The growth of organisms is constrained by environmental factors. As the Earth revolves around the sun, the alternation of seasons, the change of day and night, and the rise and fall of tides all have a cyclical impact on the growth of organisms' shells, skeletons, and other hard parts. These hard parts manifest this cyclical change through growth rhythms, becoming ancient "calendars".
During the growth process of reef-building corals, the calcareous secretions of zooxanthellae within their bodies cause their skeletons to continuously accumulate. This accumulation rate is faster during the day and slower at night, so a thin growth line is formed every day. Since seasonal temperature changes also affect the growth rate, the calcium accumulation of corals exhibits periodic changes in density, which are reflected in the growth rings on fossils, with each growth ring representing one year. The growth lines and growth rings of coral fossils record the passage of time.
American paleontologist John Wells discovered that about 380 million years ago, the annual growth rings of Dayiops spinesceramia corals contained about 400 growth lines, indicating that there were approximately 400 days in a year during the Middle Devonian period. Wells further cited growth line data from the Paleozoic era: 412 lines 450 million years ago, 398 lines 330 million years ago, and 390 lines 300 million years ago. These data show that the number of days in a year decreases as geological periods become more recent, confirming the rule that the Earth's rotation speed gradually slows down in geological history. According to calculations, the Cambrian period had 20.8 hours per day, the Devonian period had 21.6 hours per day, the Carboniferous period had 21.8 hours per day, the Triassic period had 22.4 hours per day, and the Cretaceous period had 23.5 hours per day, until the present day, which has 24 hours per day.