New complexity at the dawn of modern life
Samples recovered from arid hillsides in Southern Africa show the earliest evidence of animals forming reefs — geological structures built by dead organisms, much like the coral reefs of today. The reefs are the fossil remains of the shells of an aggregating animal called Cloudina that lived here about 550 million years ago.
It’s difficult to identify Cloudina’s closest living relatives, says Rachel Wood, a professor of geoscience at the University of Edinburg, Scotland, and senior author of a Science paper published today. “Our best guess is the Cnidarians, group that is commonly represented in modern world by coral, jellyfish and sea anemones.”
The reef is one to two meters wide, and less than half a meter high. It formed about 10 million years before one of biology’s most momentous events: The Cambrian explosion, an orgy of organic creativity when multi-cellular organisms profoundly diversified to form most of the body plans seen today — and plenty of others that have since gone extinct.
Cloudina was simple, at least judging from the fossilized remains of their shells. Big question: What did gathering together do for them? “We think aggregation allowed Cloudina to clump together in such a way that they are supported in the current, which possibly implies filter feeding,” says Amelia Penny of the University of Edinburg, first author of the new study. As the phrase implies, filtering food from the water is a common tactic among living crustaceans.
View Metazoan fossil reef complex near Rietoog, Namibia in a larger map
When life becomes geology
“Reefs” are nothing new. Starting 3 billion years ago, microbes began clumping into pedestal-shaped structures called stromatolites. But the new finding represents the oldest reef created by animals with a skeleton, in this case, an external skeleton, AKA a shell. “People have been trying to understand how this early skeletal animal grew,” says Wood. “Previous studies have suggested that it was rooted in microbes, was sticking out of a stromatolite, so if you swam over the sea floor, you would see this round, green stromatolite made of algae and bacteria, with the primitive tubes of Cloudina sticking out.”
Instead, the Cloudina grew into a large, self-sustaining structure, Wood says. “Our finding was a very large example of Cloudina growing in a thicket, not relying on microbes at all to support them. It may have started on a microbial surface and then grew, twisting and cementing into a freestanding thicket.”
In the Cloudina reefs, Wood says, “Parents are producing larvae, and the offspring are settling on the parents’ backs.” (We parents at The Why Files were tempted to make some hay with this observation. Then we had a rare attack of common sense…)
The heap of Cloudina forms a “clonal” assembly, Wood says. “It’s like a modern coral, with thousands of sea anemones each in a calcareous cup, all genetically identical. All these individuals are [physically] connected to each other. It’s not like a colony of ants or termites, which are made of individuals that are genetically related but not identical.”
The reef rose above the ocean floor, as evidenced by the absence of the sand grains commonly found in ocean bottom water.
By cementing themselves to their neighbors, the individual Cloudina presumably got several benefits:
* The reef was stronger reef and more resistant to water currents;
* The large clumps protected insiders from predators. “A reef is something that modern animals use as a strategy to avoid being killed,” Penny told us. “It can be damaged quite significantly by predators without total mortality,” because the damage largely affects individuals near the surface.
* The structure channeled flowing water toward the Cloudina’s intake tubes, helping them eat microbes and plant material in the water.
One last question
Finding unexpected complexity 10 million years before the Cambrian explosion got us to wondering. Was the Explosion, when dozens of different body plans emerged in virtually an eyeblink, dated incorrectly? No, the new study’s authors are not trying to adjust the timing of a hallowed moment of biology.
“We are not saying the Cambrian explosion has been wrongly dated,” says Wood. “It is unparalleled in the fossil record, in the different animal lineages diversifying and appearing in the fossil record for the first time.” The assembly of animals dates to the Ediacaran period, which preceded the Cambrian. “The Ediacaran is nowhere as diverse as the Cambrian,” she adds. “We are just saying we have found some unexpected ecological diversity in the earliest skeletal community, which was thought to be very simple.”
– David J. Tenenbaum
Kevin Barrett, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David J. Tenenbaum, feature writer
- Ediacaran metazoan reefs from the Nama Group, Namibia, by Amelia M. Penny et al, Science 27 June 2014. ↩