During the Cambrian explosion, a complete earthly ecosystem was formed. The structure of this ecosystem is very similar to that of modern times, that is, the pagoda-shaped structure of what we usually call "big fish eat small fish, small fish eat shrimp shrimp, and shrimp shrimp eat mud".
On the bottom of the Cambrian sea, seaweeds that symbiosis with the Chengjiang fauna grew very luxuriantly. In the sediment of the seabed live a variety of burrowing worms, such as linear animals, gill dragging animals, etc., most of which feed on seafloor sediments. There are also many reptile arthropods on the seafloor, most of which also feed on sediments on the seafloor. These algae plants, mud-eaters, etc. constitute the underlying organisms of the pagoda-shaped food chain. Sponges, spiny cell animals, goblet worms, and Ildong-shaped tentacles live on the seafloor to feed on nutrients in the filtered seawater. Crawling on the seafloor and preying on other small animals, leaf-legged animals that feed on animal carcasses, and arthropods that swim and prey on three-dimensional waters form the middle layer of the food chain. So, what are the animals at the top of the food chain in the Cambrian ocean?
The largest animal found in the Chengjiang fauna is an animal called "Odd Shrimp", which is nearly 2 meters long according to fossil fragments.
The odd shrimp has a pair of large stem-shaped eyes on its head, as well as a pair of powerful grabs and a huge round mouthpart with sharp fangs. The large eyes give the shrimp a wide field of vision, powerful grippers can be used to capture and firmly control larger animals to avoid escape, and the huge mouth with sharp teeth can tear apart the carcasses of various animals. The body of the odd shrimp is segmented and has an exoskeleton. On both sides of the ventral surface of the odd shrimp, there are paddle-like legs and limbs and strong inner limb structures, which allow them to swim quickly in the sea and use their legs to catch other animals.
It can be seen that in the early Cold Age ocean, the odd shrimp was the most ferocious animal, standing at the top of the food chain.
It is worth mentioning that the process of scientists' understanding of odd shrimp is quite interesting.
Odd shrimp were first discovered in the Burgess Shale of British Columbia, Canada. In 1892, Whiteaves first named this crustacean that looked like a shrimp "odd shrimp." Later, other fossil fragments of strange shrimp discovered one after another were described as completely different animals such as sponges, jellyfish, echinoderms, and link animals. In 1979, Briggs realized that the so-called "shrimp" was really just an appendage of an animal rather than a complete individual. In 1985, Whittington and Briggs pointed out that the structures of what were previously thought to be different animals actually belonged to odd shrimp.
Burgess shale fauna
In August 1909, the famous American paleontologist and president of the Smithsonian Society, Wellat, led the family on a trip to the Burgess Mountains in the Canadian Rockies. On the way back, his wife's horse was tripping over a rock. When he picked up the stone and looked at it, he was surprised to find that it was an animal fossil he had never seen before. The animal protrudes two pairs of antennae from its head and has a multi-segmented body. Wilkat named it "Mal Trilobite" after one of his friends.
The following summer, Wilkat returned to this place and began large-scale excavations, resulting in more than 100 well-preserved fossils of invertebrates, some like hypnotics, anemones and other coelenterates, some like link animals, some like sea cucumbers and other echinoderms...
Because the "Maltrilobite" and many different species of animal fossils discovered later were found in a formation called the "Burgess Shale Formation", these animals are collectively referred to as the "Burgess Shale Biota".
The study found that the Burgess shale biota lived around the Middle Cambrian Period (505 million years ago), which is what we often call the Cambrian explosion period. During this period, large populations of animals suddenly appeared in a relatively short period of time (perhaps only a few million years), and almost all biological species can now find their ancestors here, whereas before the animal world was composed of simple organisms such as multicellular offspring and small shells. The Burgess shale biota became important evidence of the Cambrian explosion.
The discovery of the Burgess biota shocked the scientific community at the time, and for the first time made it clear to scientists that only a few animals with skeletons in the Cambrian ocean, the vast majority of which were mollusks that were not easy to preserve, thus correcting the misconception that only a few hard animals such as trilobites remained in the Cambrian.
