DEFINITION OF METAZOA

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 are

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 probably evolved from the 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 3 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 million and 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

echinoderms, and eventually in the Ordovician were joined

by crinoids and bryozoans. Modern Metazoa include corals,

gastropods, bivalves, and echinoids.

See also SUPERCONTINENT CYCLE; VENDIAN.

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