Grzimek's Animal Life Encyclopedia: Lower Metazoans and Lesser Deuterostomes Vol 1 by Michael Hutchins
English | 26 Mar. 2003 | ISBN: 0787657778 | 560 Pages | PDF | 48.95 MB
The Metazoa, today taken as a synonym of Animalia, comprises a large grouping of organisms that may be characterized as being multicellular and heterotrophic, i.e., they do not synthesize their own food, but obtain it from external sources.
While there has been some debate in the past, it now seems overwhelmingly likely that the metazoans are monophyletic, and thus that all living examples are de scended from an animal that lived some time in the Proterozoic (probably at least 600 million years ago [mya]). Even this statement is controversial, however. Molecular evidence suggests that metazoans had emerged up to one billion years ago or more; but the fossil record is most reasonably read as implying a much later origin, with definitive metazoans not appearing before 600 mya, and perhaps even later. Unfortunately, most "lower" metazoans today lack substantial hard parts such as mineralized shells, so their fossil record is correspondingly very poor. Telling the true time of appearance of such animals just from the fossil record may therefore well
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