During the Tommotian Age (530-521 mya), named after rock exposures in Siberia, a whole range of small shelly fossils (SSF) appeared. Most of the fossils are less than a centimetre across and include a wide range of shells and skeletal pieces of calcium carbonate, calcium phosphate, or silicon dioxide.
This, the first major radiation of animals, saw the first appearance of the molluscs, which include gastropods (‘stomach foot’: slugs, snails), bivalves (clams, mussels) and cephalopods (‘head foot’: nautili, octopuses, squid). Archaeocyathids (‘ancient cups’), probably a kind of sponge, quickly diversified and became the planet’s first reef-builders.
During the Atdabanian Age (521-514 mya) named after Siberian outcrops, there was a much larger variety of marine life. The arthropod trilobite was dominant during the Palaeozoic until its extinction at the end of the Permian. The arthropods include crustaceans (crabs, shrimp); insects (cockroaches, grasshoppers); chilopods (centipedes) and diplopods (millipedes). There were also numerous echinoderms.
During the Palaeozoic the brachiopod (‘arm foot’) also was common. It looks similar to the bivalve but most brachiopods are attached to the substrate by means of a fleshy ‘stalk’, whereas most bivalves are free-moving, usually by means of a muscular ‘foot’. During the Mesozoic the bivalves largely replaced the brachiopods and surviving orders are today found only in environments of extreme cold and depth.
The Burgess Shale (about 508 mya) is an exceptional mid-Cambrian fossil locality in the Canadian Rockies in British Columbia. Named after the Burgess Pass, it was discovered in 1909 by Charles Doolittle Walcott (1850-1927). The Burgess fauna lived on banks of mud in shallow waters directly under a reef wall built by algae. Occasional mudslides swept these animals into a deeper basin where they were buried in mud and died; the lack of oxygen prevented them from decay and their soft parts were left intact.
Most fossil sites of this age yield only animals with hard skeletons, such as the trilobites and brachiopods. The Burgess site is important because it is a rare taphonomic (the study of decaying organisms over time) snapshot of the diversity of ancient life. If it were not the Burgess Shale our view of the early evolution of animals would have been slanted towards the groups that had hard parts.
About forty percent of the Burgess fossils are arthropods, but only a fraction of these are trilobites. Of the rest some are on recognised routes – echinoderms, molluscs, sponges. Others seem to have been destroyed before they could develop further. Many of the creatures were bizarre and had only the sketchiest resemblance to known animals. The final part of the Cambrian is called the Furongian Age (497-485 mya), a name derived from the Chinese word Furong (‘lotus’) and referring to the Hunan Province, known as the ‘Lotus State’. During this time the trilobites proliferated and the molluscs became more diverse. New groups such as graptolites (these fossils resemble hieroglyphs written on rock) and conodonts (‘cone teeth’: an extinct eel-like marine animal that may have been a primitive vertebrate) appeared.
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