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Title: How old are fossils?
Author: Roy, Sharat K.
Language: English
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HOW OLD ARE FOSSILS?
BY
SHARAT K. ROY
ASSISTANT CURATOR OF INVERTEBRATE PALEONTOLOGY
[Illustration]
GEOLOGY
LEAFLET 9
FIELD MUSEUM OF NATURAL HISTORY
CHICAGO
1927
LIST OF GEOLOGICAL LEAFLETS ISSUED TO DATE
No. 1. 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 .50
No. 9. How old are Fossils? .25
D. C. DAVIES, DIRECTOR
FIELD MUSEUM OF NATURAL HISTORY
CHICAGO, U.S.A.
[Illustration: LEAFLET 9. PLATE I.
A VIEW OF THE GRAND CANYON OF THE COLORADO RIVER, ARIZONA.
ZOROASTER IN BACKGROUND.
Photograph by O. C. Farrington.]
FIELD MUSEUM OF NATURAL HISTORY
DEPARTMENT OF GEOLOGY
CHICAGO, 1927
LEAFLET NUMBER 9
How old are Fossils?
“How old is that fossil and how do you know it?” is a question
frequently asked by visitors going through the hall of fossils in
the Museum. A precise answer to such a question is impossible and
an adequate one demands a longer time than can usually be afforded.
The consequences of inadequate explanation have often proved to be
unsatisfactory. The visitor becomes skeptical and instead of taking
interest in the subject, he seems to be confirmed in his doubts.
In this leaflet is given a condensed, general statement of methods
of determining the age of ancient life. The information is drawn
from the works of various authors, especially Barrell’s “Rhythms and
Measurements of Geologic Time.” It is intended for those who are
interested in the age of past life and yet do not intend an exhaustive
study of the subject, nor have free and easy access to its literature.
The birth of life was the most momentous occasion in the history of
the earth. When one considers the myriads of evidences unearthed by
paleontologists and paleobotanists, they seem to leave no room to doubt
the great conception that the life of the land has emerged from the
sea. It is, therefore, only a natural impulse to look for the remains
of this life in the rocks laid down by the ancient seas and to wonder
at the vastness of time behind them.
Since traces of the lowest forms of life have been found in
practically the oldest known sedimentary strata, the problem of
determining the age of life necessarily involves the determination of
the age of those strata. But unravelling the dead past is not an easy
task. One trying to unlock the “secrets of the cemetery of Nature’s
dead,” walks on a shadowy road. His difficulties are many. It is like
crossing a deep moat, climbing a steep wall.
Various methods have been applied to estimate the age of different
periods of the earth’s history and much progress has been made toward
a successful issue. Broadly, the procedure of different methods is the
same. They do not differ in principle. “The rates of certain changes
at the present day are determined as accurately as possible, and in
imagination, the respective processes are traced backward in time until
limiting conditions are arrived at.” Until the epoch-making discovery
of radium, the two most outstanding methods used in calculating
geologic time were (1) the rate of land erosion and deposition and (2)
the rate of derivation of salt (sodium chloride) from the land and its
accumulation in the oceans. Theoretically, it is simple to use the rate
at which sediments are being deposited or solutions gathered into the
ocean, as “geologic clocks” for estimating the length of past time. But
in practice each method encounters its own difficulties and the results
deduced give us at best only a rough idea of the immensity of time
involved. I shall not deal with these individual estimates, but give
the mean of several, which is 100,000,000 years, speaking roundly. As
stated before, it is only a rough estimate. Nevertheless, it confirms
the fact that the earth is very old—indeed much older than is commonly
believed. In 1650, Bishop Ussher, in his interpretation of the “In the
beginning” of Genesis, estimated that the earth was created 4004 years
before the birth of Christ. According to this view the earth is 5931
years old today. Many cosmogonists and even some geologists of the 19th
century held this Biblical interpretation to be the age of the earth.
Other ancient religions held that the earth was created much earlier
then 4004 B.C. Hutton, one of the founders of Geology, in his studies,
found “no vestige of a beginning—no prospect of an end.” One cannot
help sympathizing with Hutton. Whoever has made a trip to the Grand
Canyon of the Colorado River, Arizona, must remember the awe-inspiring
depth of the Paleozoic strata and thousands of feet of Proterozoic
sediments beneath them (Pl. I). If he has traveled farther north to
the Cabinet Range, Montana, he must have carried with him an undying
impression of the 35,000 ft. of the rocky monument, there built up by
the Proterozoic seas.
[Illustration: LEAFLET 9. PLATE II.