How did the "Cambrian explosion" event come about?
What factors led to the Cambrian explosion?
One view that many people agree with is the "oxygen content variation theory". The theory is that about 2 billion years ago, due to the emergence of aquatic prokaryotes that travel photosynthesis, the accumulation of free oxygen in the hydrosphere and atmosphere and the weakening of ultraviolet radiation. Between 900 million and 1.5 billion years ago, eukaryotes began to appear, leading to a further increase in oxygen levels in the atmosphere and water bodies, carbonate deposits and a decrease in carbon dioxide levels. The immediate consequences of this event are the rise and fall of global temperatures, the formation and melting of glaciers, and the rise and fall of sea levels, which triggers the diversification of the ecological environment on the earth's surface and the sharp differentiation of biological groups. Among them, oxygen content is a major obstacle to the evolution of probiotic organisms, and when this obstacle is removed, multicellular progeny animals appear immediately and evolve rapidly.
Another popular view is the "harvest principle theory". The theory suggests that the appearance of predators was the main inducing factor for the Cambrian explosion. Before the Cambrian, for up to 3 billion years, there were only simple primary producers like algae among marine organisms. With no predators, the composition of the entire Precambrian organism was extremely monotonous. The emergence of predators has opened up new space for primary producers. As a result, more and higher-level predators evolved.
In addition, there are some theories that attempt to explain the Cambrian explosion from the evolution of developmental mechanisms, arguing that during the Cambrian explosion period, when the complexity of genes reached a certain critical value, many groups of epizoans occurred explosive radiation due to genetic reconstruction.
So far, there has not been a clear and convincing explanation for the causes of the Cambrian explosions. Therefore, the Cambrian explosion remains an unsolved mystery.
How is the Chengjiang fauna perfectly preserved?
All kinds of animals in the Chengjiang fauna can be so well preserved, there must be some geological and sedimentary emergencies, so that all kinds of creatures living in the sea area of Kunming at that time were quickly buried and sealed, not only from the corrosion of bacteria, but also from the disturbance of various benthic animals.
For the burial methods and preservation of the Chengjiang fauna, some experts believe that on a muddy bottom of the seashore, the storm makes the sediment-laden seawater flow to the deep-sea basin and form a turbid flow, like a flowing submarine river; due to the invasion of the foul-smelling soil, this current becomes poisonous water, killing animals living in shallow seas, bringing their corpses to the hypoxic belt under the surface of the wave and depositing them from carnivores and burrowing animals; a few days later, the deposited fine mud envelops the animal carcasses. Protect it from pervasive bacteria. In this way, the Chengjiang fauna should be buried in a different place. However, the well-preserved specimens of brachiopods, specimens of hypermycete appendages, and specimens of the leaf-footed Luolishan insects found so far show that the Chengjiang fauna should be buried in situ.
Some experts pointed out that it may be that the great storm blew a large amount of sand from the adjacent ancient land into the sea and deposited it, resulting in the death of a large number of animals. However, this explanation does not account for the cause of death in some animals. Indeed, the bare paleocontinentals of the Cambrian period would be dusty when they struck, leaving clear waters full of clastic sediment, which may kill animals with poor swimming ability and benthic fixed animals, but it is difficult to put burrowers and strong swimming animals to death. As can be seen from the fact that different parts of a worm's body are kept in different mudstones, many worms living in the burrows seem to be moving through layers before they die. Studies of a large number of specimens have shown that the Chengjiang fauna represents what appears to be a fauna buried in situ. The emergency not only caused all kinds of animals to be buried quickly, but also caused toxic substances to be produced in the water, causing the animals to suffocate and die quickly in the struggle, and even bacteria were difficult to survive and reproduce at that time.
In recent years, some experts have pointed out that volcanic eruptions may also affect the preservation environment of the Chengjiang fauna.
New research proves that the reason why the fossils of the Chengjiang fauna are very beautifully preserved is mainly the result of the mineralization of pyrite. After the death of the animal, the mineralization of pyrite occurs rapidly. The exquisite tectonic details of the fossils of the Chengjiang fauna we see now are actually the result of the mineralization of pyrite.