The Proterozoic Eon is also called the Cryptozoic ("age of hidden life"). About 2.5 billion years ago, enough shield rock had formed to start recognizable geologic processes such as plate tectonics. Geology was about to be joined by biology to continue Earth’s progress from a molten hell to a living planet. It is generally accepted that different types of prokaryotic organisms formed symbiotic relationships. Some types, more efficient at converting energy, were engulfed by larger protective “bubbles” able to shield them from the harsh environment. As time went on the symbiotic relationship became permanent and the “energy conversion” components became the chloroplasts and mitochondria of the first eukaryotic cells. Microfossils of these early cells are called Acritarchs.
About 1.2 billion years ago, plate tectonics forced the available shield rock to collide, forming Rodinia (a Russian term meaning “mother land”), Earth’s first super continent. Rodinia’s coastal waters were filled with rounded colonies of photosynthetic algae known as stromatolites. Photosynthesis began to add oxygen to the atmosphere, putting pressure on organisms adapted to the reduction atmosphere of the early Earth.
After a brief ice age in the mid-Proterozoic, organisms underwent rapid differentiation. The Ediacaran Period , the last of the Proterozoic Era, saw the first multicellular organisms. Autotrophs and soft-bodied heterotrophs filled the continental shelf regions around Rodinia. Many were Cnidarians similar to small jellyfish with radial body symmetry and specialized cells to sting prey and convey it into the body cavity. Fossils show that significantly different populations inhabited different localities. Some benthic (sea bottom-dwelling) organisms used a muscular “foot” to cling to the ocean bottom similar to the modern sea pen. Kimbrella fossils show a clear anterior/posterior axis, bilateral body symmetry and some indications they could crawl. Some scientists classify them as being related to the mollusks.
The boundary between the Ediacaran Period of the Proterozoic Era and the Cambrian Period of the Paleozoic Era is not as clear-cut as it was once thought to be. It used to be thought that increasing oxygenation caused a mass extinction of Ediacaran forms and a geologically sudden proliferation of new complex forms. Now it is understood that there were many complex multicellular animals capable of living in the higher oxygen of the Ediacaran environment. However they were nearly all soft-bodied forms, which left few fossil traces behind for us to find. The relative abundance of Cambrian fossils represents an increase in animals with calcified body parts, which were easily fossilized, not a mass extinction of Ediacaran life as was once thought.
About 1.2 billion years ago, plate tectonics forced the available shield rock to collide, forming Rodinia (a Russian term meaning “mother land”), Earth’s first super continent. Rodinia’s coastal waters were filled with rounded colonies of photosynthetic algae known as stromatolites. Photosynthesis began to add oxygen to the atmosphere, putting pressure on organisms adapted to the reduction atmosphere of the early Earth.
After a brief ice age in the mid-Proterozoic, organisms underwent rapid differentiation. The Ediacaran Period , the last of the Proterozoic Era, saw the first multicellular organisms. Autotrophs and soft-bodied heterotrophs filled the continental shelf regions around Rodinia. Many were Cnidarians similar to small jellyfish with radial body symmetry and specialized cells to sting prey and convey it into the body cavity. Fossils show that significantly different populations inhabited different localities. Some benthic (sea bottom-dwelling) organisms used a muscular “foot” to cling to the ocean bottom similar to the modern sea pen. Kimbrella fossils show a clear anterior/posterior axis, bilateral body symmetry and some indications they could crawl. Some scientists classify them as being related to the mollusks.
The boundary between the Ediacaran Period of the Proterozoic Era and the Cambrian Period of the Paleozoic Era is not as clear-cut as it was once thought to be. It used to be thought that increasing oxygenation caused a mass extinction of Ediacaran forms and a geologically sudden proliferation of new complex forms. Now it is understood that there were many complex multicellular animals capable of living in the higher oxygen of the Ediacaran environment. However they were nearly all soft-bodied forms, which left few fossil traces behind for us to find. The relative abundance of Cambrian fossils represents an increase in animals with calcified body parts, which were easily fossilized, not a mass extinction of Ediacaran life as was once thought.
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