The rise and decline of Ediacaran biota

In 1946, Reg Sprigg discovered animal fossils in the rock of Ediacara Hills in Australia. The age of these fossils couldn't be determined Initially, but later it was found that they were from an era older than the Cambrian period. The animals that formed these fossils came to be known as the Ediacaran biota (it is also know as Vendian biota). The era they inhabited was defined as the Ediacaran period (635-538 million years ago) in 2004. This discovery shattered the long-standing belief in the scientific community that multicellular animal fossils couldn't appear before the Cambrian period.

The rise and flourish of Ediacaran fauna

The Ediacaran biota was often considered to be one of the first group of complex multicellular life on Earth. They appeared in the end of a global glaciation known as Snowball Earth, and flourished until the dawn of the Cambrian. Most of the animals in the Ediacaran biota are difficult to fit into modern animal classifications. Some scientists propose that they were failed evolutionary branch that were eliminated by nature's selection.

Most animals of Ediacaran biota were soft-bodied and lacked skeletons. Therefore, it's so hard for them to leave fossils, and the fossils that are found represent only a small portion of the creatures in that era. They had no mouths, anuses and had flat bodies, some liked leaves, and others liked flat disks. It was possible that before this time, there were not many consumers in the ecosystem, the microorganisms were not only abundant in the seawater, but also formed a thick microbial mats covering the seabed. Therefore, these animals did not develop muscles and nerves for quick movement. They simply crawled or stood upright in shallow seabed like plants to absorb inexhaustible food. Most of them completely gave up their ability to move and evolved into larger and flatter forms. Their wrinkled bodies increased surface area to make them more efficient in oxygen absorption and food particles filtration from the seawater. This peaceful submarine scene where creatures didn't compete with each other, is referred to as the "Ediacaran Garden."

The Decline of Ediacaran Gardens

The Garden of Ediacara was a peaceful and stable ecosystem that lasted for about ninety million years. However, it came to an abrupt end with the onset of the Cambrian explosion of life. The fossil record shows a dramatic turnover of species, as the Ediacaran biota disappeared and were replaced by new forms of animals that had skeletons, appendages, eyes, mouths, and guts.

Changes in oxygen levels are considered one of the main factors behind the rise and fall of the Ediacaran biota. Before the Ediacaran period was the Snowball Earth era, extreme cold and the proliferation of anaerobic bacteria suppressed photosynthesis, leading to decreased oxygen levels on Earth. The greenhouse effect recovered due to the release of carbon dioxide from volcanic eruptions, and algae in the oceans proliferated to release oxygen. Nevertheless, oxygen levels during the Ediacaran period were only about 40% of modern levels. This limited the evolution of organisms with higher metabolic rates. When oxygen levels rose to around 60% of modern levels during the Cambrian period, it was favorable for the appearance of more active animals.

The emergence of new types of animals may have altered the ecological balance and created new selective pressures for adaptation or extinction. For example, the evolution of grazers may have destroyed the microbial mats that supported the Ediacaran biota. The evolution of predators may have driven the development of skeletons, defense structures, or escape behaviors among prey species. These animals were able to move, burrow, graze, and hunt, and they transformed the seafloor from a peaceful microbial mat into a complex and dynamic environment. Animals that lacked armor and mobility were undoubtedly the first to be eliminated in the competition.

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