Steven Stanley, a paleontologist and evolutionary biologist at the University of Hawaii, has proposed a revised perspective on the Permian-Triassic mass extinction event, which occurred around 252 million years ago. This extinction event has traditionally been thought to have wiped out up to 96 percent of marine life. However, Stanley challenges this commonly accepted view, arguing that background extinctions—those that occur in between mass extinction events—should be factored into the loss of species during the period. Based on this adjustment, Stanley suggests that the actual marine extinction rate during the Permian-Triassic event was likely closer to 81 percent, a significant but less catastrophic figure than previously believed. His findings were published in a paper in the Proceedings of the National Academy of Sciences.
The Permian-Triassic extinction event, often referred to as “The Great Dying,” spanned approximately 60,000 years and was one of the most severe extinction events in Earth’s history. It affected both terrestrial and marine life, with many species unable to survive the harsh environmental changes. Earlier research indicates that a combination of intense volcanic activity, possibly multiple asteroid impacts, global warming, and increased ocean acidification caused severe disruptions to ecosystems, leading to widespread extinctions. However, Stanley argues that while the event was undoubtedly catastrophic, it may not have been as devastating to marine life as previously thought.
Central to Stanley’s argument is the concept of background extinctions, which are often overlooked in discussions of mass extinctions. Background extinctions occur gradually over time and are typically the result of environmental changes that a species cannot adapt to, such as the introduction of new predators, shifts in habitat conditions, or other ecological stresses. These extinctions happen on a smaller scale compared to mass extinctions but can accumulate over long periods, significantly reducing biodiversity before a mass extinction event even occurs.
Stanley conducted a detailed analysis of extinction patterns between known mass extinctions and observed that the number of species that went extinct during these periods correlated with the amount of time that passed between extinction events. Essentially, the longer the time between two mass extinctions, the higher the likelihood of background extinctions occurring, as species continually faced changing conditions. Stanley applied this model to the period leading up to the Permian-Triassic extinction event and found that many of the marine species believed to have died out during the Great Dying would have likely gone extinct due to background extinction events in the years leading up to it. In other words, they were already on the brink of extinction before the mass event, and their disappearance could not be solely attributed to the catastrophic conditions of the Permian-Triassic boundary.
By accounting for these pre-existing extinctions, Stanley recalculated the marine extinction rate during the Permian-Triassic event. He argues that the true extinction rate was closer to 81 percent, still a major loss, but far lower than the 96 percent figure often cited in past studies. This adjusted figure challenges the traditional narrative of the event’s severity, suggesting that while the event was certainly devastating, it was not as uniformly catastrophic for marine life as earlier studies implied.
Stanley further suggests that not all marine life was equally affected by the Permian-Triassic extinction. Some groups of organisms were far more resilient to the environmental changes caused by volcanic activity, warming, and ocean acidification. Other groups, however, were more vulnerable, and these losses were disproportionate. This indicates that the event’s effects were not uniform across all marine species. Some groups may have already been in decline due to background extinctions, while others were more capable of withstanding the changing conditions.
In his research, Stanley highlights the survival of numerous marine groups during the Permian-Triassic event. According to his estimates, approximately 90 orders and 220 families of marine life managed to survive the extinction, demonstrating a more diverse and resilient marine ecosystem than previously thought. The survival of these groups also suggests that certain environmental stresses—such as warming and acidification—had different levels of impact on different ecological niches.
In particular, Stanley’s research calls into question the severity of the event for some of the dominant marine groups at the time, such as the trilobites, which are often cited as a primary victim of the extinction. His analysis suggests that the decline of these groups may not have been solely due to the environmental catastrophe but also because they were already experiencing decline from background extinctions before the event. These findings present a more complex picture of the Permian-Triassic extinction and challenge the oversimplified view of a total collapse of marine ecosystems.
Moreover, Stanley’s work emphasizes the importance of reconsidering how we define and measure mass extinctions. By incorporating background extinctions into extinction rate calculations, we gain a more nuanced understanding of how ecosystems change over time. This shift in perspective underscores the complexity of evolutionary processes and highlights the fact that mass extinctions are not always absolute events that completely reset ecosystems. Instead, they may be the culmination of ongoing environmental stresses, which can interact with background extinctions to shape the trajectory of life on Earth.
Stanley’s argument also raises questions about how scientists study extinction events. Traditionally, mass extinctions are viewed as discrete events marked by sharp declines in biodiversity. However, as Stanley points out, the distinction between background extinctions and mass extinctions is not always clear-cut. By rethinking the boundaries of mass extinction events and incorporating background extinctions into our models, we can develop a more accurate understanding of how these dramatic episodes in Earth’s history unfolded.
The revised extinction rate of 81 percent does not lessen the significance of the Permian-Triassic event; it still represents a profound loss of biodiversity, especially in the context of marine life. However, Stanley’s findings provide a more balanced and nuanced interpretation of the event, one that accounts for the complexity of extinction dynamics. By incorporating background extinctions, Stanley’s research helps to paint a more comprehensive picture of life on Earth during this critical period of our planet’s history.
More information: Steven M. Stanley. Estimates of the magnitudes of major marine mass extinctions in earth history, Proceedings of the National Academy of Sciences (2016). DOI: 10.1073/pnas.1613094113