Why do plants have thorns? The answer lies in the Tibetan Plateau

The “goodness” of the plant world is often accompanied by a kind of “harm”. Roses, succulents, cacti… These thorny plants all maintain a “love and kill” relationship with animals and humans.

Why do plants have thorns? When did the thorn first appear? And how did it evolve? Scientists have always lacked in-depth research on these interesting questions.

Over the past few years, the Paleoecological Research Group of xishuangbanna Tropical Botanical Garden of the Chinese Academy of Sciences (hereinafter referred to as “Banna Botanical Garden”) and the Institute of Paleontology and Paleoanthropology of the Chinese Academy of Sciences jointly formed the Qinghai-Tibet Plateau Paleontological Expedition Team, which has found a fossil plant group in the middle of the Qinghai-Tibet Plateau, which has the largest number of thorn fossils in the world. Scientists have finally discovered direct evidence of the evolutionary history of thorny plants and their causes. The results of this study were recently published in Nature Communications.

Fossils of thorns that have been “snubbed”

Since 2016, the paleontological expedition team has been conducting paleontological scientific investigations in the Cenozoic sedimentary basin of the Pangong Lake-Nu River Suture Belt in the central Tibetan Plateau for many years, accumulating a large number of plant thorn fossils from the Late Eocene (about 39 million years ago) in the past few years.

There are 44 specimens of thorn fossils from the Lumpola Basin and Nyima Basin, which are divided into skin spines and branch thorns according to their shape, size, growth mode and other characteristics, with a total of 7 morphological types, which is also the largest number of fossil plants known in the world.


Fossil plant spines of 7 morphological types in the late Ephemerocene (about 39 Ma) in the central Tibetan Plateau and living spiny plants Courtesy of Zhang Xinwen

In the fossil flora of North America and Europe, there have been some fossil records of thorns, and the number is small, and scientists have not studied them in depth alone. Although the fossils of thorns on the Tibetan Plateau were brought to the laboratory of the Banna Botanical Garden, they were also “left out” for nearly three years.

“Although this batch of fossils is numerous and diverse, due to the lack of detailed characteristics, it is almost impossible to further define these fossils to genera, even families.” Even Zhou Zhekun, a researcher at the Banna Botanical Garden and a paleobotanist, couldn’t help but feel chicken ribs when he first saw them.

In 2019, these fossils were handed over to Zhang Xinwen, a doctoral student in the Paleoecology Research Group of Banna Botanical Garden, in addition to the classification information description, how to bother her can tell a new scientific story about thorn fossils.

“When we look at living spiny plants, it’s easy to think that they have important environmental indicators in ecosystems.” Zhang Xinwen, the first author of the paper, told China Science News.

She explains that thorns are specialized organs of plants that may come from specialized leaves, stems, or epidermis. Since the surface area of the spines is very small, transpiration can be reduced, which helps the plant adapt to some relatively arid environments. At the same time, thorns are also a defensive structure of plants, which can effectively reduce the frequency of eating by plant-eating animals. Therefore, the most typical habitat for spiny plants is the savanna.

“We have a full understanding of living spiny plants, and paleontology is a discipline that will discuss the ancient, 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 thorny plants “erupt”

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

In this study, the team also combined molecular phylogenetic analysis to reconstruct the phylogenetic curve of the Cenozoic Eurasian barbed plant taxa, and found that the barbed plants began to differentiate rapidly from the Eocene, and after the Late Eocene, the species diversity even showed a multiple increase, which coincided with the geological age of the thorn fossils on the Qinghai-Tibet Plateau.

“It also confirms our idea that the types of fossils we found were very diverse, meaning that spiny plants had rapidly differentiated since the Late Eocene.” Zhang Xinwen said.

But what was the reason for 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, a total of 315 copies, accounting for 38% of all plant specimens in the same layer. They also analyzed the layer-by-layer microfits of the same set of strata and found a large number of phytosilic bodies produced by herbaceous plants, such as short saddle-type phytosilicon bodies of the Thrush subfamily, cap-type phytosilic bodies of the precocious grass subfamily, and fan-shaped phytosilicon bodies of the bamboo subfamily. “The desire for these herbs to thrive meant that the environment was an open habitat rather than a closed forest.” Zhang Xinwen explained.


Typical phytosilicas and herbaceous fossils in the same layer Photo courtesy of Zhang Xinwen

In order to further verify the conjecture of this environmental type, the research team also used the atmospheric-ocean circulation model and the Triffid model of the University of Bristol in the United Kingdom to simulate the paleoclimate and paleogearget at that time, and combined with the paleoenvironment reconstruction results of the fossil flora, the paleoenvironment of the Neoteth Ocean in the Paleogene period showed a gradually drying trend in the Central Valley along the Pangong Lake-Nu River suture belt, coupled with the global climate cooling. The vegetation there has changed from closed forests in the Miocene (about 47 million years ago) to open woodland, with tall trees, medium-height shrubs, and low herbaceous plants.

Interestingly, researchers looking at the 658 animal fossil records of the Tibetan Plateau and its surrounding areas found that the diversity of large plant-eating mammals in the environment at that time also began to increase.

“Due to the existence of open woodland, large plant-eating mammals enjoy a richer source of food, which will inevitably lead to an increase in the feeding pressure of animals on plants, which in turn promotes the evolution of plant spines in the open woodlands of the central valley of the Qinghai-Tibet Plateau.” Zhang Stressed that this is about 24 million years before a similar shift 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 drought in the central valley of the Late Eocene Qinghai-Tibet Plateau and the feeding pressure of large plant-eating mammals jointly drove the rapid evolution of the functional trait thorn. It also shows that the environmental changes of the Tibetan Plateau in the geological and historical period not only have a profound impact on the plant diversity of Asia and beyond, but also shape the plant functional traits in the region.

Zhou Zhekun believes that the highlight of this research is that it combines a variety of methods and means such as large fossils, micro-fossils, phylogenetic analysis and model simulation, and is the product of the cross-fusion of paleobotany, paleovertebrate zoology, paleoclimate simulation, molecular biology and ecology. (Source: China Science Daily, Hu Minqi, Teng Wenyu)

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