May 10. Corals and the days of our lives

Corals are sensitive little critters. Little, as individuals, but their colonies can become immense, hundreds of miles long and many hundreds of feet high. Like brachiopods, corals thrived in the warm tropical seas that covered much of the combined North American-European continent of Laurasia. 

Given that I grew up in Michigan, it may not surprise you to hear that one of my favorite Devonian corals is a colonial coral called Hexagonaria. The individual chambers, maybe a half-centimeter across, are typically hexagonal (or pentagonal, or heptagonal) in outline, but many hundreds or thousands grew together to make the whole colony and eventually to contribute to reefs.

Petoskey stone (Hexagonaria), photo by jtmitchcock

The Devonian rocks of Michigan crop out in the northwestern part of the Lower Peninsula, around Traverse City. Wave-rounded cobbles of hexagonaria colonies are plentiful on beaches around the town of Petoskey, where they are called Petoskey Stones. They’re often already well rounded and smooth, but collectors sometimes polish them to bring out the interlocking hexagonal patterns. The Petoskey Stone is the state fossil of Michigan, and they lived in Middle Devonian time, about 380 million years ago.

So back to the coral’s sensitivity. The coral animal, the little polyp, builds its home, whether a single cup or a colony or an entire reef, by secreting calcium carbonate, the mineral calcite, to build the structure it lives in. It only does that during the day and stops secreting at night. Why? Because most shallow-water corals live in a symbiotic relationship with photosynthetic algae. The algae provide the coral with carbon in the form of simple sugars like glucose, which the coral uses for energy and to make the calcium carbonate that builds the skeletal structure that houses the coral animal. In return, the coral passes nitrogen to the algae. The nitrogen comes from floating animals and other nutrients that the coral’s tentacles collect from the surrounding water. It’s a complex relationship that benefits both the coral and the algae, and it’s sensitive to light which is necessary for the algae to photosynthesize.

That process of secreting calcite with a daily break in secretion makes a line in the coral structure every day, like a tree’s growth rings. Seasonal variations also generate annual lines even more like annual tree rings. Knowing this, paleontologist John Wells, who worked at Cornell University back in 1963, counted the daily and annual layers in corals – and confirmed that there were more days per year during the Devonian Period than there are today. About 400 rather than 365. And that count has decreased over time. This confirmed something that was already known, that the earth’s rotation is slowing down. Similar layering in other animals, such as mollusks, indicates the same thing.

Wells’ simple study, counting growth rings in fossil corals, has repercussions far beyond the idiosyncrasies of coral life. For example, it implies that the earth had its moon during the Devonian, because calculations of the count of days based on tidal drag came up with 399 days for the Devonian year – essentially identical to the 400 Wells estimated based on coral rings. And knowing the length of the year – when you are talking about millions of years – had an impact on the way we understand cyclic sedimentation patters and even calculations of the evolution of the earth’s orbit.

All this means that Devonian days were a little over 21 hours long, rather than the 24 of today, and in the future the day will be even longer, and the year will have fewer days. But never fear, you won’t need a new calendar for a long time. It’ll be more than 17 million years before the year is down to 364 days.—

—Richard I. Gibson

Further reading
Days are getting Longer
John Wells

Photo by jtmitchcock via Wikipedia under GFDL license