Origins 1(2):58-66 (1974).
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Growth lines found in several invertebrates show promise of serving as a basis for many avenues of investigation. Their value as an independent method for geochronometry is presently questioned, while other methods of using them are being developed.
INTRODUCTION
Periodic growth structures (lines, bands, and rings) are preserved
in the skeletons or hard parts of many organisms. Although the best known example is the
annual tree rings (not further considered in this paper), periodic growth structures have
also been long observed and studied in other organisms (Orton 1923, 1926; Ma 1937;
Davenport 1938; Stevenson and Dickie 1954; Sakai 1960). Most current interest, however,
was initiated by Wells (1963) when he described "daily" growth lines in living
and fossil corals and used these in conjunction with annual growth structures in proposing
the growth-line method of "absolute" age-dating (geochronometry). Shortly
thereafter, Wells' data was used in studies on the origin of the earth-moon system and the
rotational history of the earth (MacDonald 1964; Runcorn 1964). Most recent papers
(including Newton 1969; Runcorn 1970; Lamar et al. 1970; Pannella 1972; Scrutton and
Hipkin 1973) have used growth lines in this way, rather than as a geochronometric
technique. A third application of growth lines, antedating Wells' (1963) paper, is in life
history, ecology, and paleoecology studies (e.g., Orton 1926; Ma 1937, 1938; Davenport
1938). This application has been made more recently by Rhoads and Pannella (1970), Farrow
(1971, 1972), and Tevesz (1972).
Although all three applications of growth lines have philosophical
significance, it was their use in geochronometry that caused greatest perplexity to those
believing in the Biblical creation account and a short chronology. According to Wells
(1963) the importance of the growth-line method was its apparent independence from
radiometric age-dating methods and its capability of directly dating fossils. (Radiometric
methods for older samples can usually date only certain rocks surrounding the fossils). If
this clock independently gave ages similar to the radiometric ages, the confidence in
these ages would be increased. Although recent research does not support its use as an
independent geochronometric method, preliminary results were in partial agreement with
radiometric ages.
In this paper, I will first describe the growth-line method, then
discuss problems in its application. Finally, consideration will be given to geophysical
and paleoecological implications of periodic growth structures that might have positive
use in creation theory and in construction of a flood model.
GROWTH-LINE METHOD OF GEOCHRONOMETRY
To understand the method, the periodic growth structures and a
phenomenon called tidal friction need explanation. Periodic growth structures are produced
by a living organism and are a record of cyclic changes that occur in growth rate and/or
density and composition of growth material. The growth structures used in geochronometry
are preserved in coral skeletons, clam and brachiopod shells, fish otoliths (earbones),
and stromatolites (algae). They are observed in both living and fossil forms.
The simplest component of these periodic growth structures consists of
a dark band, high in organic content, alternating with a light band of mainly inorganic
material, usually CaCO3 (see picture on back cover).
This simplest component will here be considered as a circadian (approximately daily) line,
although often two or more of these single components are produced each day (Barker 1964).
The circadian line is usually from 5 to 60 microns wide. Other growth structures or
patterns are constructed by cyclic fluctuations in the width, density, or composition of
the circadian lines. Fortnightly, monthly, and annual patterns occur this way. These
patterns are correlated with, and are presumably a response of, the organism to various
physical environmental factors such as the light-dark cycle, tidal fluctuations,
temperature, and sedimentation.
Although sophisticated mathematics is required for a complete treatment
of tidal friction and its effects, the basic concept can be understood intuitively. As the
earth rotates, the moon, through gravitational attraction, continuously raises tidal
bulges on the earth. Due to tidal friction, the bulges lag in time and are carried forward
by the rotation of the earth, causing a misalignment of the tidal bulges with respect to
the line of centers of moon and earth. (Tidal friction is mainly friction in the tidal
currents between water molecules and between the water and the coastline or ocean bottom.)
This misalignment produces a torque between the earth and moon causing a deceleration of
the earth's rotation and an increase in the moon's angular momentum. The result is a
slight increase in day length and a slight decrease in the number of days per month and
days per year. (See Goldreich 1972 for a detailed but non-mathematical treatment of the
effects of tidal friction.)
