541 million years ago to 485 million years ago – Cambrian Period
After the Cambrian explosion, a variety of organisms flourished on the seabed, and some fauna are still active today.
Cambrian saw an explosion in the diversity of life on Earth and the birth of many major fauna, some of which are still active today. Most of them live in water, and many live in shallow seas, such as software animals, worm-like animals, and sponges.
Exactly what ignited the Cambrian's astonishing explosion of new life is unclear. Maybe it's because the oxygen levels in the atmosphere have increased, or because it's related to a warming climate. Many of these newly born creatures are hard-bodied arthropods, the ancestors of insects, spiders and crustaceans. Their hard bodies not only defend against attacks from other creatures to avoid being eaten as food, but also grow into larger sizes as body skeletons.
Anomalocaris (the largest predator in the Cambrian Period) were widely distributed in the Cambrian oceans, and fossils of it have been found in China, North America and Australia.
A novel group of arthropods called trilobites appeared during this period, and was one of the earliest animals with compound eyes (Anomalocaris). They are known for leaving behind a large number of fossils, ranging in length from only one centimeter to more than seventy centimeters.
Many of the trilobite samples were excavated from the Burgess Shale formation in British Columbia, Canada, and the local fossils are nearly perfectly preserved, and the fossil imprints include not only the hard parts of the insect's body, such as the shell and teeth, but also the muscles, gills and digestive system, allowing experts to get many important clues about the trilobite's lifestyle.
Because the fossils at this site preserve the soft parts of the insect's body, scientists can also study other invertebrates found here, such as the Crumillospongia, a large sac-shaped sponge that is closely related to the bath sponge that humans continue to collect to this day.
In the Cambrian ocean, one of the most feared hunters is the Anomalocaris. It has a hard shell and preys on trilobites and other arthropods, worms and software animals. These predators are constantly evolving, and so are their prey. Wind worms (Wiwaxia) that live on the ocean floor develop scales and spines to protect themselves.
The hallucigenia (a footed and extremely clawed animal), living in the ocean about 530 million years ago, paleontologists mistakenly thought that the two rows of thorns on its body were legs used for walking, and mistakenly used the legs used for walking as ornaments, believing that such a fantasy creature "can only be dreamed of by dreams", so it was named grotesque.
The hallucigenia, which is covered in thorns, has a similar trick - it stands up with seven pairs of feet and raises two rows of spines on its back; the grotesque worm is completely blind (some scholars believe it has eye spots), so it relies on the spines on its back as protection.