Refigium amidst ruins: Unearthing the lost flora that escaped the end-Permian mass extinction
by: Huiping Peng, Wan Yang, Mingli Wan, Jun Liu, and Feng Liu
Summarized by: Jenna Allen, an environmental science and biology undergraduate student at Binghamton University. Currently, she is experiencing existential dread regarding her future but enjoys reading and painting in her free time.
What data were used? A team of scientists collected rock samples from the Permian–Triassic Boundary, 251.94 to 251.88 million years ago (Ma), of the South Taodonggou Section in the Xinjiang Province of northwestern China for pollen and spore fossils, also known as palynomorph fossils. Unlike many other organisms, plants do not have bones or teeth, which means they are less likely to fossilize. Fortunately, pollen produced by plants have hard outer layers that increase their preservation potential. Scientists can identify and match the pollen and spores present in a given rock layer to specific plants. Using that information, as well as the type of rock the fossils are found in, they can reconstruct environments from hundreds of millions of years ago. Plant and animal fossils, however rare, are still useful and were collected whenever possible in this study for a total of 129 palynomorph samples, 39 fossilized tree trunks, and 20 animal fossils.
What was the goal of the paper? The Permian–Triassic mass extinction was the worst of the five major mass extinctions in Earth’s history, with 80% of known species going extinct. With this study, scientists aimed to better understand the effect this extinction event had on terrestrial plant communities. They looked at (a) the response of terrestrial plants to changing environmental conditions, (b) the role of isolated plant populations, also known as refugia, and (c) the amount of time it took for terrestrial ecosystems to recover after the extinction event.
Methods: The palynomorph samples were collected across the Permian–Triassic boundary and had to be removed from the sedimentary host rock for identification and dated in order to compare what plants were present before, at the start of, and after the extinction event. To isolate the palynomorphs to study them, the samples were crushed, treated with a chemical solution to remove them from the rock itself, and filtered through a mesh screen. Out of the 129 samples, only 15 of them were well preserved enough for fossilization and identification. Of these 15, only 12 had enough individual palynomorphs to be used in the statistical analysis used to determine the relationship between samples. The samples were dated using a type of radiometric dating called U-Pb (uranium-lead) zircon dating. Radiometric dating is based on known rates of decay, or half-lives, of radioactive elements on Earth. Scientists can measure how much of a certain radioactive element is left within a sample and compare that with the half-life to figure out the age of the rock. The plant and animal fossils that were also collected were dated to reconstruct a timeline from 252.10 to 251.67 Ma, a period 59–160 thousand years before the extinction to 160 thousand years after the extinction.
Results: Scientists, using an analysis that plots data points by similarity, grouped the 12 samples into three assemblages. Each group has a unique composition of plants that represent a specific environment and were named after the plants that produced the most common palynomorphs in the group’s samples. The first is the Cyclogranisporites sp.-Lycopodiumsporites reticulumsporites (CR) assemblage from 212.10 to 252.00 Ma, 59 to 160 thousand years before the mass extinction. Most of the palynomorph fossils were spores for plants called lycopsids. Lycopsids, which include clubmosses, spikemosses, and quillworts, are found in humid lakeplain environments. Just below the CR assemblage, the scientists found a herbivorous mammal-like Turfanodon fossil, leading them to believe that they were living with the lycopsids before the extinction (Fig. 1). The second group is the Alisporites landianus-Chordasporites australiensis (LA) assemblage from 251.99 to 251.93 Ma, right at the start of the mass extinction. Instead of lycopsid spores, the LA assemblage samples had pollen from seed producing trees and ferns, which are better suited to drier environments. This means that at the start of the extinction event the lake dried up and the plants that could live there changed. The third group is the Klausipollenites schaubergeri (Ks) assemblage from 251.72 Ma, 160 thousand years after the extinction. Similar to the first group, spore fossils make up the majority of palynomorph fossils which indicates a return to humid conditions and a decline in seed producing plants.
Why is this study important? The pattern observed in these samples is different from previous studies of the Permian–Triassic Boundary in nearby regions and more broadly across the world. This study suggests that the region was protected from drastic environmental changes, such as severe drought or excessive rainfall. While the increase in pollen at the start of the extinction event did indicate a shift toward a drier environment, the spores returned within the next 160 thousand years, which is quick in geologic time and means that the drought was neither excessive nor long lasting. Until recently, it was believed that terrestrial ecosystems took millions of years to recover to pre-extinction levels of complexity. This was based on the response of marine ecosystems, which are more likely to fossilize and are therefore more heavily researched and better understood.
Broader Implications of this study: These findings demonstrate a need to reexamine currently held beliefs surrounding the terrestrial response to the Permian–Triassic mass extinction and the importance of isolated plant communities, or refugia, on terrestrial ecosystem recovery. These questions become ever more important as terrestrial ecosystems throughout the world experience changing conditions. Understanding past responses to climate change can help scientists predict what may happen in the future, for plants, animals, and people.
Citation: Peng, H., Yang, W., Wan, M., Liu, J., & Liu, F. (2025). Refigium amidst ruins: Unearthing the lost flora that escaped the end-Permian mass extinction. Science Advances, 11(11), eads5614. https://doi.org/10.1126/sciadv.ads5614