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of oxygen by the Proterozoic. About 2 billion years ago, when the oxygen-
absorbing banded iron formations had ceased being deposited, oxygen began
to replace carbon dioxide in the ocean and atmosphere.
About 1.5 billion years ago, the previously sparse geologic record of pre-
served cellular remains vastly improved as evolution suddenly sped up. However,
nearly another billion years elapsed before multicellular animals called meta-
zoans appeared in the fossil record. By then, the dissolved oxygen content of the
sea was about 5 to 10 percent of its present value. Furthermore, the increased
level of oxygen apparently sparked the evolution of many unique animals.
The triggering mechanisms for such a rapid evolutionary phase included
ecologic stress, geographic isolation caused by drifting continents, and climatic
changes. Organisms no longer relied entirely on surface absorption of oxygen.
Gills and circulatory systems evolved when oxygen levels reached about half
their present values near the end of the Proterozoic. Afterward, an explosion
of species created the progenitors of all life on Earth today (Table 4).
During the latter part of the Proterozoic, around 600 million years ago,
individual cells joined to form multicellular animals called metazoans.The meta-
zoans gave rise to ever more complex organisms, which were ancestral to all
47PROTEROZOIC METAZOANSTABLE 4 CLASSIFICATION OF SpeciesGroup Characteristics Geologic Age
Protozoans Single-celled animals: foraminifera and radiolarians, about Precambrian to recent
80,000 living species
Porifera Sponges: about 10,000 living species Proterozoic to recent
Coelenterates Tissues composed of three layers of cells: jellyfish, hydra, Cambrian to recent
coral;about 10,000 living species
Bryozoans Moss animals: about 3,000 living species Ordovician to recent
Brachiopods Two asymmetrical shells: about 260 living species Cambrian to recent
Molluska Straight, curled, or two symmetrical shells: snails, clams, Cambrian to recent
squids, ammonites; about 70,000 living species
Annelids Segmented body with well-developed internal organs: worms and Cambrian to recent
leeches;about 7,000 living species
Arthropods Largest phylum of living species with over 1 million known: Cambrian to recent
insects, spiders, shrimp, lobsters, crabs, trilobites
Echinoderms Bottom dwellers with radial symmetry: starfish, sea cucumbers, Cambrian to recent
sand dollars, crinoids; about 5,000 living species
Vertebrates Spinal column and internal skeleton: fish, amphibians, reptiles, Ordovician to recent
birds, mammals; about 70,000 living species