In a Cambrian reef community, an Opabinia tries to catch some Marrella, while Hallucigenia explores the sponge maze.

The Cambrian was a very early period in the history of life from around 538 to 485 million years ago and was witness to a massive diversification of animal species compared to earlier periods. Different niches were explored, new survival strategies invented and complex body plans created. Nearly every major animal group (including the first vertrebrates) emerged during this time, although most of these early ancestors were small and "primitive". Predators arised, armed with claws, teeth and improved eyes, and forced their prey to adapt by becoming agile, armored or good at hiding, in a so-called "evolutionary arms race". Countless interesting fossils were found in the Burgess Shale in Canada, the Maotianshan Shales in China, the Sirius Passet in Greenland and others. The radiation event is also known as the "Cambrian Explosion" and took place over several million years. However, the validity of an "explosion" has been questioned recently, since more and more fossils from even older layers have been found and hint at quite some diversity before the "Cambrian Explosion".

Opabinia is a perfect example for the weirdness of the Cambrian fauna. It was an arthropod (group that includes insects, crustaceans, millipedes and others), but probably not really related to any of these. It was only 7 cm (2.8 in) long and probably a good swimmer. With its proboscis ending in a claw-like structure, it could grab prey and put it to its mouth which was positioned below its head. Especially the five eyes make Opbiania look really alien-like.

Marrella was a free-swimming arthropod as well and only 2.5 cm (1 in) long. It is the most common animal from the Burgess Shale.

Hallucigenia was another strange-looking creature, whose relationship is still unclear. It was around 5 cm (2 in) long and had 7 pairs of spines on its back, which were initially thought to be its legs.

Other animals on this picture include sponges (Corals existed, but were rare and didn't form reefs yet) and brachiopods (which look like clams but are not related).

Digital Painting by 'Paleo Pete' Peter Nickolaus, 2023. E-Mail: contact.paleopete@gmail.com