COMPARISON OF AN ANCIENT AND A MODERN MARINE ANIMAL OF ALLIED GROUPS.
1, A CAMBRIAN TRILOBITE (AFTER WALCOTT). 2, A MODERN HORSESHOE CRAB.]
The present rate of denudation in the Hudson Bay region is one foot in
about 47,000 years. How long then, has it taken the seas to lay down
these miles of sediments, which are but a small fraction of the whole
geologic column? Having considered all this, can we estimate, even
approximately, the vast length of time that has elapsed between these
periods of sedimentation, the so-called “gaps” or “breaks”—the torn,
illegibly written pages of the history of the earth? Indeed, there are
moments when all may feel that it is much beyond their comprehension.
But, man, by nature, is at once humble and exalted. He is willing to
admit his defeat, yet his thirst to conquer new knowledge, to know the
truth, is never satiated.
The reason for the failure to arrive at an absolute result is not very
far to seek. In computing geologic time, one has to calculate that
which has elapsed, by some process in nature that takes place in one
direction only and that does not change its rate when conditions alter.
Whatever the method applied, be it the deposition of sediments, or the
gathering of solutions, or the losing of heat by the sun and the earth,
its rate of action should be uniform and uninterrupted. It should be
independent of the changing conditions of the earth. Uniformity of
the rate of action is the criterion for precise calculation. But we
know that the past was quite different from the present. Different
conditions have existed at different times during the earth’s history.
The configuration of the earth, its climate, humidity, temperature and
many other factors have varied from time to time and with them the rate
of erosion, deposition and solution has either accelerated, diminished
or ceased. The doctrine of uniformitarianism cannot be assumed in a
changing world, even though our knowledge of the earth of the past can
only be gained from a fuller study of the earth of the present.
When we consider the rate of sedimentation as a method for estimating
geologic time, we take the total observed thickness of the geologic
column, (estimated to be 70 miles) and divide it by the rate at which
the sediments are now being laid down. But do we know this rate? By
taking the average rate of sedimentation of nine large rivers[1] now
in existence and assuming it to be the rate at which sediments were
deposited in the past, an approximate conclusion can be arrived at.
But it will be noticed from the figures of the rate of sedimentation
of different rivers that they are widely variable, the highest being
many times greater than the lowest. Can we then use an average of nine
figures so out of proportion and yet expect a reliable quantitative
value? Furthermore, the rate of deposition along the coast, near the
mouth of a large and active river, is much higher than on a coastline
where no rivers empty their sediments into the sea. The rate is also
largely controlled by the character of the sediments deposited.
Sandstone and shale are more rapidly deposited than limestone.
Moreover, just as there is no knowledge of the duration of time of
erosion between periods of sedimentation, there is also no record of
the amount of detritus that has fallen off the edge of the continental
shelf during widespread emergence of the continents.
[1]
-----------+-----------+-------------+-----------+------------------
River | Drainage | Total tons | Ratio of |Height in feet of
| areas in | annually | sediment |column of sediment
| square | | to water | with a base of
| miles | | in weight | 1 square mile
-----------+-----------+-------------+-----------+------------------
Potomac | 11,043 | 5,557,250 | 1: 3,575 | 4.0
Mississippi| 1,244,000 | 406,250,000 | 1: 1,500 | 241.4
Rio Grande | 30,000 | 3,830,000 | 1: 291 | 2.8
Uruguay | 150,000 | 14,782,500 | 1:10,000 | 10.6
Rhone | 34,800 | 36,000,000 | 1: 1,775 | 31.1
Po | 27,100 | 67,000,000 | 1: 900 | 59.0
Danube | 320,300 | 108,000,000 | 1: 2,800 | 93.2
Nile | 1,100,000 | 54,000,000 | 1: 2,050 | 38.8
Irrawaddy | 125,000 | 291,430,000 | 1: 1,610 | 209.0
-----------+-----------+-------------+-----------+------------------
Babb. Science, Vol. XXI, p. 343, 1893.
We have also no record of the vast quantity of shallow water sediments
that were stirred up by the penetration of storm waves, and carried to
abyssal depths by the currents and tides.
Similar uncertainties beset us when we consider the rate of chemical
denudation, that is, the rate at which salts have been dissolved from
the lands and accumulated in the oceans, as a measure of geological
time. Here, again, we take the total amount of salts that is in the
oceans today and divide it by the present rate of annual supply. We
know with reasonable accuracy the quantity of salts in the oceans
and if it were possible to assume the present rate of supply to be a
true mean for all geological time, a satisfactory age of the oceans
might be obtained. But it cannot be assumed as such. An assumption of
this nature will only lead us from the domain of exactness to that of
uncertainty. Aside from various other factors, neither the area of
the continents, nor their relief was in the past the same as today.
