n the early Cambrian, Earth was generally cold but was gradually warming as the glaciers of the late Proterozoic Eon receded. Tectonic evidence suggests that the single supercontinent Rodinia broke apart and by the early to mid-Cambrian there were two continents. Gondwana, near the South Pole, was a supercontinent that later formed much of the land area of modern Africa, Australia, South America, Antarctica and parts of Asia. Laurentia, nearer the equator, was composed of landmasses that currently make up much of North America and part of Europe. Increased coastal area and flooding due to glacial retreat created more shallow sea environments.
A fossilized <i>Spartobranchus tenuis</i> from the Burgess shale in Canada. The animal contains features of modern acorn worms and modern tube worms called pterobranches.
A fossilized Spartobranchus tenuis from the Burgess shale in Canada. The animal contains features of modern acorn worms and modern tube worms called pterobranches.
Credit: JB Caron
At this point, no life yet existed on land; all life was aquatic. Very early in the Cambrian the sea floor was covered by a “mat” of microbial life above a thick layer of oxygen-free mud. The first multicellular life forms had evolved in the late Proterozoic to “graze” on the microbes. These multicellular organisms were the first to show evidence of a bilateral body plan. These near-microscopic “worms” began to burrow, mixing and oxygenating the mud of the ocean floor. During this time, dissolved oxygen was increasing in the water because of the presence of cyanobacteria. The first animals to develop calcium carbonate exoskeletons built coral reefs. [Image Gallery: Cambrian Creatures: Primitive Sea Life]
The middle of the Cambrian Period began with an extinction event. Many of the reef-building organisms died out, as well as the most primitive trilobites. One hypothesis suggests that this was due to a temporary depletion of oxygen caused by an upwelling of cooler water from deep ocean areas. This upwelling eventually resulted in a variety of marine environments ranging from the deep ocean to the shallow coastal zones. Scientists hypothesize that this increase in available ecological niches set the stage for the abrupt radiation in life forms commonly called the “Cambrian Explosion.”
A fossilized <i>Spartobranchus tenuis</i> from the Burgess shale in Canada. The animal contains features of modern acorn worms and modern tube worms called pterobranches.
A fossilized Spartobranchus tenuis from the Burgess shale in Canada. The animal contains features of modern acorn worms and modern tube worms called pterobranches.
Credit: JB Caron
At this point, no life yet existed on land; all life was aquatic. Very early in the Cambrian the sea floor was covered by a “mat” of microbial life above a thick layer of oxygen-free mud. The first multicellular life forms had evolved in the late Proterozoic to “graze” on the microbes. These multicellular organisms were the first to show evidence of a bilateral body plan. These near-microscopic “worms” began to burrow, mixing and oxygenating the mud of the ocean floor. During this time, dissolved oxygen was increasing in the water because of the presence of cyanobacteria. The first animals to develop calcium carbonate exoskeletons built coral reefs. [Image Gallery: Cambrian Creatures: Primitive Sea Life]
The middle of the Cambrian Period began with an extinction event. Many of the reef-building organisms died out, as well as the most primitive trilobites. One hypothesis suggests that this was due to a temporary depletion of oxygen caused by an upwelling of cooler water from deep ocean areas. This upwelling eventually resulted in a variety of marine environments ranging from the deep ocean to the shallow coastal zones. Scientists hypothesize that this increase in available ecological niches set the stage for the abrupt radiation in life forms commonly called the “Cambrian Explosion.”
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