One of the greatest crises in Earth's history has been marked by a major shift in shellfish, with brachiopods (lampshells) being widely replaced by bivalve species such as oysters and clams. This was the result of the devastating end-Permian mass extinction, which effectively reset the evolution of life about 250 million years ago.
Research by paleontologists in Bristol, UK, and Wuhan, China, sheds new light on this crucial shift in marine ecosystems as they transition from ancient times to modern times.
There are abundant terrestrial and marine life, forming a unique ecosystem. In modern oceans, the seafloor is dominated by animals such as bivalves, gastropods, corals, crustaceans, and fish. But these ecosystems all date back to the Triassic, when life came back from the brink. Only one in 20 species survived that crisis, and debate persists about how new ecosystems were constructed and why some groups survived while others did not.
Before the extinction, brachiopods were the dominant shelled animals, however, bivalves flourished after the extinction, better adapting to their new environment.
"A typical example is the replacement of brachiopods by bivalves," explains Guo Zhen of Wuhan and Bristol Universities, who led the project. "Paleontologists used to say that bivalves were better competitors and therefore somehow beat out brachiopods during this time of crisis. There is no doubt that brachiopods were the dominant shelled group before they became extinct, and bivalves took over after that."
Co-author Joe Flannery-Sutherland said: "We wanted to explore the interactions between brachiopods and bivalves through their long history, particularly during the Permian-Triassic transition. So we decided to use a computational method called Bayesian analysis to calculate origins, extinctions and fossil preservation rates and test whether brachiopods and bivalves interacted. For example, did the rise of bivalves lead to the decline of brachiopods?"
Professor Michael Benton, from Bristol's School of Earth Sciences, said: "We found that, in fact, both groups showed similar trends in diversification dynamics throughout the crisis period. This means they were not really competing or preying on each other, but it is more likely that both responded to similar external drivers, such as sea temperatures and brief crises. But the bivalves eventually prevailed, and the brachiopods retreated to deeper waters, where they were still present but in reduced numbers."
Professor Chen Zhongqiang from Wuhan commented: "It's great to see how modern computational methods can solve such a long-standing problem. We always thought that the mass extinction at the end of the Permian marked the end of brachiopods, and that's exactly what happened. But both brachiopods and bivalves seemed to be hit hard by the crisis, and both recovered in the Triassic, but bivalves were better able to adapt to high ocean temperatures. So, that gave them an advantage, and after the Jurassic, their numbers exploded, while the brachiopods didn't play much of a role."
Guo Zhen said: "During the research, I had to examine and sort through more than 330,000 records of brachiopod and bivalve fossils, and then perform a Bayesian analysis on the Bristol supercomputer, which took several weeks. However, I like this method because it repeats everything millions of times to account for various uncertainties in the data and provide a lot of rich information about what is going on."
Professor Benton concluded: "The mass extinction at the end of the Permian was the most severe ever, and it dramatically reset evolution. In fact, 50 million years after the crisis, the Triassic marked a revolution in life on land and in the sea. Understanding how life recovered from near extinction and then laid the foundations for modern ecosystems is one of the great questions in macroevolution. I'm sure we haven't said the last word here yet!"