The Cambrian era saw the sudden appearance of representatives of all modern animal phyla, except bryozoans. The period is marked by the emergence of shield-shaped arthropods called trilobites and early sponges. During the period, a variety of invertebrate animals and some early vertebrates evolved. About half of the Cambrian fauna became extinct during the Cambrian-Ordovician extinction events at the end of the period.
Cambrian organisms lived during the Cambrian era, about 542 to 488 million years ago. The Cambrian era is famous for the Cambrian explosion, a sudden period of adaptive radiation during which representatives of all modern animal phyla (except bryozoans) appeared. This occurred about 530 million years ago and lasted for about 10 million years.
The beginning of the Cambrian period has traditionally been defined as the emergence of shield-shaped arthropods called trilobites and early sponges (and the first cliff-builders) called archaeocyathids (literally “ancient cup”). Its onset has since been redefined as an extinction event marking the Cambrian-Precambrian boundary. The timing of this event has been very precisely defined using the changes in carbon isotope ratios that occur during this period, which indicate a sharp decline in planetary biomass.
The Cambrian faunas were the first truly diverse faunas to evolve. The beginning of the Cambrian period is marked by the raised trilobites and archaeocyathids, as well as the small, slightly older shellfish fauna, the earliest known organisms with a hard shell, thought to be simple brachiopods. In the early Cambrian, distinctive fossil traces appear, including vertical burrows and other traces thought to have been left by the first animals to develop hydrostatic skeletons. A hydrostatic skeleton is not a hard skeleton at all, but rather a fluid-filled body cavity that is attached to and pulled in by surrounding muscles.
During the Cambrian Period, a variety of invertebrate animals and some early vertebrates evolved in the form of jawless fish. These include jellyfish, early crustaceans, sponges, Priapulida, the very common Marrella (relative of the trilobites), the tiny arachnomorph Yohoia, the soft trilobite Naraoia, and “strange wonders” including the bizarre five-eyed Opabinia, a vaguely crayfish-like arthropod with a clawed appendage extending from its mouth, the huge (by Cambrian standards) Anomalocaris, which was the first apex predator, the spiny velvet worm Hallucigenia, and the strange, ruler-like Odontogriphus.
About half of the Cambrian fauna became extinct during the Cambrian-Ordovician extinction events at the end of the period. Brachiopods, conodonts and trilobites have been heavily affected. Trilobites were never so numerous or diverse again.
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