Origin of the metazoans

Key points

• Relatively few basic body plans have appeared in the fossil record; most animals have a triploblastic architecture, with three fundamental body layers.

• Molecular data show there are three main groupings of animals: the deuterostomes (echinoderm-hemichordate-chordate group), the spiralians (mollusk-annelid-brachiopod-bryozoans-most flatworms-rotifers (platyzoans) group) and the ecdysozo-ans (arthropod-nematode-priapulid plus other taxa group). Together, the spiralians and ecdysozoans are usually called the protostomes.

• Five lines of evidence (body fossils, trace fossils, fossil embryos, the molecular clock and biomarkers) suggest that the metazoans had originated prior to the Ediacaran, 600 Ma.

• Snowball Earth by coincidence or design was a pivotal event in metazoan history; bila-terians evolved after the Marinoan glaciation.

• The first metazoans were probably similar to the demosponges, occurring first before the Ediacaran.

• The Ediacaran biota was a soft-bodied assemblage of organisms largely of uncertain affinities, reaching its acme during the Late Proterozoic, which may represent the earliest ecosystem dominated by large, multicellular organisms.

• The Tommotian or small shelly fauna was the first skeletalized assemblage of metazoans; this association of Early Cambrian microfossils contains a variety of phyla with shells or sclerites mainly composed of phosphatic material.

• The Cambrian explosion generated a range of new body plans during a relatively short time interval.

• The Ordovician radiation was marked by accelerations in diversification at the family, genus and species levels together with increased complexity in marine communities.

Consequently, if my theory be true, it is indisputable that before the lowest Silurian [Cambrian of modern usage] stratum was deposited, long periods elapsed, as long as, or probably far longer than, the whole interval from the Silurian age to the present day; and that during these vast, yet quite unknown, periods of time, the world swarmed with living creatures.

Charles Darwin (1859) On the Origin of Species

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