Pre-cambrian
Cambrian period
cambrian
Fossils of fish appeared approximately 500 million years ago during the Cambrian period. This period saw the rapid diversification of life forms in the oceans, including the development of early fish species.
The Ordovician period came after the Cambrian period.
Trilobites
Trilobite
The presumed animal fossils first appeared during the Archaean Cambrian period. There is no clear indication why they appeared this late into history.
Cambrian period
cambrian
Large numbers of fossils begin to appear around 530 million years ago. It is called the Cambrian Explosion, because it occurred in the Cambrian period. The Cambrian period was the first part of the Paleozoic era.
Fossils of fish appeared approximately 500 million years ago during the Cambrian period. This period saw the rapid diversification of life forms in the oceans, including the development of early fish species.
First of all, the Cambrian Explosion is designated as a huge diversification event taking place between 543 to 506 MYA, when most of the phyla extant (living) today appeared. So the first chordates, shelled organisms and arthropods were seen in the Cambrian period. Therefore, complex bilateria dominated the Cambrian scene.
Cambrian Period Actually the correct answer to this question is the "Pennsylvanian Period".
Trilobites are one of the index fossils commonly found in Cambrian rock layers. These ancient arthropods lived in the seas during the Cambrian period and are used by geologists as markers for dating rocks from that time period.
The sudden appearance of most of the major animal phyla within the fossils early in the Cambrian period. This caused a sudden diversification of life.
The Ordovician period came after the Cambrian period.
The first vertebrates evolved during the Cambrian Explosion about 525 million years ago. The Cambrian Explosion, obviously, occurred during the Cambrian period. The Cambrian period was part of the Paleozoic era.