New discovery! The origin story of angiosperms hidden in "stones"

Produced by: Science Popularization China

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

Producer: China Science Expo

What did you eat for breakfast? Bread, fried dough sticks, porridge, soy milk, or wontons, steamed buns, eggs, milk?

Let's take a look at what these things are made of. There are plants such as wheat, rice, beans, and various vegetables, as well as meat, eggs, and milk from animals such as chickens, pigs, and cows.

These plants are all angiosperms. The reason why these animals can produce meat, eggs and milk is that they eat a variety of feeds every day, and most of the feeds also belong to angiosperms.

Diversity of angiosperms

Angiosperms are also called flowering plants. The flowers you give and receive come from them, but angiosperms are used for more than that.

Angiosperms are the main cornerstone of modern terrestrial ecosystems, and their photosynthetic products are the main source of energy for the Earth's biosphere. An important feature that distinguishes them from other plants is that they have flowers and seeds wrapped in fruits.

Without angiosperms, the world would lose a lot of delicious food and beautiful scenery, and perhaps even humans would not exist on Earth. Therefore, the importance of angiosperms to us humans can be seen.

Camellia: One of the angiosperms

(Photo source: veer photo gallery)

How many flowers are there in the world? Where do they come from? Botanists are just as confused as we are.

There are currently 300,000 to 400,000 species of angiosperms known to humans, and the important feature that distinguishes these plants from each other is their flowers. As for the answer to the second question, that's a long story.

The mystery of the origin of angiosperms

To answer the second question, we need to first understand paleobotany, which is the study of plants in geological history based on their fossilized remains.

As the dominant plant group in today's ecosystem, angiosperms have inevitably become one of the research focuses of paleobotany, and the issue of their origin is also a focus of botanical attention.

The origin of angiosperms has a certain relationship with Darwin, the founder of the theory of evolution. Darwin is an advocate of the gradualism in biological evolution and believes that the origin and occurrence of all organisms is a slow process.

However, this was very different from the fossil record that people knew at the time. One of the difficult-to-explain phenomena was that "angiosperms suddenly appeared in large numbers in the strata in the middle Cretaceous period."

This phenomenon made Darwin's theory of gradualism untenable and troubled him greatly, so he called it the "annoying mystery."

Charles Darwin

(Photo source: veer photo gallery)

There are two schools of thought on the origin of angiosperms in modern paleobotany research. One school believes that angiosperms originated in the Early Cretaceous period about 130 million years ago, and this view has gained an advantage in the academic community. The other school believes that the origin of angiosperms is earlier than the Cretaceous period because "the fossil records that have been confirmed now will definitely not be the entire and earliest records of a certain group, and will be broken sooner or later."

Studies have shown that even in the Yixian Formation of the Early Cretaceous, the diversity of angiosperms was already very high.

Therefore, in my opinion, there must have been angiosperms before the Cretaceous that have not yet been recognized. Although it is easy to hold this view, it is not easy to prove it.

When the “first flower” Paleocarpus was published, it was claimed to be from the Jurassic period (before the Cretaceous period). However, today, most people in the academic community believe that Paleocarpus is from the Cretaceous period. This misunderstanding of the age of Paleocarpus seems to confirm the impression that there were really no angiosperms before the Cretaceous period! Comparing the current state of knowledge with that of Darwin’s time, people will find that the so-called Darwin’s “nasty mystery” has not been solved at all, but the time has been moved from the middle Cretaceous period 100 million years ago to the early Cretaceous period 130 million years ago!

Regarding the origin time of angiosperms, the mainstream view internationally is that it was in the Early Cretaceous period about 130 million years ago, or slightly earlier , but definitely not in the Jurassic period.

Criteria for determining angiosperms

There has always been controversy surrounding the criteria for identifying angiosperms.

As a term, "angiosperm" was coined in 1852 by the German botanist Paul Hermann (1646-1695) to refer to plants whose seeds are enclosed in a container. This is also the definition used by many paleobotanists (including the author of Paleofruit).

Later studies showed that some gymnosperms actually also wrap their seeds after pollination, so it is clear that the original definition was not quite appropriate.

Tomlinson and Takaso published an article in 2002, arguing that the important feature that distinguishes angiosperms from gymnosperms is that the ovules (predecessors of seeds) are wrapped during pollination. Botanists who study modern angiosperms have found that different angiosperms wrap ovules to different degrees: the wrapping is better in more evolved groups (completed by plant tissues), while in more basal angiosperms, this wrapping is completed by both plant tissues and mucus. This feature can well confirm the angiosperm attributes of a plant.

The new discovery used the standard that the ovules are wrapped during pollination.

Taiyuan Sui: Tracing the Origin of Angiosperms

Our protagonist today is a fossil plant from the Early Permian (270 million years ago) in Taiyuan, Shanxi: Taiyuananostachya ovuilifera .

Two Taiyuan ears on the same specimen

(Image source: author)

At first glance, this fossil plant appears to be a common spike of reed or cone of gymnosperm in the Paleozoic era, which is why early paleobotanists were vague about its specific classification.

After careful observation, the author found that the central axis of the fossil plant's "cone" was covered with nearly spherical ovules or seeds, and these seeds or ovules were wrapped in lateral units (= fruits) and connected to the outside world only by a tube filled with mucus.

This observation confirmed that the fossil contained both enclosed ovules and enclosed seeds, thus ruling out the possibility of any ophiopogons and gymnosperms and confirming its angiosperm attributes.

This seemingly simple judgment plays an important role.

A: Multiple connected fruits with seeds inside; B: Seeds exposed at the base of a broken lateral unit (= fruit); C: The bulging base of a complete lateral unit (= fruit) reveals the seeds inside.