AsianScientist (Feb. 16, 2017) – In the immediate aftermath of an ancient mass extinction event, sponges were among the first animals to recover. These findings, based on the Anji Biota fossils, have been published in Current Biology.
The end-Ordovician crisis, 445 million years ago, was the second-largest mass extinction even since the rise of animals, resulting in the death of 85 percent of the species at the time. It was the result of a sudden, intense ice age, followed by an equally rapid warming, and corresponding changes in ocean chemistry and circulation.
The plankton recovered quite quickly, but little is known about life on the deeper parts of the sea floor. Till now, the only fossil deposit from this interval is the peculiar glacial lagoon environment of South Africa’s Soom Shale.
Now, a joint team of researchers from the Nanjing Institute of Geology and Palaeontology (NIGP) Chinese Academy of Sciences has revealed a new fossil fauna preserving delicate skeletons and soft tissues, from the immediate aftermath of the Ordovician mass extinction.
Called the Anji Biota, the fossils were discovered in the bamboo forests of Zhejiang Province, China, in a narrow band of mudstone exposed at several sites up to ten kilometers 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 localized. It doesn’t look like a survival fauna at all; these simple animals were flourishing.
Sponges were not quite the only animals on the sea floor, however. Together with thousands of sponges, a few conical-shalled nautiloids were also recovered, and a single fossil sea scorpion complete with legs. The sea scorpions were a very rare group in the Ordovician, and well-preserved specimens are almost entirely limited to these sites of exceptional preservation.
“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,” explained study lead author Dr. Joe Botting from NIGP.
Sponges are known today as ecosystem engineers, encouraging biodiversity by stabilizing sediment and providing habitats. In the case of the end-Ordovician crisis, such an abundance of sponges over wide areas might well have helped the ecosystem to recover.
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. There are lessons for the present, as well. 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.
The article can be found at: Botting et al. (2017) Flourishing Sponge-based Ecosystems After the End-Ordovician Mass Extinction.
Source: Chinese Academy of Sciences.
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