Metazoa are complex multicellular animals in which the cells are arranged in two layers in the embryonic gastrula stage. The Metazoa are extremely diverse and include 29 phyla, most of which are invertebrates. The phylum Chordata is an exception.
The Metazoa appeared about 620 million years ago and experienced a rapid explosion around the Precambrian-Cambrian boundary, probably associated with the formation and breakup of the supercontinents Rodinia and Gondwana and the rapidly changing environments associated with the supercontinent cycle. They evolved from eukaryotes, single-celled organisms with a nucleus, that appeared around 1,600 million years ago. Prokaryotes are older, probably extending back past 3,800 million years ago.
some of the oldest soft-bodied Metazoa are remarkably well preserved in the Ediacarian fauna from southeast Australia and other locations around the world. These fauna include a remarkable group of very unusual shallow marine forms, including some giants up to a meter in length. This explosion from simple, small single-celled organisms that existed on Earth for the previous three billion years (or more) is truly remarkable. The Ediacarian fauna (and related fauna, collectively called the Vendoza fauna) died off after the period between 620-550 million years ago, as these organisms show no affinity with modern invertebrates.
After the Ediacarian and Vendoza fauna died off, other marine invertebrates saw a remarkable explosion through the Cambrian. These organisms in the Cambrian included shelly fossils, trilobites, brachiopods, mollusks, archeocyathids, and echino-derms, and eventually in the Ordovician were joined by crinoids and bryozoans. Modern Metazoa include a variety of organisms including corals, gastropods, bivalves, and echinoids.
See also Cloud, Preston; evolution; fossil; Grabau, Amadeus William; historical geology; life's origins and early evolution; Phanerozoic.
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