Page 304 - Introduction to Paleobiology and The Fossil Record

THE BASAL METAZOANS: SPONGES AND CORALS 291

Box 11.9 Corals and the Earth’s rotation

Through time, the Earth has changed its rate of rotation, and days have become longer. This extraor-
dinary discovery has come from detailed analysis of the growth bands on coral epithecae. Well-

preserved corals often display fine growth lines, grouped together into thicker bands; the former are

thought to reflect daily growth while the latter bands are monthly growth cycles, controlled by the
lunar orbit. A set of more widely spaced bands may represent yearly growth. In a classic study, John
Wells of Cornell University counted the growth lines on a variety of Devonian corals (Fig. 11.33)
and suggested that the Devonian year had about 400 days. The implication that Devonian days were
shorter suggests the Earth’s rate of rotation is decreasing due to the gravitation pull of the moon.
Ivan Gill of the University of New Orleans and his colleagues (2006) have taken the story much
further. Using a range of more sophisticated techniques, including the scanning electron microscope
and backscattered electron imaging, it is now possible to identify with much more precision microscale
banding in some coral species that could ultimately act as proxies for daily changes in our environ-
ment, highlighting short-term climatic and other events. Moreover, this style of banding can help
decipher a great deal more about the detailed mechanisms and timing of skeletalization within the
corals as a whole.

Figure 11.33 Devonian banded coral, Heliophyllum halli (×3). (Courtesy of Colin Scrutton.)