Metabolic Rates Helped Ancient Bivalves Outcompete Brachiopods
March 26, 2014

Metabolic Rates Helped Ancient Bivalves Outcompete Brachiopods

Lee Rannals for - Your Universe Online

Scientists at Stanford University are challenging a popular theory about how bivalves outgrew brachiopods. The results of their study have been published in the journal Proceedings of the Royal Society B.

About 250 million years ago, brachiopod groups largely disappeared along with 90 percent of the planet’s species. However, during this time bivalves flourished, branching out into a variety of shapes and sizes and spreading into freshwater habitats. Scientists theorized that bivalves simply outcompeted brachiopods, but the latest study says it was more of an issue of energy.

"Brachiopods and bivalves used about same amount of food during Paleozoic, but bivalve food intake has increased by a factor of 100 since then," Jonathan Payne, a paleobiologist at Stanford University, said in a statement. "There's no way that increase is the result of bivalves simply displacing brachiopods. Even if bivalves had entirely supplanted brachiopods, their metabolism would have only doubled."

The team conducted a search of scientific literature and recorded the body sizes of more than 6,000 ancient and modern genera of bivalves and brachiopods. Some of the fossils used in the study dated back to 500 million years ago.

"If scientists want to know how evolution works, one of the best ways is to study changes in animal shape over Earth's history," Payne said. "By moving from form to function as we have done in this study, fossils give us a very direct way of comparing how ancient ecosystems worked compared to their modern counterparts."

The researchers found that ancient brachiopods possessed relatively little soft tissue and would have had low metabolic rates compared to ancient bivalves. Essentially, Payne said that there is a lot less meat inside a brachiopod.

"That's one reason that we eat bivalves, and not brachiopods. There also aren't there [sic] many brachiopods around, and some of them may be toxic to humans,” he added.

The study provides a deep time perspective of how life arose and declined during this period due to energy consumption.

"Today's ocean reflects these evolutionary stories of the past. We can look at the modern oceans and see the end of the story–lots of bivalves and few brachiopods–but now we know more about the introduction and climax of the narrative,” study coauthor Craig McClain, an oceanographer at the National Evolutionary Synthesis Center in Durham, NC, said in a statement.

According to the researchers, the evolutionary strategies used by brachiopods that made them successful for hundreds of millions of years could have been their own demise due to environmental changes.

"We think that about 250 million years ago, a series of large volcanic eruptions in Siberia ejected something on the order of 6 million kilometers of basalt rock. That's enough to cover all of Western Europe in a quarter-mile deep layer of basalt," Payne said.

These eruptions would have altered the chemistry of the oceans and increased their acidity, which would have negatively affected Brachiopods.

"Bivalves have gills and an active circulatory system, while brachiopods don't. That means that in general, bivalves are more effective at regulating their internal chemistries relative to the external environment."