Consequently, the stream gradient and its power of dissolving salts
from the land surfaces have not been the same. It is also not known how
much salt the ocean derived from the shore line and from beds beneath
the ocean, nor how much of the rock-salt beds on the earth that has
been precipitated out of ocean water.
It is plain, therefore, that the rate of any process that is controlled
by so many conditions cannot be used (even making generous allowances
for irregularities and inaccessible data) as a reliable guide to
evaluate geologic time. “It is a clock,” says Harker, “which now
hurries and now creeps or stands still, and it cannot be trusted as a
timekeeper.”
Any estimate based on the temperature of the earth, or of the sun,
encounters similar practical difficulties, for the temperature of a
body may not be constant. It may rise or it may fall. Further, the rate
of change of temperature is controlled by a variety of conditions, such
as the amount of energy radiated, the supply of energy and so forth.
Nor is there any record of the immense quantity of heat produced by
igneous agencies and radio-activity.
Another estimate, one of the earliest, was based on the rate of life
transformation in successive periods. The geological series were
divided into twelve periods and it was believed that 20,000,000 years
were required for an entire change in the species of each period, or
240,000,000 years in all. This does not include the time in which we
have no record of plant or animal life.
There is no reasonable debate as to the passage of one species to
another. It is clearly manifested in the succession of fauna found
today all over the world in the sedimentary rocks. Even the most casual
student of paleontology is convinced of this glaring truth. “The brutal
cogency of a slab of fossils could be hated and fought, but could not
be gainsaid.” But when we are confronted with the question of setting
a standard of measuring geologic time by means of this paleontological
record, more precisely, through this biological process, we cannot
help pondering over the grave uncertainty of the result. When we fix
our gaze upon a trilobite, a three-lobed, crab-like creature (Pl. II
fig. 1) that ruled the seas in the dim days of the Cambrian period (p.
11) and see that it was equipped with gills and swimming organs, with
powers of digestion and excretion, with specific organs of circulation
and reproduction and with motor and sensory nerves, and compare it
with one of its tribe, a present day horseshoe crab, (Pl. II fig. 2)
we do not find any noticeable progress in structure, in intricacy or
in the degree of specialization. Yet the time that has elapsed since
the Cambrian is, according to a moderate estimate, nearly 600,000,000
years! (p. 11). Geologic record testifies that evolution awaits
environmental change, that animals in some way adjust themselves to
their environment, either by discarding or modifying old characters or
by acquiring new ones. Yet, what are known as “immortal” types, such as
the brachiopods, Lingula, Crania and Terebratula (Pl. III figs. 1-3)
or the pelecypods, such as Pecten, Pinna and Arca (Pl. III figs. 4-6)
or the gastropods, such as Pleurotomaria, Natica and Trochus (Pl. III
figs. 7-9), have withstood all possible environmental changes and have
steadfastly held their own ever since we have records of their very
early appearance on earth. On the contrary, we have records of types
that have yielded so rapidly to change that their evolution is almost
explosive. It is almost incomprehensible how, within such a limited
period of time, fishes have changed into amphibians, amphibians into
reptiles, and reptiles into birds and mammals (Pl. IV figs. 1-4). With
these conflicting evidences staring us in the face, with the knowledge
that the entire organic world has been subject to earth-wide periods
of long stagnation and rapid intensive change, one may well ponder
whether it is within our power to establish a standard for measuring
geologic time on the evidence of life transformation. The study of the
succession of faunas—the change of one species to another, can only
indicate the magnitude of time involved. It cannot afford any basis,
whatsoever, for a concrete expression of geologic time.
During the last three decades, a number of radio-active changes of
one chemical element into another have been discovered and studies of
certain minerals and rocks containing various radio-active elements
have created means to calculate their age with remarkable accuracy.
“A study of the various radio-active elements contained in minerals
and rocks,” says Harker, “has shown that it is possible, in certain
favorable cases, to calculate directly their age in years.”
The radio-active minerals are commonly found in igneous rocks. They
are widely distributed all over the world. The parents of the whole
series of radio-active elements are uranium and thorium. They possess
the highest atomic weights of all known elements. Each of these
parental elements transforms itself through a succession of changes.
