The Sponge – a Lone Survivor
(March 24th, 2017) In Earth’s entire history, five mass extinctions have killed millions of species. In the hostile post-extinction world, one phylum has, however, managed to conquer the sea floors all over the world – the kingdom of sponges.
Fossils are like windows into the past. Thus, palaeontologists are always very pleased when they find deposits with organic remains, which give them an idea about how life used to be. Especially interesting are fossils after mass extinctions.
A mass extinction is defined as an event, in which more than three-quarters of all species on earth die in a geologically short interval. As far as we know, this happened five times in the past 540 million years. In the Late Ordovician, 445 million years ago, the earth suffered the second biggest mass extinction since the rise of animals: a sudden and intense ice age killed 85% of all species. Then, Earth rapidly became warmer and the fauna started to recover. At first, the number of plankton rose, then one phylum took over almost exclusively – the sponges. This is what scientists in China and Britain recently found, when they examined “exceptionally preserved fossil fauna” from South China. Botting et al. identified over 75 sponge species – unusual for post-extinction ecosystems. Normally, the hostile environment would allow only small and stunted organisms to survive and thrive – but not a flourishing, diverse kingdom of sponges. Nevertheless, the deposits from the Zhejiang Provence showed something else: they contained fossils of large and structurally complex sponges that were as diverse as modern communities. But why and how did the sponges recover so quickly and so well?
The authors assume that the ecological collapse increased one important factor: the sponges' food source. “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 increased enormously by the death and destruction all around them,” says lead author, Joe Botting, in a press release.
Interestingly, the abundance of sponges in a post-extinction world seems to be a recurring phenomenon. Botting et al. compared their findings with reports of other extinction events and found a high number of sponges in every fossil sample. The palaeontologists, thus, think that sponges play a crucial role after mass extinctions: they help the ecosystem to recover. Like an ecosystem engineer, sponges stabilise the sea floor surface with their skeletons, allowing sessile suspension feeders, such as brachiopods, corals and bryozoans to thrive again.
The past showed that after every mass extinction a few species survived and thrived. Unfortunately, there is evidence that we are now facing a sixth mass extinction. This extinction is different from the others because it is caused by only one species – the Homo sapiens, who recklessly exploits, pollutes and destroys the earth and its inhabitants. Biologists are optimistic that the looming disaster can still be stopped – if not, there will, at least, be again millions of sponges, flourishing on the sea floor.
Photo: NOAA Photo Library