species) and Ordovician
(950 species). In the Silurian, though they were still common, the
trilobites were nevertheless on the decline (485 species), and this
ebbing of their vital force is seemingly shown in many picturesque
forms replete with protuberances, spines, and exaggeration of parts.
As a rule, in evolution, one finds that when an organic stock is
losing its vital force there arises in it an exaggeration of parts,
as if heroic efforts were being made to maintain the race. Spinosity
in animals is often the prophecy of tribal death. In the Devonian,
the variety and number of the trilobites were greatly reduced
(105 species), at a time when the ancient types of fishes, which
undoubtedly fed on these crustaceans (trilobites), began to be common
in the seas. In the later Paleozoic seas, the trilobites were relics,
or animals surviving from a time better suited to their needs, and one
by one they vanished, until a little before the close of the Paleozoic
era none were left." (Schuchert.)
[Illustration: Fig. 56.--A giant, sea scorpion of Devonian time. Length
nearly 3 feet. (After Clarke and Ruedemann, New York State Museum.)]
An extraordinary type of Arthropod which ranged throughout Paleozoic
time and became extinct at its close was the so-called "sea scorpion,"
closely related to the modern scorpion. Their five or six pairs of
appendages all came out from the head portion, one pair in some cases
having been developed as powerful pincers. Their culmination in size
was reached during the Devonian when some forms grew to the astonishing
length of over eight feet! Such gigantic creatures must have been
tyrants of the seas until they were subdued by the oncoming powerful
fishes. True scorpions are known from rocks as old as the Silurian.
Lobsters and crabs made their appearance during the Mesozoic era.
Since insects constitute the highest subdivision of Arthropods, they
include the very highest forms of animal life except the Vertebrates.
The oldest known fossil insects are from Pennsylvanian strata, more
than 1,000 species having been described from rocks of that age.
They were all simple or primitive types like cockroaches and dragon
flies, and were remarkable for size. Giant cockroaches got to be four
inches long. One form of dragon fly, with a spread of wing of over two
feet, was probably the largest insect which ever lived (Plate 15).
Development of insect life was especially favored during the great
Coal Age because of the prolific vegetation, but more than likely
insects originated somewhat earlier. Early in the Mesozoic era a great
progressive change began to come over insect life and higher forms
gradually evolved until by the close of the era many of the highest
types like flies, ants, and bees were common. As might be expected, the
highest insects did not develop until after the appearance of the true
flowering plants in later Mesozoic time, butterflies apparently not
having evolved until early in Cenozoic time. Many of the thousands of
known species of fossil insects are from strata of Tertiary age during
which time they may have been even more numerous than to-day, although
there are about 400,000 species now living. An almost incredible case
is a Tertiary stratum only a few feet thick in Switzerland from which
nearly 1,000 species of insects have been unearthed. Another famous
locality is Florissant, Colorado, where during early Tertiary time
there was a small lake into which showers of fine volcanic dust fell
and entombed vast numbers of insects, more than 2,000 species having
been unearthed. Still another extraordinary occurrence is along the
shores of the southern Baltic Sea where more than 2,000 species of
insects have been found in a fossil resin called amber. The insects
were caught in the still soft sticky resin while it was exuding from
the trees, and thus we have the insects, fully two or three million
years old, literally embalmed and marvelously preserved, often in
beautifully transparent amber.