The Ediacaran, a geological period spanning from 635 to 538 million years ago, saw the emergence of the first complex multicellular animals after Earth was ruled for almost 3 billion years by microorganisms.
First described from the Ediacara Hills in Australia, Ediacara-type fossils have been found in Newfoundland, England’s Charnwood Forest, Namibia, Russia and China.
The Ediacara fossils includes many weird organisms of unknown affinity, like Dickinsonia, an egg-shaped, segmented hybrid between a worm and a jellyfish, Charnia, a segmented and branched organism resembling superficially modern sea pens, or Tribrachidium, showing a threefold rotational symmetry not found in any modern creature.
The beginning of the Ediacaran is marked by the Marinoan glaciation, a worldwide glaciation lasting from 654 to 632 million years ago. Its possible role in the emergence of the Ediacaran fauna has long been debated, with some researchers suggesting that the melting ice released nutrients into the sea, providing a fertile ground for complex life to evolve.
In a new study, Chinese researchers used cyclostratigraphy to exactly date the aftermath of the Marinoan glaciation and the emergence of the Ediacara fauna, suggesting that oxygen pulses played a mayor role.
Cyclostratigraphy analyzes astronomically-forced cycles preserved in sedimentary rocks. When integrated with radioisotope geochronology, a technology that uses the radioactive decay of elements to date rocks and minerals, it can produce a continuous, high-resolution geological time scale.
Approximately 580 million years ago, South China was part of the northern coastline of the supercontinent Rodinia and positioned near the equator. The erosion of the still barren land produced a succession of black mudstone and white limestone layers. By studying this succession, the researchers discovered cycles lasting from 2.4 million years to 103,000 years, likely a result of orbital changes and shifting climate patterns.
According to the new analysis, the Marinoan ice age was followed by a rapid sea-level rise, leading to the deposition of a thick layer of cap carbonate in just one to 10 million years. This marks a dramatic shift from a frozen world to a hot, high-carbon dioxide environment.
The emergence of the earliest Ediacara-type fossils was dated to 619 to 587 million years, with species becoming progressively more complex over time. The rapid deglaciation influenced oceanic currents, leading to periodic pulses of oxygen reaching the deep sea. The emergence of new species, so the study’s conclusions, coincides with these oxygenation events.
The flattened body of many Ediacaran fossils suggests they adsorbed oxygen and nutrients directly from the water through their body surface. But this adaption made them vulnerable to low oxygen levels. A combination of volcanic eruptions, tectonic plate motion, maybe even an asteroid impact, at the end of the Ediacaran caused a drop in global oxygen levels, leading to the first mass extinction around 539 to 500 million years ago
This extinction may have helped pave the way for the evolution of animals as we know them today.
The study, “Astronomically calibrating early Ediacaran evolution,” was published in the journal nature communications.