The final product of uranium is the formation of the metal lead and
the gas helium. These transformations take place in one direction
only, that is, from an element of higher atomic weight to an element
of lower atomic weight. It has also been demonstrated beyond question
that these transformations are unalterable by any process whatsoever
and that they are independent of temperature, pressure or any other
physical or chemical state. Temperatures up to 2,500 C. and pressures
up to 600 tons per square inch have not been found to influence the
rate of transformation. Time estimated on the basis of these processes,
therefore, offers a more reliable result than that obtained by any
other method hitherto known. Detailed descriptions of how the metal
uranium slowly and regularly breaks down in a descending series into
the metal lead and the gas helium, will be found in the literature on
radio-activity. For our purpose, it suffices to say that according to
Barrell, an atom of uranium which breaks up will ultimately give rise
as a stable product to eight atoms of helium and one atom of lead.
Since the rate of transformation is known, data for calculating the age
of the mineral and with it the rock formation of which it is a part,
can be obtained by measuring the quantity of helium and lead in the
rock and comparing it with the quantity of uranium in the same volume
of material. But, as helium is a gas, it is likely that a certain
portion of it leaks out and consequently the estimate of age on the
basis of how long helium had been in contact with uranium and lead is
to be regarded as a minimum estimate. For example, the age of the
mineral thorianite that occurs abundantly in the sands and gravels of
Ceylon has been estimated to be 280,000,000 years, but the mineral
is doubtless much older, as, ever since it was broken away from its
original home in the pegmatite dikes of Ceylon, it lay exposed to the
action of weathering and it was, therefore, very likely that during all
these years a certain percentage of its helium contents had leaked away.
But estimates based on the lead ratios of radio-active minerals offer
results consistent among themselves. That is, whenever fresh, primary,
uranium-bearing minerals of the same geological age have been examined,
the lead ratios are always found to remain constant. The value of the
ratios increases or decreases as the geological age of the respective
mineral increases or decreases. In other words, the lead ratios are in
keeping with the geological age.
The procedure of applying the lead ratio in calculating geological time
can be briefly stated thus: The rate of production of lead from uranium
can be readily calculated. The rate at which helium is generated is
accurately known and the quantity of lead liberated in the same time is
approximately 6.5 times that of helium. In a year one gram of uranium
produces 1.25 × 10⁻¹⁰ grams of lead, and at this rate 8,000[2] million
years will be required for the production of one gram of lead.
[2] A more recent and accurate computation reduces this 8,000 million
years to 7,500 million years.
There is no serious difficulty in applying this method for measuring
geologic time, except that it is necessary to determine whether
the lead is of radio-active origin or original lead. The presence
of original lead is likely to mar the constancy of the lead ratio
essential for accurate results. But ordinary lead need not be confused
with uranium lead, as the atomic weight of ordinary lead is 207.1 and
that of uranium lead is 206.2. Values between these two figures imply
a mixture of two types of lead. For reliable calculations, a series of
fresh, primary minerals of the same geological age showing a constant
lead ratio of atomic weight 206.2 needs to be examined.
The following table shows the geologic time that has elapsed
between the first evidences of life and the present, as calculated
by Barrell from radio-active data. The figures are his minimum and
maximum estimates. It will be noticed from the figures in the table
that the earliest life of which we have fossil records is about
1,500,000,000 years old. From this, it could be safely concluded that
the inception of life on earth must have taken place much earlier.
It is quite significant that each geological era, occasionally a
geological period, has its characteristic grouping of life developed
from the life of preceding periods. As we climb higher in the
geological column, life becomes more and more complex and specialized.
From the one-celled life of the Archeozoic it passes through the
invertebrates—fishes—amphibians—reptiles—birds and mammals to man
of the Recent time.
Since Barrell’s publication of the estimates of geologic time as
measured by means of radio-activity, some further studies have been
made along the same line, but, as no generally accepted results have
shown any marked differences from Barrell’s results, it has been deemed
advisable to use his age data as perhaps our present most adequate
guide as to the length of geologic periods.
Although the measurable forces of radio-activity give on the whole a
remarkably satisfactory time gauge and are doubtless more accurate
than any method here discussed, it must not be considered that the
ages given (p. 11) are absolute. The knowledge of geological time is
of more importance for the comparative than for the absolute magnitude
of the results obtained. Just as the study of astronomy gives us the
conception of the vastness of space, so does the study of geology
reveal to us that of the immensity of time.
SHARAT K. ROY.
[Illustration: LEAFLET 9. PLATE III.
“IMMORTAL” TYPES.
1, LINGULA. 2, CRANIA. 3, TEREBRATULA. 4, PECTEN. 5, PINNA. 6, ARCA.
7, PLEUROTOMARIA. 8, NATICA. 9, TROCHUS.
