New progress in angiosperm research! Chinese scientists discovered the earliest fossil evidence

Produced by: Science Popularization China

Author: Wang Xin (Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences)

Producer: China Science Expo

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Angiosperms are the most diverse group in the world today, with at least 300,000 species. We humans rely on angiosperms for food, clothing, shelter, transportation and medicine to a large extent. For example, most of the staple food and vegetables we eat every day are angiosperms. In fact, as early as millions of years before the origin of humans, angiosperms have been an indispensable and important component of the earth's ecosystem.

Classification of angiosperms

(Image source: Nature)

The complex relationship between angiosperms and insects

Due to the diversity and importance of angiosperms, angiosperms and animals (including insects and humans) have formed complex interactive relationships during the long process of evolution.

Insects, which have developed and grown together with angiosperms, are also a group with extremely high diversity in the biological world. During the long process of evolution, they have formed an inseparable ecological relationship with angiosperms. Some insects have even formed a one-to-one corresponding benign interactive relationship with specific groups of angiosperms, which is beneficial to both insects and angiosperms.

But it should be noted that the relationship between most insects and angiosperms is a feeding relationship, which not only includes situations where insects feed on plant leaves (or other tissues), but also situations where plants capture and eat insects (of course, the latter situation is relatively rare).

Insects feed mainly on the leaves of plants. This common behavior is a disaster for agricultural production. Humans will intervene in the feeding relationship between insects and crops by spraying pesticides to prevent insects' behavior from harming human interests.

In modern ecosystems, angiosperms do not have effective defense and resistance mechanisms to deal with a certain type of insect, so they are often attacked by more than one insect. Considering that there are hundreds of thousands of insect species in the world, it is "natural" that the leaves of modern angiosperms suffer from multiple pests, which reflects the complexity of modern ecosystems.

Leaves bitten by insects

(Photo source: veer photo gallery)

But this is only the current state, not the permanent state. In my research on paleoecology, I found that the interaction between ancient plants and contemporary animals is constantly changing during the geological history period.

Complex relationships revealed from a single fossil

Recently, many interesting advances have been made in paleoecological research on fossil plants. There are feces and gnawing traces left by insects in Permian wood, traces of insect gnawing have been found on many fossil leaves from the Mesozoic Era, and Cenozoic plants (especially angiosperms) are often attacked by insects... In general, with the passage of time, the interaction between angiosperms and insects has become more frequent and complex.

One sign of the complexity of this ecological relationship is the diversity of insect gnawing traces. The earliest angiosperm leaf fossils were often attacked by the same insect, so there was only one kind of gnawing trace. But in modern times, a plant may be attacked by multiple insects, and a single insect may also gnaw on different plants. This ecological relationship from simple to complex has an evolutionary process.

How to confirm the process and degree of the complexity of this ecological relationship? When did this process of complexity begin? Although people can use evidence from various aspects to speculate, fossils are the final source of information that can be conclusive. Therefore, finding corresponding fossil evidence has become an important task for paleobotanists.

In July 2019, Professor Han Gang of Hainan Tropical Ocean University and Professor Zhang Lijun of the Shenyang Geological Survey Center of the China Geological Survey obtained several plant fossils from Arong Banner, Hulunbuir League, Inner Mongolia through friends.

Professor Han Gang and I had collaborated on research for a long time before, and he contacted me as soon as he got the fossils. Among these fossils is a large leaf with traces of insect bites on it.

I realized that this fossil had potential ecological significance, so I contacted Professor Han Gang to send the fossil to the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, for further observation and research. In early August, the fossil was sent to me. As expected, after careful observation, I confirmed my initial judgment: according to the international classification of insect bite marks, it can be found that this leaf fossil less than 10 cm long and 5 cm wide has three different insect bite marks!

