Scientists have long thought of the Cambrian Explosion 541 million years ago as the flowering of complex life on Earth. Strangely shaped, large soft-bodied organisms were known to have lived in the period just prior -- the Ediacaran -- but they made few hard parts and scientists have debated whether any or how many were ancestors of the life that followed.
Now a team of scientists from the United Kingdom and Namibia who just published their results in Science last week say that the Namibian fossil beds demonstrate that some animals had already acquired the habit of building large, fortified, complex reefs by 548 million years ago, at least seven million years before the Cambrian. Since scientists had previously believed that reef-building was a response to the increased crowding and predation produced by the explosion of diversity in the Cambrian, these results, if true, imply that these forces had been building and operating long before life's big bang.
Here is one imagining of what these reefs may have looked like:
Here is a closeup, with a cross-sectional view near the front.
As you can see, Cloudina, the reef-former at center, appears to be a collection of stacked funnels that form a "cone-in-cone" structure. Some have suggested that the animal that lived inside those cones was a relative of modern corals.
To the right are alien-looking Namacalathus, whose skeleton resembled a lollipop with portholes. There's no telling what the animal that lived inside that thing looked like.
Reefs are aggregations of animal skeletons superglued together by the reef-forming organisms into one large structure. Until now, Cloudina fossils had been interpreted as laying on the seabed in large filter-feeding bunches, attached to their substrate at the pointy end but separate from each other. But in the fossils the team examined, the funnels of Cloudina appeared to be cemented together -- elevating them to reef status -- by little wedges of the mineral calcite (CaCO3, though in life it may have been another form of calcium carbonate called aragonite).
Here are the putative connections in cross section. The Cloudina tubes look like fried eggs or danishes, while the cement connections are indicated at the tips of the arrows.
The scientists believe these connections between individuals existed prior to fossilization because minerals called cement botryoids formed during the process of fossilization grew on their surface. The connections between Cloudina tubes had been noted before, but were interpreted as inorganic chemical precipitation. But because these features seem to have been present before fossilization, the authors think it is more likely that they were the product of active growth by Cloudina. They may have been similar to structures seen in worm snails or biomineralizing polychaete worms today.
Because the scientists didn't find much sand or silt between the tubes of fossilized Cloudina, they also believe they were elevated above the seafloor. Within the fossil matrix, the tubes all point the same direction, suggesting a filter-feeding habit in which all the animals faced into the prevailing current to maximize their meals.
Reefs like Cloudina probably appeared because reef builders gained a cooperative advantage not only from their new fortified castles, but also by their elevation into waters further from the sea floor carrying more drifting edibles thanks to stronger currents and fewer competitors. Plain old competition for space probably also played a part; as any New Yorker knows, if you can't build out ... build up. Clustering into aggregates or reefs whose members are glued together also helps produce yet more substrate for young organisms to grow in a nurturing environment and helps protect the herd from strong currents and storms.
But reefs were an important innovation in the evolution of life because they changed the game for everyone, not just for the reef builders. They altered the shape of the seabed, creating many new niches in their nooks and crannies and changing the flow of water. This probably spurred the evolution of many other new forms of life.
Scientists have long supposed that external skeletons evolved specifically in response to massively increased predation during the Cambrian Explosion by the ancestors of life alive today (the bilaterians, or animals with left and right sides). But fossils like Cloudina -- and several other Ediacarans that show evidence of hard skeletons -- belie this assumption. They demonstrate that predation was already important in the run-up to the Cambrian. In fact, fossils of Cloudina from China even show tiny boreholes where something -- no doubt something terribly exciting, whose identity we may never know -- successfully drilled into their shells to drain the contents. You can see these same sort of boreholes in seashells today, produced by predatory snails. As there were no snails in the Ediacaran, we must remain tantalized by the Cloudina borer's identity and appearance.
Cloudina's own appearance may also have been a response to such predation. The irregular thickness of its skeleton may have frustrated the Unknown Borer, making it difficult to evolve drilling equipment that could reliably perform a shell-breach. The holes, when found, are also a relatively consistent distance away from the openings of the funnels, further implying that whatever it was that was attempting the break-in knew enough to stay well away from whatever it was that Cloudina could do to it from the mouth of its funnel. One would imagine that Cloudina had a crown of tentacles like today's corals for whacking pesky borers, but the truth could well have been something so wildly different and surprising from that we would be shocked. Deep time is frustrating like that. No one would ever have predicted the appearance of the Ediacaran fauna from the subsequent 541 million years of fossils, and yet ... there it is.
Until this discovery, the oldest animal reefs were thought to be made by archaeocyath sponges in the early Cambrian 530 million years ago. If supported, this discovery would push animal reef building back another 18 million years. Ediacaran reefs had been thought to be confined to simple bacterial constructions like the stony mounds called stromatolites or thrombolites. Cloudina, together with oddball Namacalathus and a crust-forming, fissure-dwelling animal called Namapoikia, paint a far more complex picture with an almost modern (although admittedly not quite yet shipwreck-worthy) appearance. And since the fossil record is spotty at best, it's likely the true diversity of reefs on Ediacaran Earth exceeds even what we are seeing -- or can imagine.
Penny A.M., Wood R., Curtis A., Bowyer F., Tostevin R. & Hoffman K.H. Early animals. Ediacaran metazoan reefs from the Nama Group, Namibia., Science (New York, N.Y.), PMID: http://www.ncbi.nlm.nih.gov/pubmed/24970084