The Lower Cambrian fauna is so characteristic that it's usually referred to as "small shelly fauna" and is used to define the Cambrian's only stage, the Tommotian. According to newer research results, the sudden appearance of shelly organisms derives from a critical oxygen level in the earth's atmosphere (approx. 1% of today's level), enabling animal life for the first time, accompanied by a lowering in CO2 concentration at the same time. The use of oxygen for respiration produces 13 times the energy that bacteria and single-celled organisms gain from the use of the same amount of lactic acid for respiration. Only this fact enabled the energy-consuming formation of hard body parts. The CO2 lowering made it easier for some organisms to shift to the production of carbonate exoskeletons, as there was no danger any more that they might have been dissolved by the aggressive sea water.
During the entire Cambrian, trilobites were widely abundant and make up great index fossils.
(ages according to FAUPL 2000)
Mobergella holsti
This animal with a cap-shaped phosphatized shell is believed to be the ancestor of the molluscs.
Tommotian, Daenisch Nienhof
Cruziana isp.
Trilobite feeding traces can mainly be found in clastic, marginal shallow marine environments - a facies that often prevented the preservation of hard body parts of the trace makers. It's possible to distinguish a cambrian and an ordovician-silurian Cruziana type. Unlike the pictured cambrian Cruziana type, the ordovician one shows distinct feet imprints as well as imprints of genal angles or spines.
Middle Cambrian, Utah
The casts have probably been caused by protomeduses that packed sand into their body cavities to achieve a more stable position on the sediment surface. In contrast to the generally round and sculpture-less casts from the Precambrian, now also rectangular and star-shaped fossils occur.
The cast on the lower right shows a clear division into 4 areas separated by deep incisions, and a central hollow. This is an evidence for a more complex differentiation of the soft body.
Cambrian, Aachen