Counting the Days
Keeping Time Is As Old As Life
Life on earth experiences time not as a continuum, but divided into pieces that can be counted, and these counts tap out the rhythms to which all life dances, rhythms kept by multiple timekeepers, large and small, fast and slow. The earth spins on its axis dividing time into night and day; it travels around the sun to create the years; the moon imprisoned in earth's orbit gives us the tides and the lunar phases, and the earth wobbles on its axis. The layering of all these motions upon one another creates complex and shifting rhythms.
Creatures have been dancing to this motion and counting its rhythms since life began. Who doesn't know about the annual rings of trees (at least in the temperate zones)? Many other life forms record time on one or several scales. Mollusks such as the quahog, Mercenaria mercenaria below, hold a particular charm because the records they make can be read in fossils millions of years old. This record of time is created by the mantle, a magical organ that builds the shell layer by layer. Like the mantle of fashion, it covers the top of the body and leaves the foot to engage the substrate. The mantle tissue removes carbon dioxide from the water, reducing the solubility of dissolved calcium carbonate, and depositing it as layer after crystalline layer (limestone) on the outer surface of the mantle, creating the protective shell in or under which the animal lives.
Quahog shell, Mercenaria mercenaria, Cedar Key, Florida
This shell deposition is discontinuous, as the mantle extends or contracts in a daily, tidal, annual, lunar or combination rhythm (depending on the species and where the mollusk lives). Deposition of layers at the shell edges enlarges the shell, and deposition on the inner wall thickens it (see Wikipedia article). The layered nature is readily seen in the cross-section below. The one on the right has been etched with acid to reveal the layered structure more clearly.
The episodic deposition produces the ridges spreading outward like ripples on a pond in the Mercenaria clam shell below, and in the cross sections above. Beginning with the newly settled, tiny clam with its simple, hinged shell, every episode of deposition resulted in a new, visible ridge as the clam grew.
Mercenaria mercenaria shell, showing non-continuous growth ridges
But what rhythm do these layers tap out? A closer look shows that shell deposition rate varied in a regular way. The darker valleys in the surface correspond to the translucent zones in the cross section above and have more closely spaced layers. They were probably deposited in the winter when low temperatures reduced deposition rates. Ridges midway between the “winter valleys” are wider and were probably laid down in the summer. Counts of the ridges between the “winter valleys” usually revealed 13 or 14 ridges, the number of lunar months (28 days) in a year. Our clam thus formed a distinct layer every lunar month. But why? There are 26 tidal cycles per year at Cedar Key Florida where this clam was at home, but of course only 13 full or new moons. Perhaps the clam did something else on every full or new moon? Spawn?
Mercenaria shell recording lunar months
Mollusks first appear in the fossil record of the Cambrian Period, about 500 million years ago, and we can presume that they started counting right from the start. Like the rings in a tree trunk, the thickness of the layers varies with the seasons, with closer spacing in the cool season as in our Mercenaria. My crude methods detected lunar months, but could not detect daily layers, but detailed studies of some mollusk shells do, with closer-spaced daily series marking the years. The number of layers between these annual markers are a count of the number of days in a year. Seventy-million-year-old fossil shells showed that days were 20 minutes longer than today, and those from the early Carboniferous period some 350 million years ago revealed that the year contained around 385 days, 20 more than the current 365. About 900 million years ago, days were only 18 hr. long, with 486 of them squeezed into a year. In other words, the earth's rotation around its axis is slowing down, a fact that has been confirmed by several other methods. The cause of this slowdown is tidal friction with the moon and the sun, friction that will continue until the earth stops rotating.
Mollusk shells, both ancient and recent, are storytellers. The measure and rhythm of their lives is printed in their shell for us to see, and perhaps to transport our imaginations to another era long past. Sometime in the future, when the earth stops rotating and there will no longer be days to count, will mollusks be around to record this in layer-less shells?