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Examining the Fossil Record
Throughout Earth’s history, five major mass extinction events have occurred, including the well-known Cretaceous-Tertiary (K-T) mass extinction, which wiped out over seventy-five percent of living organisms at the time, including the dinosaurs. However, some scientists see the present as a sixth, human-caused mass extinction, as today’s extinction rates are thousands of times greater than the natural rate. Consequently, it has become increasingly urgent for researchers to understand how mass extinction events impact populations and how species respond to them.
While previous mass extinction events occurred millions of years ago, geologists can consult the fossil record to study them. Analyzing physical characteristics of fossils enables geologists to identify now-extinct organisms, predict the planet’s climate patterns over millions of years, and discover ecological relationships that may still exist today. Geologists can also estimate the ages of fossils through investigation of radioactive decay in the rocks that encase them. By noting the ages of fossils belonging to the same species, researchers can piece together timelines of origination and extinction for those species. When many species disappear from the fossil record at around the same time, geologists can conclude that a mass extinction has occurred.
The Early Burst Model
While studying the mass extinctions themselves is important, it is also necessary to examine the behavior of the species that remain after the events. Immediately following mass extinction events, many niches, or areas within an ecosystem in which organisms specialize, are vacated. For instance, a mass extinction of lions would open up a larger niche occupied by cheetahs, since lions and cheetahs occupy the same area and target the same prey. Mass extinctions therefore provide the surviving species considerable opportunity to differentiate with new traits that allow them to fill other niches.
The commonly accepted representation of such development is the early burst model, a hypothesis originating in the 1940s where survivors of mass extinctions quickly radiate into many new morphologies (physical forms) to fill the now-empty niches in the environment. A key example is after the K-T mass extinction, when surviving mammals began to differentiate very quickly to fill the open ecospace. By adapting traits to better suit various conditions, different groups of mammals were able to expand into their respective niches. There are now thousands of mammal species currently alive, dominating far more ecospace than they had before the K-T extinction.
Diversity and Disparity
Christopher Whalen, a researcher at the Yale Department of Earth and Planetary Sciences, sought to put the early burst model to the test using ammonoids, an extinct group of marine molluscs related to octopi and squids. Whalen investigated two main characteristics of the ammonoid populations: diversity (the number of species a group of organisms contains) and disparity (the extent to which morphology differs from species to species). By examining the physical shape of ammonoid fossils from directly after mass extinctions, Whalen was able to judge how unique each species was from one another and determine whether disparity indeed increases rapidly in relation to diversity after mass extinctions, as the early burst model predicts.
Specifically, the researchers factored in various dimensions of the spiral ammonoid shell to quantify morphological differences among specimens. This was a key advantage to using ammonoids to study disparity patterns: ammonoid shells come in relatively simple geometric shapes, allowing for a straightforward method of calculating disparity among species. Also, since the shell accounts for the majority of the ecological fitness of the organism due to its effect on the hydrodynamics of the ammonoids, measuring shell dimensions allows the researchers to study a characteristic that is at the focal point of ammonoid evolution.
So, if the early burst model argues that disparity increases quickly after mass extinction events, then where does diversity fit into the equation? As the number of species increases, it becomes more difficult for a new species to develop a morphology that is different from all others. It is then necessary to take into account the number of species currently present to temper the expectation for disparity at that time. “As in rolling dice, the more times you do it, the more times you are going to repeat,” Whalen said. “When you have fewer species, differences among them are easier to achieve.”
To factor in diversity, Whalen developed a null model to predict disparity patterns based on diversity patterns throughout history. The model randomly assigned morphologies to each species and calculated the resulting ammonoid disparity over time. This simulation was repeated thousands of times, shuffling morphologies among species to produce a new, slightly different disparity pattern each time. Whalen then calculated the median disparity pattern to predict how disparity was expected to look given the diversity pattern. By comparing the actual ammonoid disparity over time to the null model’s expected disparity, the researchers could find whether disparity increased sharply after mass extinctions events, as predicted by the early burst model.
Results and Implications
If the early burst model held true, the actual disparity would outpace the null model’s expected disparity after each mass extinction event. However, Whalen instead discovered a surprising trend: ammonoid disparity after most mass extinctions actually lagged behind diversity. The early burst model not only failed to explain ammonoid population behavior, but it predicted the exact opposite of the true outcome. These findings are especially noteworthy given the characteristics of ammonoids. As an r-selected group, meaning they rapidly reproduce in high quantities and follow “boom and bust” population dynamics, ammonoids are particularly adept at recovering quickly from mass extinctions. Therefore, Whalen suggested that ammonoids are a group that should be favorable for supporting the early burst model. “The fact that the model does not hold up [for ammonoids] is good evidence that it is suspect in many, if not all situations,” he said.
However, Whalen does offer one caveat: we may simply not be looking in the right place. It is possible that studying too general of a group of organisms leads to no clearly identifiable early burst trend, which could still appear in specific subsets of that same group. “It is possible that early burst could exist at those finer levels, but that is not something you can determine based on this ammonoid data,” Whalen said.
As gaps in the fossil record have been filled through the decades following the development of the early burst model, so too have gaps in our knowledge about ancient organisms. By studying Earth’s past mass extinction events, we can draw a clearer picture of what the aftermath of human impact on ecosystems will look like. We may also be able to more accurately predict how vulnerable species will recover from the sixth major mass extinction, if at all.
Whalen, Christopher D., et al. “Paleozoic Ammonoid Ecomorphometrics Test Ecospace Availability as a Driver of Morphological Diversification.” Science Advances, vol. 6, no. 37, 2020, doi:10.1126/sciadv.abc2365.
Whalen, Christopher D., and Derek E. G. Briggs. “The Palaeozoic Colonization of the Water Column and the Rise of Global Nekton.” Proceedings of the Royal Society B: Biological Sciences, vol. 285, no. 1883, 2018, p. 20180883., doi:10.1098/rspb.2018.0883.