The increase in day length is estimated at two thousandths of a second
per century (2 msec/century). This is based on observed perturbations in the orbits of
artificial satellites and from comparing reported with expected times of ancient eclipses
or other astronomical events (Newton 1969; Scrutton and Hipkin 1973). Although the
magnitude of change seems insignificant and undetectable, it would be significant in two
instances. First, since the increase in day length is additive, the cumulative
time shift would be significant even in historical time. For example, the time shift would
be 36 seconds in a century, one hour in a millennium, and about 6 hours in 2500 years. An
eclipse 2500 years ago would then be reported as occurring 6 hours earlier than expected
from present observations and calculations. This type of information was used originally
to establish the 2 msec/century value.
The second instance where the change in day length would be significant
occurs when the geologic (radiometric) time scale is accepted. One hundred million years
ago the day would have been 0.55 hours (2 msec/century × 1,000,000 centuries = 2,000,000
msec = 0.55 hours) shorter than the present day, giving 374 days/year (8766 hours/day ÷
23.45 hours/day = 374 days/year). At 600 million years (the oldest radioactive dated
material where abundant well-defined fossils occur), the day would have been 3.3 hours
shorter, giving 424 days/year. It is on this basis that the growth-line method was
proposed.
Wells (1963) suggested that if both daily and yearly growth structures
could be identified in fossils, the fossil's age could be determined by counting the
number of daily bands per yearly band. Thus, from our previous example, a fossil
containing 374 daily bands per yearly band would be 100 million years old, and one
containing 424 daily bands per yearly band would be 600 million years old. Other ages
could be obtained in the same way from other daily bands per yearly band values.
Using fossil coral specimens from the Devonian and Pennsylvanian
geologic strata, Wells (1963) counted respectively 385 to 410 and 385 to 390 daily bands
per yearly band. From the radiometric dates assigned these strata, the expected number of
daily bands would have been 399 and 392 respectively. Approximately 360 daily bands per
yearly band were counted in one species of living West Indian coral. Although interesting,
this data is inconclusive because of the small number of specimens used and the large
range in values
Since this initial paper, most work has been done on clams and results
are expressed in terms of changes in days per month rather than days per year, since
complete monthly sequences are more commonly preserved in the fossil record. This data is
summarized by Pannella (1972). While some of the growth-line ages agree with the
radiometric ages, there are two serious anomalies in the present data. One of these is
apparent in Figure 1 where the slope of the computer-fitted curve often deviates from the
predicted trend. A more striking, but less well substantiated, anomaly based on
Precambrian stromatolite data (Pannella 1972) suggests a great reduction of the 2
msec/century value in Archeozoic time if the radiometric ages are correct. These anomalies
indicate that either the deceleration of the earth's rotation rate has not always been
constant (i.e., it has deviated from 2 msec/century), the radioactive dates are not
correct, or insufficient data has been collected. Pannella maintains that while more data
is necessary to establish the exact shape of the curve, the present data do strongly
support a non-uniform deceleration rate. Implicitly, the radiometric dates are accepted as
correct and the growth lines are not used as a geochronometric technique.
FIGURE 1. Variations in the number of days per month during geologic time according to Pannella (1972). Vertical bars indicate ±1 standard error. The computer best fit curve is a fourth order polynomial. (Redrawn from Pannella 1972).
PROBLEMS WITH THE GROWTH-LINE METHOD
Already alluded to, the most serious problem in applying the
growth-line method concerns the magnitude and constancy of the deceleration of the earth's
rotation rate. The ancient astronomical records on which the historical deceleration
values (2 msec/century) are based are not easily interpreted. Some are definitely not
reliable. Newton (1969) discusses this problem at some length. The evidence does indicate,
though, that the deceleration of the earth's rotation rate has not been constant even
within relatively recent historic time. Newton states that "ancient astronomical data
show with high confidence that the amount of tidal friction ten centuries ago was twice
what it is now." No method exists for independently (i.e., independent from growth
lines) determining the magnitude of tidal friction or deceleration prior to the ancient
astronomical observations. Geophysicists are now using the growth-line data to obtain
prehistoric deceleration rates (Newton 1969; Runcorn 1970). However, if the growth-line
data is used to determine deceleration values, it cannot be used as an
independent geochronometric method as this would involve circular reasoning. It could be
used in geochronometry by calibration with radiometric dates, but then the independence
between the two methods is lost and the value of the method greatly reduced.
The use of daily bands per monthly band in most of the recent work
poses a second problem. While the rate of revolution of the earth around the sun is
assumed to have remained constant, the rate of revolution of the moon around the earth has
probably not remained constant. Exactly how it has changed is not known. This introduces a
second variable and the possibility of confusion of variables.
