These leisurely animals that spend their days lying down actually died of "involution" in the Cambrian period?

During the Ediacaran period 570 million years ago, the Charnwood Forest area in England was once a vast ocean. The nearby volcanoes were very active, and large amounts of volcanic ash were constantly injected into the ocean, bringing rich nutrients to the seawater and making the ocean full of vitality. In the following 500 million years, with the splicing and breakup of supercontinents, this area also experienced a long period of sea-land changes. The fine sand on the seabed solidified into rocks, and was lifted by tectonic movements to the surface.

Children's great discovery

One afternoon in April 1957, a primary school student named Roger Mason and his two friends were rock climbing in Charnwood Forest. At that time, Mason had already developed a strong interest in geology. He knew many types of rocks and knew that the Charnian series rock formations in the Charnwood Forest area were formed in the ancient Precambrian period. While playing, Mason accidentally saw a fossil that looked like a leaf on the surface of a stone. When Mason showed this stone to Professor Trevor Ford, a paleontologist at the University of Leicester, Ford couldn't believe his eyes. In the cognition of scientists at the time, it was impossible for such complex organisms to exist in the Precambrian period, but this "leaf" fossil in his hand truly denied this impossibility.

Precambrian "leaf patterns" | Smith609 / Wikimedia Commons

Under the leadership of Mason, Ford went back to the location where the fossils were originally found and continued to search. Finally, they found 12 disc-shaped fossils and 6 leaf-shaped fossils on the rock layer of fine-grained siltstone rich in volcanic ash. In 1958, Ford published this great discovery of paleontology in the Journal of the Yorkshire Geological Society. In order to commemorate Mason's contribution to the discovery of fossils, Ford named the fossils that look like leaves Charnia masoni and the disc-shaped fossils Charniodiscus spp. - the protagonist of our species calendar today.

Charon's disk worm | tina negus / Wikimedia Commons

Leaf-like animals

The name of Charon's Disk Worm can easily lead readers to misunderstand their morphology. This is because when Ford first named Charon's Disk Worm, he only found some disc-shaped fossils with concentric circles. These disc-shaped fossils should have been some spherical structures before being buried and flattened by sand and mud. Ford speculated that they were probably the holdfast structures of some kind of organisms. With the discovery of new fossil records around the world, scientists can finally restore the full picture of Charon's Disk Worm. They have leaf-like bodies, with many opposite or alternate pinnates on both sides of the petiole, and a soft spherical holdfast at the bottom of the petiole. They can grow to more than one meter tall, which is definitely a giant in the Ediacaran ocean.

A flattened fossil of Charon's disk. Ghedo / Wikimedia Commons

When Ford first discovered Charon's disk worms, he suspected that they might be a type of algae, but at the same time he realized that no living algae had such a complex structure, so he conservatively speculated that they might be an unknown type of algae. However, algae need to photosynthesize, and Charon's disk worms live in the deep sea where sunlight cannot reach. So, what exactly are Charon's disk worms? This question puzzles paleontologists. Some scientists think that they look like modern soft corals - sea pens, but later the discovery of more and more Charon's disk worm fossils proved that they were not any known organisms.

Sea pens of the genus Virgularia, living animals | Nhobgood Nick Hobgood / Wikimedia Commons

Although the body of Charon's disk looks very complex, its self-organization is relatively simple. Each part of its body repeats the same body pattern, forming a multi-partitioned flat body that is entangled and stitched together. Each branch is actually just a smaller version of the whole. No species in the current animal kingdom grows in such a simple fractal way. Charon's disk body is soft, without mineralized shells and bones, and without organs for movement, feeding or digestion that modern animals have. They rely on the pressure of the liquid in their body to support their body, like an open umbrella, to maximize the surface area of ​​their body, so that as much of the body surface as possible is in contact with the seawater, so that they can obtain oxygen and nutrients from the seawater by osmosis.

Charon's disk worms flourished in the oceans of the Ediacaran period, but disappeared before the Cambrian explosion. After decades of research, paleontologists speculate that they were animals that obtained energy by filtering organic matter in seawater, but their position in the evolutionary tree of life is still controversial.

The fragile world of the Precambrian

Because Charon's disk worm looks like a fern leaf, it and many of its sister groups are called rangeomorphs. Frond fossils are the most representative fossil type of the Ediacaran period, and have been found and reported in Ediacaran strata around the world, especially in Newfoundland, Canada, England, the White Sea in Russia, and the Ediacaran Mountains in Australia, where a very rich fossil record is preserved.

These frond-like fossils are fixed in groups on the sandy seabed, forming ancient "undersea forests" that sway with the seawater. These spectacular "undersea forests" have caused a lot of confusion for paleontologists over the past half century, and have also completely changed people's understanding of the history of biological evolution - the Precambrian ocean was not a desert of life, but was once home to countless complex life forms.

Ediacaran organisms | Maulucioni / Wikimedia Commons

The Ediacaran-type organisms represented by the frond-shaped fossils have no bones, soft appearance, no mouth or intestine, and a flat body. Most of them do not move when they are alive, and after death they are preserved in the rock layer in the form of soft bodies. They have no weapons such as claws or thorns, nor do they have defensive shells. These soft "water bags" either lie on the bottom of the water to graze bacteria and algae on the surface of the sediment, or "stand" in the sea water to filter suspended organic matter. There is basically no conflict and complex interaction between them, and everyone lives in peace, so scientists call the Eden-like ocean of this period the "Ediacaran Garden."

Ediacaran animals Dickinsonia | Aleksey Nagovitsyn / Wikimedia Commons

These mysterious creatures of the Ediacaran period cannot establish a definite evolutionary connection with later creatures. They are the blind end of many extinct "dead" creatures on the evolutionary tree. Some scientists believe that Ediacaran animals were an unsuccessful evolutionary attempt of multicellular animals: they used a body structure design different from that of modern animals and explored for the first time the possibility of tissue differentiation between multiple cells of themselves; but because their body functions were too simple, they could not adapt to the great changes in the marine environment and the selection pressure brought by later predators, and thus withdrew from the stage of history forever, and only a very few lucky ones survived to the Cambrian period.

Reconstruction of Charon's disk | Andrey Yu. Ivantsov et al. / Paleontological Journal (2016)

Back to the present, the place where the fossils were discovered in Charnwood Forest has become a famous fossil protection site, and the earliest fossil records are now preserved in the local Leicester Museum. The little boy Mason who discovered the fossils, who loved geology, later became a famous geology professor. He came to China, thousands of miles away from the UK, to work and teach students metamorphic rock petrology at the university. The gray-haired old professor often shares his childhood story of encountering Ediacaran fossils with students on the campus of China University of Geosciences.

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