Why do plants grow thorns? The answer lies in the Qinghai-Tibet Plateau

The "beauty" of the plant world is often accompanied by a kind of "harm." Roses, succulents, cacti... these thorny plants all maintain a "love-hate" relationship with animals and humans.

Why do plants have thorns? When did thorns first appear? How did they evolve? Scientists have always lacked in-depth research on these interesting questions.

Over the past few years, the paleoecological research group of the Xishuangbanna Tropical Botanical Garden (hereinafter referred to as "Banna Botanical Garden") of the Chinese Academy of Sciences and the Qinghai-Tibet Plateau paleontological expedition team jointly established by the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences have found a fossil plant community in the central part of the Qinghai-Tibet Plateau, which has the largest number of thorn fossils known in the world. Scientists have finally found direct evidence of the evolutionary history and causes of thorny plants. The research results were recently published in Nature Communications.

The “neglected” fossil thorn

Since 2016, the paleontological expedition team has been conducting paleontological scientific surveys in the Cenozoic sedimentary basin of the Bangong Lake-Nujiang suture zone in the central Qinghai-Tibet Plateau year after year, and has accumulated a large number of plant thorn fossils from the Late Eocene (about 39 million years ago) over the years.

There are a total of 44 thorn fossil specimens from the Lunpola Basin and the Nima Basin. According to their shapes, sizes, growth patterns and other characteristics, they are divided into skin thorns and branch thorns, with a total of 7 morphological types. This is also the fossil plant group with the largest number of thorn morphologies known in the world.

Seven morphological types of plant thorn fossils and living thorny plants from the late Eocene (about 39 Ma) in the central Qinghai-Tibet Plateau. Image courtesy of Zhang Xinwen

There have been some records of thorn fossils in the fossil flora of North America and Europe, but the number is small, and scientists have not conducted in-depth research on them separately. Although this batch of thorn fossils from the Qinghai-Tibet Plateau was brought to the laboratory of the Xishuangbanna Botanical Garden, they have been "ignored" for nearly three years.

"Although there are a large number of thorn fossils with various shapes, it is almost impossible to further identify these fossils to genus, species, or even family due to the lack of detailed features." Even Zhou Zekun, a researcher and paleobotanist at the Xishuangbanna Botanical Garden, felt that they were useless when he first saw them.

In 2019, these fossils were handed over to Zhang Xinwen, a doctoral student in the paleoecology research group of the Xishuangbanna Botanical Garden. In addition to the classification information description, what bothered her was how to tell a completely new scientific story about the thorn fossils.

"When we see living thorny plants, we can easily associate them with important environmental significance in the ecosystem," Zhang Xinwen, the first author of the paper, told China Science Daily.

She explained that thorns are a specialized organ of plants, which may come from specialized leaves, stems or epidermis. Since the surface area of ​​thorns is very small, they can reduce transpiration, thus helping plants adapt to some relatively dry environments. At the same time, thorns are also a defensive structure of plants, which can effectively reduce the frequency of herbivorous animals gnawing. Therefore, the most typical living environment of thorny plants is tropical savanna.

"We have a full understanding of living thorny plants, and paleontology is a discipline that discusses the past with the present, so we boldly speculate that the large number of thorns is also related to these environmental factors." Zhang Xinwen said that the research team decided to start the research from the ecological significance of thorns.

Why do thorny plants explode?

So far, scientists have not found the earliest fossil records of spiny angiosperms, but according to the results of molecular phylogenetic analysis, they first appeared in the Late Cretaceous, when spiny plants began to appear with the explosion of angiosperms.

In this study, the research team also combined molecular phylogenetic analysis to reconstruct the phylogenetic accumulation curve of the Eurasian thorny plant group in the Cenozoic. The results showed that thorny plants began to differentiate rapidly since the Eocene, and after the late Eocene, species diversity even increased exponentially, which coincides with the geological age of this batch of thorn fossils on the Qinghai-Tibet Plateau.

"This also confirms one of our ideas. The types of thorn fossils we found are very diverse, which means that thorny plants have rapidly differentiated since the late Eocene," said Zhang Xinwen.

But what exactly caused the rapid increase in the diversity of thorny plants at that time? Is it related to the ecological environment?

In the same layer, in addition to finding a large number of thorn fossils, the researchers also collected a wealth of herbaceous plant fossils, totaling 315 pieces, accounting for 38% of all plant specimens in the same layer. They also conducted a layer-by-layer microfossil analysis of the same set of strata and found a large number of phytoliths produced by herbaceous plants, such as the short saddle-shaped phytoliths of the subfamily of Artemisia, the cap-shaped phytoliths of the subfamily of Poa, and the fan-shaped phytoliths of the subfamily of Bambusoideae. "For these herbaceous plants to thrive, it means that the environment at that time was an open habitat, not a closed forest," Zhang Xinwen explained.

Typical phytoliths and herbaceous plant fossils in the same layer. Photo courtesy of Zhang Xinwen

To further verify the hypothesis about this environmental type, the research team also used the atmosphere-ocean circulation model and Triffid model of the University of Bristol in the UK to simulate the paleoclimate and paleovegetation at that time. Combined with the paleoenvironmental reconstruction results of the fossil flora, it was proved that with the retreat of the Neotethys Ocean in the Paleogene, the central valley along the Bangong Lake-Nujiang suture zone showed a trend of gradual drying. Coupled with the global climate cooling, the vegetation there changed from the closed forest of the Middle Eocene (about 47 million years ago) to open woodland, with tall trees, medium-height shrubs, and low herbaceous plants.

Interestingly, after consulting 658 animal fossil records from the Qinghai-Tibet Plateau and its surrounding areas, the researchers found that the diversity of large herbivorous mammals in the environment at that time also began to increase.

"Due to the existence of open woodlands, large herbivorous mammals enjoyed a richer food source, which inevitably led to an increase in the feeding pressure of animals on plants, and thus promoted the evolution of thorns in open woodland plants in the central valley of the Qinghai-Tibet Plateau." Zhang Xinwen emphasized that this was about 24 million years earlier than a similar transformation that occurred in Africa.

"This research story illustrates the importance of fossils in understanding the evolution of plant functional traits." Zhang Xinwen said that the study proved that the aridification of the central valley of the Qinghai-Tibet Plateau in the late Eocene and the feeding pressure of large herbivorous mammals jointly drove the rapid evolution of the functional trait of thorns. It also shows that the environmental changes of the Qinghai-Tibet Plateau during the geological history period not only had a profound impact on the plant diversity in Asia and even the wider region, but also shaped the functional traits of plants in the region.

Zhou Zekun believes that the highlight of this research lies in the combination of multiple methods and means such as macrofossils, microfossils, phylogenetic analysis and model simulation. It is the product of the cross-integration of paleobotany, vertebrate paleontology, paleoclimate simulation, molecular biology and ecology.