Sponges may be simple creatures, but they ruled the world some 445 million years ago after the second-largest mass extinction in Earth’s history, according to a new study of a fossil treasure-trove in China.
The end-Ordovician crisis resulted in 85 per cent of species dying out. It was the result of a sudden, intense ice age, followed by an equally rapid warming, and corresponding changes in ocean chemistry and circulation, researchers said.
The plankton started to recover quite quickly, but until now we have known little about life on the deeper parts of the sea floor, they said. Researchers from Chinese Academy of Sciences and National Museum Wales in the UK revealed a new fossil fauna preserving delicate skeletons and soft tissues, from the immediate aftermath of the Ordovician mass extinction.
The Anji Biota was discovered in the bamboo forests of Zhejiang Province, China, in a narrow band of mudstone exposed at several sites up to 10 kilometres apart.The fauna is extraordinarily diverse, with nearly 100 species found in the first phase of collecting.
The surprise, though, is that this diversity is almost entirely composed of sponges. The Anji Biota records an astonishing range of different sponge species, in many different major groups, with a total diversity exceeding that of equivalent modern faunas.
Most post-extinction survivor ecosystems are made up of small, stunted species that managed to thrive and are found everywhere. In the Anji sponge fauna, the sponges are large and complex, and although some species formed forests on the sea floor, many others were very scarce or extremely localised. It does not look like a survival fauna at all; these simple animals were flourishing.
Together with thousands of sponges, a few conical-shelled nautiloids were also recovered, and a single fossil sea scorpion complete with legs. “We think the sponges thrived because they can tolerate changes in temperature and low oxygen levels, while their food source (organic particles in the water) would have been increased enormously by the death and destruction all around them.,” said lead author Joe Botting.
Sponges are known today as ecosystem engineers, encouraging biodiversity by stabilising sediment and providing habitats, researchers said. In the case of the end-Ordovician crisis, such an abundance of sponges over wide areas might well have helped the ecosystem to recover, they said.
The team also notes that mass sponge remains have been recorded after other mass extinction events, suggesting that this is a common pattern after ecological collapse. If the past is anything to go by, then as marine ecosystems begin to collapse due to human activities, we should expect to see sponges rule the seas once again, researchers added.