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How primitive plants evolved to survive Earth's most catastrophic extinction event
Earth responded to its most severe past warming event by evolving a new and bizarre type of photosynthesis that allowed a group of primitive plants to survive.
Research led by the University of Leeds has revealed how lycophytes - a type of ancient plant - not only survived a mass extinction 250 million years ago but then came to dominate the recovering landscapes.
During the Permian-Triassic mass extinction, which is also known as the "Great Dying," global temperatures rose dramatically with most forests collapsing under extreme heat and vast areas of land becoming barren.
The study, which is published today (20 April) in the journal Nature Ecology and Evolution , concludes that lycophytes conserved water and tolerated heat by opening their stomata at night instead of during the day, storing CO2 as an acid to use in the daytime for photosynthesis.
The researchers believe lycophytes may have been the first plants to use this mechanism revealing a biological innovation that was able to keep Earth's biosphere active with the plants able to remove carbon from the atmosphere, ultimately combating the effects of the warming event.
Today, plants using CAM photosynthesis make up only a small proportion of global vegetation and are most common in hot and dry environments such as deserts.
Lead author of the study, Dr Zhen Xu from Leeds' School of Earth and Environment, said: "Our results suggest that under future warming, plants with CAM photosynthesis traits could become far more important.
"If the world experiences sustained extreme heat, plant communities may shift toward species that are better able to tolerate high temperatures and water stress."
Lycophytes are spore-bearing vascular plants (a type of plant characterised by the presence of tissues for transporting water and nutrients). There are more than 1,200 species of the plant still in existence. They can survive in many places but are most diverse in tropical regions.
To understand how lycophytes survived when so many other plants perished, the researchers first studied their evolutionary relationships to find their closest relatives, such as the quillworts that can still be found around the world, including in Scotland. They then studied carbon isotopes (variants of carbon atoms) in fossil plants from South China from the late Permian to the Middle Triassic period. Different types of photosynthesis leave different carbon isotope signatures, so this can reveal how ancient plants functioned.
They found that lycophytes had carbon isotope values that were noticeably different from other plants during the Permian–Triassic extinction period. This difference became smaller once environmental conditions had improved.
The team then compared where the lycophyte fossils were found with climate model simulations. The results suggest that these plants lived in places where surface temperatures likely exceeded 50 °C.
The researchers believe increased knowledge about Earth's geological past can help to inform predictions about future climate resilience, something which they say is becoming increasingly important in a warmer world.
Co-author of the study, Professor Barry Lomax of the University of Nottingham, said: "The analysis pulled together many separate scientific disciplines to test how this group of enigmatic plants not only survived the great dying but also how they were able to thrive in a highly stressed environment.
"By linking these data together, we are able to further understand plant adaptation to past climate emergencies deepening our understanding of the resilience of the Earth system to climate perturbations."
Professor Benjamin Mills from Leeds' School of Earth and Environment added: "Understanding how plants' diverse physiological strategies shaped ecosystems in the past helps us to anticipate how vegetation might reorganize in the future, and because plants are the base of terrestrial food webs, changes in dominant plant strategies can alter the functioning of the entire Earth system."