Presently insufficient data is available on growth lines in living
organisms to properly evaluate their meaning in fossil species. It is not always simple to
determine of what a daily, monthly, or yearly band consists (e.g., the bands in the back cover picture). Subdaily bands often occur. These may be
confused with daily bands. Apparently lines are at times missing; thus some researchers
urge the use of maximum line counts rather than average counts (Clark 1968; Mazzullo
1971). However, this has problems since in one recent species anywhere from 283 to 425
circadian bands per yearly band were found (Farrow 1971). Maximum counts would give high
values. If large ranges (283 to 425) in values are typical, large sample sizes (many
fossils) would be necessary to get sufficiently good resolution for the method to be
meaningful (i.e., the inherent variability or range in values within a single species is
of the same magnitude as the range expected from changes in deceleration rate in many
hundred million years). In fact, sufficiently well-preserved fossils are rare and
subjective bias can be introduced in interpreting unclear growth patterns. Objective
methods of identifying and counting these bands are not now available. Environmental
factors change the nature of the lines in ways that are far from being completely
understood. Some of these problems could disappear with future work, others seem
insurmountable.
ALTERNATIVE INTERPRETATIONS OF GROWTH-LINE DATA
The use of growth lines in geochronometry does not seem feasible and
is not advanced as such in recent papers on the subject (Pannella 1972; Scrutton
and Hipkin 1973). However, other types of information that they might provide could be
useful. The available data on growth lines does need to be interpreted in terms of a flood
model, and they may be useful in constructing such. One possible explanation of an
increase in number of circadian increments per month or year towards the bottom of the
geologic column could be related to the depth at which the preflood living specimen
occurred. Although depth is known to have an effect on the nature of the growth line in
recent organisms (Rhoads and Pannella 1970), no present evidence is available on the
relation between depth and number of lines. If valid, this explanation would fit with
Clark's ecological-zonation model (Clark 1967, pp. 76-80).
Another possible explanation would be a relatively rapid change in the
earth's rotation rate at about the time of the flood. Changes in the magnitude of tidal
friction are usually explained in terms of changes in the extent of shallow seas or extent
of ice cover — particularly in the Antarctic (Goldreich 1972; Newton 1969). Rotation
rate could also change by other means than tidal friction, e.g., changes in the moment of
inertia could occur through accretion or through redistribution of the earth's mass. It
would be surprising if some such activities did not occur at or following the flood, given
the violent nature of the event as recorded in inspired writings. While the present
growth-line data is not easily interpreted in terms of this explanation, the anomalies
that occur in the data do partially support it.
Another way the growth lines may be useful in the support or
construction of a flood model is through their use in paleoecology. Growth lines are often
sensitive indicators of environmental conditions. Present research in this area is mainly
concerned with gathering sufficient data on growth lines in living organisms to allow
interpretation of fossil ones. It is suggested that such information as water temperature,
depth, and age and season of death may be contained in the growth-line patterns (Barker
1964; Rhoads and Pannella 1970). Clark (1968) suggests that by comparing the individual
growth patterns within a fossil assemblage, the assemblage may be determined as a
community with a catastrophic death (growth patterns have same endpoint), as a normal life
span community (overlapping growth patterns), or no community, e.g., not in position of
growth (no correlation between growth patterns). In a flood-model explanation of the
geologic column, the first and third cases should have greatest prevalence.
CONCLUSIONS
Relatively little work has been done with these invertebrate growth lines. Their use in geochronometry, geophysics, or paleoecology is still very much in the initial state; therefore, caution should be used in saying just how they can or cannot be used. Present information does not support their use as an independent geochronometric method (Pannella 1972). They may be useful in developing geophysical theory (regarding movements of the earth and moon), although the precision and resolution of the method has not yet been sufficient for really significant contributions in this way either (Scrutton and Hipkin 1973). Further research is necessary to determine their potential as paleoecological indicators.
REFERENCES
FRONT COVER. Cross-section through right and left valves of the clam Protothaca tenerrima. Notice similarities in growth-line patterns between the valves (38× nonpolarized negative). See article by Conrad D. Clausen for further details.
BACK COVER. Higher magnification of part of front-cover picture (290× polarized positive). Groups of wide bands represent periods of rapid growth, while groups of narrow bands represent slow growth which probably reflect a fortnightly tidal cycle.
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Geoscience Research Institute. All rights reserved.
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