Fossilized leaves from the Platanaceae family from the Late Cretaceous, 80 million years ago

(Image source: Reference 1)

Relationship progression from the Late Cretaceous

Obviously, this is a discovery of great academic value. In order to do further research, I decided to arrange a field survey with Professors Han Gang and Zhang Lijun as soon as possible to determine the stratigraphic information of the fossils. In late October, we came to Ganhe Farm in Arong Banner, Hulunbuir League. In geology, this area is called Dayangshu Basin, and some stratigraphic studies have been conducted by predecessors.

The rock stratum in which we discovered the fossils belongs to the Nenjiang Formation of the Late Cretaceous. This conclusion was supported by our subsequent isotope dating research and the research results of Researcher Li Gang from the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, on the associated limbed shell fossils.

After confirming the stratigraphic age, for the sake of caution, we decided to consult Dr. Wang Hongshan, a curator at the Florida Museum who has long focused on the study of leaf fossils. Dr. Wang was very enthusiastic and provided important help to our work.

Through preliminary research, we believe that the fossil is most similar to Arthollia, a plant of the Platanaceae family found in the Late Cretaceous in northeastern China, the former Soviet Union. This result is not surprising: the geographical distance between the two regions is not too far.

The first type of bite mark on the leaf: the leaf tissue on both sides of the main vein is completely eaten away

(Image source: Reference 1)

The Platanaceae family is the largest group of angiosperms, the basal group of eudicots. Although the diversity of this plant is not very high now, in the Cretaceous, the Platanaceae family was very rich and diverse. It was once widely distributed in the Northern Hemisphere, and even served as the dominant group in some layers of the Dakota flora in the United States. Therefore, it is reasonable for the leaves of the Platanaceae family to appear in the Great Poplar Basin.

In order to ensure the accuracy of our identification, we also compared the fossils with leaves of modern Platanaceae plants. During this process, Dong Xiaoyu from the Nanjing Zhongshan Botanical Garden Herbarium provided us with valuable help.

The second type of bite marks on leaves: the leaf tissue between the veins is completely eaten away

(Image source: Reference 1)

Insects are extremely diverse in the world today, with hundreds of thousands of species, not counting extinct species. Different insects have different mouthparts and feeding methods, which will leave different bite marks on plant leaves. We worked with Zhang Haichun, a fossil insect expert from the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, to participate in this study. The three bite marks we found on the fossils prove that at least three different insects used the leaves of the Platanaceae family as their food source.

The third type of bite marks on leaves: the edge of the leaf is eaten away

(Image source: Reference 1)

In previous reports, it is common for angiosperm leaves to be eaten. However, in general, there are only traces of bites from one type of insect on a leaf, which is a relatively simple food chain relationship in ecology.

However, the food chains in modern ecosystems are intertwined into complex food webs, and there is little or no fossil evidence to prove the origin and history of such food webs. It is very rare to find three types of insect bite marks on the same fossil leaf.

The current fossils come from the Late Cretaceous period 80 million years ago, which is the earliest relevant fossil record so far, indicating that the ecosystem related to angiosperms had begun its process of becoming more complex towards modern times as early as the Late Cretaceous period.

Conclusion

The interaction between contemporary angiosperms and insects forms a complex food web. This food web now appears to have started at least 80 million years ago in the Late Cretaceous. Our discovery provides evidence for this, but we still know very little about whether there was an earlier history and how the food web evolved since then. Hopefully, future research will reveal more truth in this regard.

Note: The author of this article is a researcher at the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences. He has been engaged in the origin and evolution of angiosperms for a long time. Based on fossil materials found in China, he has discovered and published many angiosperm reproductive organs before the Cretaceous (including the Jurassic Schweiner fruit, Nanjing flower, Pan's true flower, etc., and the Permian Taiyuan spike). The author's interests also include cytoplasm fossils and prime number research.

References:

1.Gang Han, Haichun Zhang, Lijun Zhang, Lingji Li and Xin Wang. The First Sign of the Complication of Angiosperm-Related Ecosystem. Sci J Biol & Life Sci. 3(5): 2024. SJBLS.MS.ID.000571. DOI: 10.33552/SJBLS.2024.03.000571