Yes, they can! Not in minutes or hours, but in days—a “coral calendar.” This is interesting, but also very important for what it reveals when compared to other natural indicators of the passage of time.
In the past 50 years, the geologic time scale has been verified by scores of radioactive mineral dates from rocks of all ages, throughout the world. Although these dates were determined with several different elements disintegrating at different rates, they reinforce each other. That is sufficient to convince physicists, and most earth scientists, that the enormous numbers on the geologic time scale are true. But what about those of us who wonder how the physicists can be so sure that the radioactive disintegration process occurs at a predictable rate? Must we simply “take it on faith”?
It would surely be convincing if there were some entirely different method of arriving at the same figures for the ages of the geologic periods. The time scale is printed in earth and life science texts, in reference books, and in articles of lay interest as well. Many theories and conclusions are based on the figures in the geologic time scale, and are considered controversial by many people who have no faith in a time scale apparently constructed on only one kind of age determination.
In the early 1960’s, Professor John Wells of Cornell University, a specialist in fossil and living corals, carried out a research project that presented a convincing argument for the truth of the time scale. It is not exactly news, but this work is still not widely known. It reveals a story well worth repeating. 
Dr. Wells was investigating the growth habits of living corals. With the aid of a microscope, he determined that these marine animals formed a layer of shell each day (they do not grow at night), and that it is possible to distinguish each day’s layer from the layer of the day before. Corals may live for many years, and the daily growth pattern is constricted annually by seasonal temperature changes. Dr. Wells verified that there were approximately 360 daily shell layers between the deeper annual ridges on the shells. He then turned his attention to fossil corals, particularly those of Devonian age in the Finger Lakes region of New York State (see Figure 1).
The study of many specimens produced an interesting discovery—there were 396 days in the year during Devonian times! Apparently, the earth was then spinning faster on its axis. Why would it rotate more slowly today? Being an earth scientist, Dr. Wells knew that it could be attributed to the moon, which causes tides in the ocean. These tides exert a braking effect that gradually slows the earth’s rotation and results in fewer days in a year.
Dr. Wells’ colleagues in the astronomy department knew how to calculate the effect of the tides with some precision. If the moon has been orbiting the earth since before the Devonian Period, and if its affect on the ocean has been constant, then here is a quite different method of calculating the day-length of the Devonian year, and incidentally of verifying the number of days in the year as revealed by the coral shell.
Too many coincidences would have to be involved to explain away what is now obvious:
- We do know how to date the rocks, and have essentially the correct figures.
- We do know the effect of the moon on the earth’s rotation period, and it acts at a constant rate, as shown by the nearly straight line in the graph (see Figure 2).
- We do know the daily and annual growth habits of ancient corals.
- We can judge the past by the present, as postulated by James Hutton, the eminent 18th century Scottish geologist. His Principle of Uniformitarianism, “the present is the key to the past”, has a basis in fact.
Dating the Devonian rocks at 345 million years, by the radiogenic method, and calculating the moon’s breaking effect for that length of time, it turns out that astronomy agrees with paleontology closely enough to rule out coincidence. Not being content with that fine interdisciplinary exercise, Dr. Wells counted the daily growth layers on fossil corals of other periods from different localities. See the chart in Figure 2 for the results.
While the exact boundaries of geologic time periods continue to be modified and refined with modern techniques, it’s still remarkable to note the simple evidence of Earth’s evolving time preserved so delicately in the shell of the lowly coral. If you would like to give your students a firsthand look at the growth rings in corals, see the WARD’S Fossil Specimen section under the Phylum Cnidaria (corals) in the Earth Science catagory.
By: Fred C. Amos (WARD’S alumnus)
Technical Advisor: John W. Wells, Professor Emeritus, Cornell University
References
- Kulp, J. Lawrence, “Geologic Time Scale”, Science, v. 133, No. 3459, p. 1105-1114, April 16, 1961.
- Wells, John W., “Coral Growth and Geochronometry”, Nature, v. 197, No. 4871, p. 948-950, March 9, 1963.
