The Growth-Line Method of Age Dating

"The [Adventist] church recognizes that the creationist position faces some very difficult problems. . . . There is the problem of the age rings on clamshells. . . . on some fossil clams [these] seem to indicate that these clams were living on the earth 450 million years ago--just as the evolutionists insist they did."1 This quotation is from one of at least two articles 1, 2 in recent Adventist literature that have referred to clam growth rings as a difficult problem for creationists. What are these rings? How are they used in age dating? Are they a serious problem for those who accept the book of Genesis as historically accurate?

-assistant professor of biology, Graduate School, Loma Linda University at the time this article was written

"The [Adventist] church recognizes that the creationist position faces some very difficult problems. . . . There is the problem of the age rings on clamshells. . . . on some fossil clams [these] seem to indicate that these clams were living on the earth 450 million years ago--just as the evolutionists insist they did."1 This quotation is from one of at least two articles 1, 2 in recent Adventist literature that have referred to clam growth rings as a difficult problem for creationists. What are these rings? How are they used in age dating? Are they a serious problem for those who accept the book of Genesis as historically accurate?

In 1963 J. W. Wells of Cornell University first proposed age dating fossils by growth rings or growth lines.3 This new method (which will here be called the growth-line method) of age dating was important because it was apparently independent of the various radiometric time clock methods and because it could date a fossil directly. (Radiometric methods for older samples can usually date only certain rocks surrounding the fossils.) If the growth-line method gave fossil ages similar to those obtained by the radiometric clocks, it would definitely help establish the validity of the radiometric ages. The partial agreement of preliminary growth line age determinations with radiometric ages has caused some perplexity among those believing in the short Biblical chronology. Recent research, however, does not support its use as an independent age-dating method, as will be seen.

This article will describe how the growth-line method works, the problems in applying it to age dating, and how growth lines could provide scientific evidence in support of the Biblical Flood account.

Growth-Line Method

First, two concepts need explanation—periodic growth bands and tidal friction. Many living organisms produce periodic bands or lines as they grow. Growth lines found in coral skeletons, clam and brachiopod shells, fish earbones (otoliths), and certain algae (stromatolites) have possible use in age dating. The growth-line patterns in these organisms are considerably more complex than the more familiar features of tree rings.

The basic unit of these patterns consists of a dark band, high in organic (protein) content, and a lighter color band consisting mainly of mineral material (calcium carbonate) (see photo). This basic unit we will term an "approximately daily (circadian) band," although an organism may produce two or more of these basic units each day. Circadian bands usually vary from one ten thousandth to two thousandths of an inch in width. Longer, more complex, growth patterns are usually formed by periodic fluctuations in the width or nature of the circadian bands. Fortnightly, monthly, and annual patterns often occur in 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.

The second concept, tidal friction, requires sophisticated mathematics for a complete explanation, but the basic concept is quite simple. As the earth rotates, the moon raises tidal bulges on the earth. Due to friction, these bulges lag in time, causing the earth's rotation to slow down and the moon's motion to speed up. As a result the days have gradually lengthened and the number of days per month and days per year has gradually decreased.

Scientists estimate that the day length is growing longer at the rate of two thousandths of a second per century. They base this figure on the observed changes in the orbits of artificial satellites and comparison of actual with expected times of ancient eclipses or other astronomical events. 4 Although such a change seems insignificant and undetectable, it could be significant in some cases. For example, if the geologic (radiometric) time scale is accepted, 100 million years ago the day would have been 33 minutes shorter than the present day, giving 374 days per year. At 600 million years (the oldest radiometric dated material where well-defined fossils occur), the day would have been 3.3 hours shorter, giving 424 days per year.

With these figures in mind Wells proposed the growth-line method as an independent time clock. He3 suggested that if both daily and yearly bands 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 examples) 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 calculated in the same way.

Using fossil coral specimens from the Devonian and Pennsylvanian geologic strata, Wells 3 counted respectively 385-410 and 385-390 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. The correlation was close. Approximately 360 daily bands per year of growth were counted in specimens of one re cent coral species living in the West Indies. Although interesting, Wells's data is inconclusive because of the small number of specimens used and the large range in values.

Most of the recent-work of this type has been done with 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. While some of the growth-line ages that have been reported agree with the radiometric ages, these data contain two serious irregularities where the growth-line ages differ greatly from the radiometric ages. 5 These irregularities indicate that (1) the earth's rotation has not slowed down at a constant rate, (2) the radiometric ages are correct, or (3) insufficient data has been collected. C. Pannella, 5 formerly at Yale University, maintains that the differences are real (i.e., that sufficient data has been collected to verify that the irregularities do occur) and the radiometric ages are valid. He believes the irregularities indicate that the earth has not always slowed down at the same rate. 5

Problems With Method

The most serious problem in using the growth-line method concerns the amount and constancy of the slowing down of the earth's rotation. The ancient astronomical records, which offer historical evidence that the earth's rotation is slowing down at the rate of two thousandths of a second per century, are not easily interpreted. Some of these ancient records are definitely not reliable. R. R. Newton, of Johns Hopkins University, discusses this problem at some length. 4 The evidence seems to indicate that the earth's rotation has not slowed at a constant rate 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." As far as I am aware no method exists for independently (i.e., independent from growth lines) determining the amount of tidal friction or slowing down prior to the ancient astronomical observations.