(Figs. 1, 2, 7, after Hall, 3, 9, after Zittel, 4, 6, after Dall, 5,
Pal. N. J. I, 8, after Cragin).
Drawings by Carl F. Gronemann.]
THE GEOLOGICAL TIME TABLE
1.0 = One million years
------------+-----------------+---------------+--------------------
| | TIME SCALE |
ERAS | PERIODS |(After Barrell)| CHARACTERISTIC
| +-------+-------+ LIFE
| |Minimum|Maximum|
------------------------------+-------+-------+--------------------
PSYCHOZOIC | Recent | | |
------------+-----------------+ 1 | 1.5 | AGE OF MAN
| Pleistocene | | |
+-----------------+-------+-------+--------------------
| Pliocene | 7 | 9 |
+-----------------+-------+-------+
CENOZOIC | Miocene | 19 | 23 | AGE OF MAMMALS
+-----------------+-------+-------+ AND MODERN
| Oligocene | 35 | 39 | FLOWERING PLANTS
+-----------------+-------+-------+
| Eocene | 55 | 65 |
------------+-----------------+-------+-------+--------------------
| Cretaceous | 95 | 115 |
+-----------------+-------+-------+
| Comanchian | 120 | 150 |
MESOZOIC +-----------------+-------+-------+ AGE OF REPTILES
| Jurassic | 155 | 195 |
+-----------------+-------+-------+
| Triassic | 190 | 240 |
------------+-----------------+-------+-------+--------------------
| Permian | 215 | 280 | AGE OF AMPHIBIANS
+-----------------+-------+-------+ AND
| Pennsylvanian | 250 | 330 | ANCIENT FLORAS
+-----------------+-------+-------+--------------------
| Mississippian | 300 | 370 |
+-----------------+-------+-------+ AGE OF FISHES
PALEOZOIC | Devonian | 350 | 420 |
+-----------------+-------+-------+--------------------
| Silurian | 390 | 460 |
+-----------------+-------+-------+ AGE OF HIGHER
| Ordovician | 480 | 590 | SHELLED
+-----------------+-------+-------+ INVERTEBRATES
| Cambrian | 550 | 700 |
------------+-----------------+-------+-------+--------------------
| | |
PROTEROZOIC | | 925 | AGE OF PRIMITIVE
| | | INVERTEBRATES
------------+ Systematic +---------------+--------------------
| classification | LONG EROSIONAL INTERVAL
------------+ variable +---------------+--------------------
| | | DAWN OF UNICELLULAR
ARCHEOZOIC | | 1500 | LIFE, ALGAL FORMS
| | | REPORTED
------------+-----------------+---------------+--------------------
BIBLIOGRAPHY
BARRELL, J.—Rhythms and Measurements of Geological Time.
Bull. Geol. Soc. Am., Vol. 28, 1917, pp. 745-904.
BECKER, GEO. E.—The Age of the Earth.
Smithsonian Misc. Coll., LVI., No. 6, 1910.
CHAMBERLAIN, T. C.—Diastrophism and the Formative Processes, XIII.
The Time over which the Ingathering of the Planetesimals
was Spread.
Jour. Geol., Vol. XXVIII, 1920, pp. 675-81.
HARKER, A.—Geology in relation to the exact sciences, with an
excursus on geological time.
Nature, Vol. 95, 1915, pp. 105-109.
HOLMES, A.—The Age of the Earth.
Harper and Bros., London and New York, 1913.
JOLY, J.—Radioactivity and Geology.
Van Nostrand, N. Y., 1909;
An Estimate of the Geological Age of the Earth.
Trans. Roy. Soc. Dublin, VII. 1899, pp. 23-66;
The Age of the Earth. Phil. Mag., 6th ser.,
Vol. XXII, 1911, pp. 359-80.
SOLLAS, W. J.—Presidential Address.
Quart. Journ. Geol. Soc., Vol. 65, 1909, pp. cxii;
Proc. Geol. Soc. of London, Sess. 1908-9, pp. i-cxxii.
WALCOTT, C. D.—Geologic Time, as indicated by the sedimentary rocks
of North America.
Smithsonian Rep. 1893, pp. 301-334;
Jour. Geol. Vol. III, 1893, pp. 639-674.
[Illustration: LEAFLET 9. PLATE IV.
EVOLUTION OF FISH TO MAMMAL-LIKE REPTILE.
1, FISH. 2, AMPHIBIAN. 3, REPTILE. 4, MAMMAL-LIKE REPTILE.
(Figs. 1, 2, after Klaatsch, 3, based on Williston, 4, based on
Gregory).
Drawings by Sharat K. Roy.]
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