The first 20-million years of the Cambrian appears to be an empty void between the Ediacarans from the previous post and the well known Cambrians. But pull out the looking glass, and we find bustling animal life in this hole. Organisms that were first overlooked because of their size, but now they are recognized as important, and include possible precursors to the Cambrian fauna:
The small shelly fossils, “shellies” among friends, are a very diverse group of…small shelly fossils. They are usually below 2 mm in size, and composed of phosphate. Some look like mini-versions of known organisms, like the shells of brachiopods and molluscs, some even with spiral shells, and may also be their precursors.
Some are spikes or possible armour of larger organisms, like Microdictyon, which first was found just as a bunch of loose spikes. Their nature were revealed only after Burgess lagerstatten brought the to light the whole organism, where the spikes sat above the branch of each appendage. Many small shellies are still a mystery that awaits finding their place.
The shellies appear above the Cambrian border, and radiate out, becoming most diverse during early Cambrian; 529-621 m.a., and thus lived together with the Chengjiang and Sirius Passet faunas. Then, they gradually declined until they disappeared by the mid Cambrian. They are like snapshots, through small windows into what happened in the micro world in the late Precambrian and Early Cambrian.
The shellies are also known as the Tommotian fauna, after the Russian name for a part of the Cambrian of ca 529-521 m.a. It is based on a continuous sequence of Ediacaran to Cambrian rocks in eastern Siberia. The presence and preservation of the shellies coincide with a period of many phosphate deposits in the sedimentary record around the globe, so it seems that a special, phosphate rich sea water chemistry enabled them to exist and become fossils. When phosphate levels in the seas became lower, the shellies died out – or, at least, their fossils disappeared.
The small shelly fauna may have been a genetic playground for evolution, where it made the genetic recipes that would shoot off the Cambrian explosion.
Two of the most important of the small shellies are Cloudina and Namacalathus. They occur in the latest few million years of the Ediacaran. Not in the same rocks as the typical Ediacarans, but on stromatolite reefs.
Stromatolites are among the oldest proofs of life we know. Stromatolites are heaps, which grow on the seafloor when a film of slimy algae catch and cement tiny particles in the water. Layer on layer of algae and particles then build up a bump or stack, which could reach meter high. Stromatolites were “everywhere” in shallow water in the late Precambrian, but when trilobites and other arthropods developed teeth and started gnawing on them, they became much rarer from the Cambrian.
Stromatolites today mostly occur in shallow bays or lakes where the water is too salt for grazers to live. Most famous is the tidal bay Shark Bay in Australia. Stromatolites also occur in some fresh-water lakes. Open water stromatolites exist only one place, in Exumas Cays in the Bahamas.
Modern day stromatolites in Shark Bay, Australia.
(Paul Harrison, see page for license, via Wikimedia Commons)
Back in the Precambrian, stromatolites were as ubiquitous as blue mussels today. Stromatolites were the reefs. On these reefs grew some tiny tubes. These funnel-shaped tubes were 2-7 mm in diameter, stacked like a tiny palm trunk, and up to 15 cm in length. They are the first things we can reasonably call “animals”.
Cloudina are widespread around the world in late Ediacaran rocks. The funnel segments seem to fit into each other like a stack of buckets, suggesting that the tiny organism may have lived in the upper cup.
Cloudina fossil drawing. Note how the cups sit into each other, looking like a palm trunk.
(derivative work: Smith609 (talk)Cloudinadraw.png: w:User:Graeme Bartlett, CC BY-SA 3.0, via Wikimedia Commons)
Even more strange is Namacalathus. As the name suggests, it was first found in the Nama group in Namibia, but later has also been found in Oman, Canada, Siberia and Paraguay.
Namacalathus has a single, unsegmented stem, 1-2 mm in diameter and 1-2 cm high, with a rounded cup shaped “head”, with around six openings. The skeleton was probably calcite, and the animal sat in the cup.
Namacalathus; drawing showing the holes in the cup “head” at the top.
(Cetomedes, Public domain, via Wikimedia Commons)
The images reveal that Namacalathus lived inside the cup, and seem to have had a gut cavity, covered by a membrane that could be opened. I.e., a primitive stomach with a mouth. If the interpretation is correct, Namacalathus may be the ancestor of all Lophotrochozoa, a “sack group” comprising mollucs, brachiopods, worms and many other less known phyla.
What are we to make of all these fossils? On one hand, the Burgessian fossil material of the Cambrian is rich, almost information overload. But the holes below the Cambrian are frustrating, and the ancestors to Cambrian animals are fragmented. It feels like the development of animals is hidden behind a high wall, where we can only peek in through a few random narrow holes.
What does the this mean for evolution?
The Ediacarans and the small shelly fauna may have been a genetic playground for evolution, where it made the genetic recipes that would shoot off the Cambrian explosion. But it needed one more ingredient before animal life could shoot off, the energy to run the hardware. Once that energy was in place, the Cambrian explosion could shoot off.
In the last blog post in this series, we will look at that critical ingredient: Oxygen.