The growth-line data previously alluded to are interpreted as indicating a changing slowing down-rate, and geophysicists are, in fact, now using the growth-line data to establish this rate. How ever, if the growth-line data are used to calculate how the earth has slowed down, they cannot be used as an independent age-dating method, for this would involve circular reasoning. They could still be used in age dating by calibration with the radiometric ages, but then the independence between the two methods would be lost and the value of the method greatly reduced.

The data presently available on growth lines in recent living organisms is insufficient to properly evaluate their meaning in fossil species. It is not always simple to determine what is a daily, monthly, or yearly band. Subdaily bands often occur and may be confused with daily bands. Since growth lines are occasion ally missing, some researchers urge the use of maximum line counts rather than average counts. However, this can lead to unsure conclusions, since in one recent species anywhere from 283-425 circadian bands per yearly band have been found. 6

If such large ranges (283-425) are typical, large sample sizes (many fossils) would be required to get adequately precise averages for the method to be meaningful (i.e., the range of normal variation could be much larger than the change in the average to be expected over many millions of years). An adequate sample size is difficult to obtain because sufficiently well-preserved fossils are rare. Preservation of growth lines in fossils is often not good and the researcher can let his bias help interpret 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 we do not now completely understand. Some of these problems could disappear with future work. Others seem insurmountable.

Alternative Interpretations

The use of growth lines in age dating fossils does not seem feasible, and it has not been advanced as such in recent papers on the subject. 5 ' 6 ' 7 However, the growth lines might provide other types of useful information—particularly in the context of the Biblical Flood. For example, could the apparent changes in number of circadian lines per month or year provide information about antediluvian and flood conditions or events? Might they indicate an abrupt change in the earth's rotation during the Flood? Such change would be in harmony with the violent nature of the Flood as revealed in inspired writings. While the growth-line data are not at present easily interpreted in terms of an abrupt change, the irregularities that occur in the data do partially support it.

Different depths at which the antediluvian marine animals lived might also be a cause of variation in the number-of circadian lines per month and year. Depth is known to have an effect on the nature of the growth lines in re cent organisms—however, no data is presently available on the quantitative relationship between depth and number of lines. If valid, this explanation would fit H. W. Clark's8 interpretation of fossil distributions. He explains these in terms of the ecological distribution (e.g., depth) of organisms at the time of the Flood, rather than as the consequence of prolonged evolution.

Growth lines may also provide other information about the natural history or ecology of fossilized organisms (paleo-ecology) since they 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 growth lines. It is suggested that such information as depth (previously alluded to) and temperature of water, age of the organism, and season of the year when death occurred may be reflected by the growth-line patterns. More importantly, by comparing the growth patterns of fossils within a bed of fossils, the fossil group might be determined as (1) a community with a catastrophic (flood) death (growth pat terns have the same endpoint), (2) a normal life-span community (overlapping growth patterns), or (3) no community, e.g., not in position of growth (no correlation between growth patterns). In a catastrophic explanation of the geologic record (as compared with a long time sequence explanation), the first and third cases should occur most frequently.


Relatively little work has been done with invertebrate growth lines; therefore caution should be used in saying just how they can or cannot be used. Present information does indicate, how ever, they will not be useful as an independent age-dating method. 5

As in so many instances where there is apparent conflict between science and revelation, more data or a careful analysis of the already available data suggests alternate interpretations. "True science and Inspiration are in perfect harmony." 9 The use of inspired testimony regarding the natural world enables the Christian scientist to reach greater understanding than would be possible from nature alone.

All problems regarding growth lines (or other aspects of science) should not be expected to become completely resolved, for Cod has not promised to remove the possibility of doubt. It is sufficient that we make decisions on the weight of evidence, 9 in confidence that with adequate investigation the weight of evidence will be on the side of Inspiration.


1. Signs Report to the Nation: Adventists and Evolution: "Glorious victory or sad mistake?" Signs of the Times 99 (6):12, 13 (1972).

2. Donald E. Hall, "The 23-hour day," Spectrum 3:39-51 (1971).

3. J. W. Wells, "Coral growth and geochronometry," Nature 797:948-950 (1963).

4. R. R. Newton, "Secular accelerations of the earth and moon," Science 766:825-831 (1969).

5. G. Pannella, "Paleontological evidence on the earth's rotational history since early Precambrian," Astrophysics and Space Sci. 16:212-237 (1972).

6. G. E. Farrow, "Periodicity structures in the bivalve shell: experiments to establish growth controls in Cerastoderma edule from the Thames estuary, Paleontology 74:571-588 (1971).

7. C. T. Scrutton, and R. G. Hipkin, "Longterm changes in the rotation rate of the earth," Earth-Science Rev. 9:259-274 (1973).

8. H. W. Clark, Genesis and Science (Southern Publishing Asso., Nashville, Tenn., 1967), pp. 76-80.

9. Ellen G. White, Messages to Young People, 1948, p. 190.

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-assistant professor of biology, Graduate School, Loma Linda University at the time this article was written

September 1974

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