After the first mass extinction, the world’s oceans filled with sponges.
The history of the Earth knows five major mass extinctions – we say “major” because besides these five there were also others, smaller ones, which wiped out relatively few species. The very first on the list is the Ordovician-Silurian extinction, which happened 440 million years ago – then 85% of species disappeared (they usually specify that they were marine, but in those days life for the most part took place in the sea). The reason was sudden climate changes: after a sharp sudden cold snap, an equally rapid warming occurred. Those who survived were given carte blanche, and the sponges took full advantage of it.
Thickets of sponges on the seabed. (Photo by Klaus Stiefel / Flickr.com.) Remains of sponges from the Anji biota, 444 million years old. (Photo by JP Botting/Current Biology, 2017) ‹ › View full size
Employees of the Nanjing Institute of Geology and Paleontology, together with British colleagues, studied the so-called Anji biota – a complex of sedimentary rocks with the remains of ancient organisms discovered in Zhejiang province in eastern China. This paleontological find should have left traces of those who managed to survive the Ordovician-Silurian extinction – and among the hundred or so species that could be identified, seventy-five were sponges.
In general, few remains from that era have reached us: in addition to the newly discovered Anji biota, there are also sedimentary rocks of Sum-Sheil from South Africa. Until recently, paleontologists could only guess what happened in nature after the first mass extinction. All that was known was that the plankton recovered quite quickly after it; There were very few finds from which larger organisms could be judged.
Therefore, the discovery of the Anji biota was a great success – judging by what was found in it, sponges of all shapes and sizes reigned in the deserted seas: an article in Current Biology says that some of them grew entire forests on the seabed. (Although sponges were not the only survivors: the researchers also managed to find several nautiloid cephalopods and one crustacean scorpion.)
Sponges are fairly simple creatures, they do not yet have specialized organs, and they feed by sitting in place and filtering the surrounding water for living and dead organic matter. However, the ancient ecosystem was able to recover from extinction, not least thanks to them: after sponges, other sessile filter feeders became stronger on the seabed – brachiopods, corals and bryozoans.
By the way, the same explosion in the number of sponges was also noted after other mass extinctions – apparently, after large-scale environmental disasters, the restoration of multicellular life usually began with them.