Fossils of giant clams tell of dinosaur-era oceans

Dale Gnidovec

When someone mentions the Cretaceous Period, what pops into many people’s minds are the dinosaurs, especially Triceratops and Tyrannosaurus, often fighting each other, although that is an unlikely scenario: Why would a carnivore go after the most heavily armed herbivore in its environment when there was plenty of easier prey?

But the dinosaurs didn’t live alone; there were many other animals living then. At that time, the center of North America was covered by ocean water that stretched from the Arctic to the Gulf of Mexico.

Known as the Western Interior Seaway, it was filled with life. Reptiles ruled in the form of mosasaurs (large swimming lizards related to the modern Komodo dragon), longnecked plesiosaurs, and short-necked pliosaurs that resembled, and probably acted like, modern killer whales. Swimming among the reptiles were ammonites, relatives of the modern chambered nautilus, and on the sea bottom, giant clams.

What were conditions like in that ocean? What was the depth, water temperature, and oxygen level? And how can we tell those things about an ocean that existed 85 million years ago?

The giant clams can answer those questions, as shown by a paper published recently in the journal Cretaceous Research, which reported on specimens collected at Monument Rocks National Natural Landmark from the famous chalk beds of the Niobrara Formation of western Kansas.

Inoceramus platinus was a Inoceramus platinus was a large, flat clam that lived on the bottom of the Western Interior Seaway. This fossil shell exhibited in Hays, Kan., is about 3 feet wide and 3 feet long. large, flat clam that lived on the bottom of the Western Interior Seaway. It was only an inch or two thick; many adults had shells that were 3 feet across, and some reached nearly 7 feet. They grew quickly, as shown by the growth lines that can be seen under a microscope, and their shells recorded the conditions in which they grew.

Clam shells are made of calcium carbonate, which consists of an atom of calcium, an atom of carbon and three atoms of oxygen. That oxygen was the key because, depending on the water temperature they are living in, clams incorporate different relative amounts of the isotopes oxygen-16 (“normal” oxygen) and oxygen-18 (which has two extra neutrons).

The oxygen isotopes indicated that the Cretaceous clams were living in water that averaged 63 degrees Farenheit but fluctuated seasonally between 55 and 78 degrees. The scientists suggested that those variations were due to changing current directions produced by salinity differences caused by evaporation during dry seasons and precipitation during wet ones.

Those temperature estimates agree not only with those calculated from other mollusks, but also with landplant fossils from the same time and with computer models of the climate.

Yearly growth lines indicated that the giants grew more than 5 inches per year, one of the fastest growth rates for any clam — fossil or modern. Perhaps that was because they lived in an oxygen-poor environment, as shown by the presence of pyrite and by the almostcomplete lack of other bottom-dwelling organisms, so there was little competition for food.

Another possible reason for their fast growth is the clams might have hosted photosynthetic algae, as many giant clams do today, which helped provide them with nutrients.

Clams can tell you a lot about the past.