A sudden drop in the diversity of species was caused by appearance of first animals.
Not only had a group of researchers fr om NSU and the Trofimuk Institute of Petroleum Geology and Geophysics SB RAS (Novosibirsk) in cooperation with a scholar from the Zavaritsky Institute of Geology and Geochemistry of Ural Branch RAS (Yekaterinburg) found important evidences of the first mass extinction in the Earth’s history, but they managed to date this anomalous event. Supposedly, as soon as the first animals appeared on the planet, they destroyed the habitat of the existing Ediacaran biota.The article Ediacaran biota in the aftermath of the Kotlinian Crisis: Asha Group of the South Urals was published in the prestigious Precambrian Research - an international journal with IF 5.66, which publishes studies on all aspects of the early stages of the composition, structure and evolution of the Earth and its planetary neighbors.
Some details of the research are shared by Dmitriy Grazhdankin, Doctor of Sciences in Geology and Mineralogy, a Research Associate at the Institute of Petroleum Geology and Geophysics, Novosibirsk, the head of the Laboratory of Precambrian Paleontology and Stratigraphy and a senior instructor at the Chair of Historical Geology and Paleontology at NSU.
The Earth’s geological history is considered to have seen five massive extinctions of living organisms. The oldest one was previously dated as far as 450-440Ma. According to our results, says the researcher, it was not the first extinction. We discovered that about 100 mln years before, there was another global event,Kotlinian Crisis, characterised by a sudden drop in diversity of the Ediacaran biota. A whole kingdom of living organisms disappeared leaving just vanishing traces of their existence. At about 530 Ma, the biosphere renovated itself, but it was an absolutely different story. That crisis is unique in terms of the total destruction of the existing food chain, which was replaced by the modern one based on algae and plants instead of bacteria.
In fact, the earliest ancestors to modern-day animals developed before the Cambrian explosion. The so-called explosion period, as far back as 540 to 530 Ma, envelops the time on Earth when most animal groups first appeared, but there had been some giant soft-bodied organisms living about 580 Ma that differed from any modern life form. Those globally distributed Ediacaran organisms represented the biodiversity of the period. The multicellular quilted organisms ranging from 1cm to 1.5 m in size flourished in shallow waters dominated by microbial mats and slime matrix. All the Ediacaran (Vendian) biota disappeared from the record 550 Ma million years ago, at the end of the Ediacaran period, leaving only curious fragments of once-thriving ecosystems. The main challenge for researchers around the globe was putting together the missing parts of the geological records related to this most enigmatic period. We know that the period of 550-530 Ma witnessed a global lowering of sea level, but the records have almost no information about the related processes. In order to prove the fact of Ediacaran biota dying about 550 Ma, Siberian scientists spent 10 years investigating formations in the South Urals, on the territory of Bashkortostan.
The fact that some fossils have not been recorded by geologists does not necessarily mean the extinction of the corresponding species. A good example of thecrossopterygian fish demonstrates that instead of dying off about 70 Ma according to the last records and fossil discoveries, the fish has survived in the Indian Ocean up to now. It encouraged us to search for marine deposits that were a favorable environment for the biota and could preserve their fossils.
It is in the South Urals wh ere such deposits can be found due to the existence of a sea basin in this area about 550-540 Ma. The Asha Group of the South Urals is dominated by laminated shales and thin siltstone–sandstone alternations interpreted as a prograding low-energy inner shelf depositional system. A more accurate dating of the layers became possible in the presence of millimetre-scale alternating layers of volcanic tuffs. The latter contain zircons allowing for uranium–lead dating (U–Pb zircon dating), a reliable technique that characterizes the age distributions of zircon crystals in a rock by their age of formation as determined by the ratio of uranium to lead. We obtained the age of the Asha Group, which is constrained by a U–Pb zircon date of 547.6 ± 3.8 Ma from an ash bed in the lower part of the sedimentary succession. It took 10 years of research to investigate and describe the sediments in the river basins of the South Urals. The ecosystem discovered contained various organisms including palaeopascichnids and frondomorphs, macroalgae, horizontal burrows, but no Ediacaran biota.
In spite of the Asha Group being part of a Late Proterozoic taphonomic window favouring fossilization of soft-bodied macroorganisms, we discovered low biodiversity of Ediacaran macroorganisms. Neither dickinsoniomorphs, nor tribrachiomorphs, nor bilateralomorphs have been encountered in the strata younger than ~ 550 Ma, which proves that they had become extinct by the time.
The scientist has also supposed why such an extinction event could take place. The demise in biodiversity of the soft-bodied organisms was accompanied by an increase in the diversity of burrowing organisms, the structural and behavioural complexity of their burrows, and the depth of burrowing. It means that the first burrowing organisms, animals, appeared 10 mln years earlier than it was previously considered, about 550 Ma.
All organisms contribute to environmental transformation, but due to animals moving in active search of food, boosting the processes of nutrient and energy transfer from one feeding level to another and controlling the whole food chain in general, they became so powerful in constructing ecosystems. All modern ecosystems are built by animals. As soon as the first species appeared, they started their work. Having learnt to burrow, the animals destroyed the microbial substrates necessary for Ediacaran biota. The most devastating wave of the late Ediacaran extinction, the Kotlinian Crisis, wiped out dickinsoniomorphs, bilateralomorphs and tribrachiomorphs. Thus, the kingdom of Ediacaran biota vanished and gave rise to a new ecosystem.
