Nondescript sea sponges turned out to be the most persistent animals on Earth - they survived all the major mass extinctions of flora and fauna and served as the main "restorers" of the destroyed ecosystems of the planet, according to an article published in the journal Current Biology.
“We assume that sponges are so resilient due to the fact that they can live in very wide temperature ranges and in conditions of almost zero oxygen concentration. In turn, their food source is organic particles in the water, which should have increased dramatically in the ocean after the mass death of other animals,”said Joseph Botting of the National Museum of Wales in Cardiff, UK.
Scientists identify the five largest mass extinctions of species in the history of life on Earth. The last of these, the Cretaceous-Paleogene extinction, occurred about 65.5 million years ago and led to the extinction of dinosaurs and all large land and marine reptiles. It is believed that each such event is accompanied by an evolutionary explosion - the rapid spread, physical growth and specialization of surviving species that occupy the vacant ecological niches.
The Ordovician Extinction, the second-strongest disaster in Earth's history, occurred approximately 443 million years ago and killed 85% of the species and 60% of the genera of marine invertebrates. The hardest hit were bryozoans, bivalve molluscs and corals - in some cases their generic diversity was reduced by two-thirds. Apparently, the reason for this extinction was the drop in the concentration of carbon dioxide in the Earth's atmosphere and the subsequent cooling.
As Botting says, our ideas about this extinction are extremely sketchy, since the rocks of that period are almost not preserved on Earth. The only known deposits of this time were found on the territory of South Africa almost 20 years ago, and they are imprints of soil from shallow waters with the remains of living creatures that got there. We hardly know what changes were taking place at great depths.
Botting and his team obtained the first information of this kind, having discovered a whole "cemetery" of ancient underwater organisms, excavating rocks of the Ordovician period in the southeast of China, in the province of Zhejiang. In fact, the entire territory of China at that time was a shallow sea in the vicinity of the Earth's equator, divided by many large and small islands.
During the Ordovician extinction, this sea, as the excavations of the authors of the article showed, was literally covered with a whole forest of sea sponges. According to scientists, they managed to find in the bamboo forests of modern China a giant layer of rocks 10 kilometers long, completely filled with the remains of sponges, the number of species of which, according to the most rough and preliminary estimates, exceeds a hundred.
Interestingly, both small and fairly large sponges were present in these deposits, which is considered extremely atypical for mass extinctions and the periods of time immediately after them. Apparently, the sponges not only quickly recovered after the disaster, but also began to quickly fill empty ecological niches in shallow waters and in the depths of the seas. In some cases, the diversity of species in these deposits exceeded that which exists in the seas of the Earth today.
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Sponges were not the only inhabitants of the seas of that time - in addition to their prints, scientists managed to find in the Zhejiang province several fragments of nautilus shells, as well as full prints of the shell of a crustacean and prints of a large sea slug.
Scientists believe that such a variety of sponges indicates that they have been playing the role of the main "engineers" of the Earth's ecosystems for several hundred million years, maintaining their stability and restoring them after extinction. It is possible that a nuclear disaster or the next mass extinction will again lead to the fact that not cockroaches or rats will survive, but sponges. They will dominate the seas of the Earth until its ecosystems recover for the sixth time, the authors of the article conclude.