Cambrian Explosion

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Cambrian, 543 to 520 million years ago. (The entire Cambrian period ranged from 543 to approximately 490 million years ago.) First and fore­most, the Cambrian explosion is marked by the global appearance of or­ganisms with skeletal hard parts in the fossil record in contrast to the strictly soft-bodied creatures prior to this. Initially, these skeletal structures were simple in design, such as minute cylindrical tubes, tiny cones, and rudi­mentary jaw like appendages. However, they evolved rapidly into larger and more elaborate structures comparable to the exoskeletons of many living invertebrate groups. These early skeletons were constructed from a diverse array of materials that form the building blocks of skeletons to this day, in­cluding calcium carbonate, calcium phosphate, and silica.

Coincident with the appearance of skeletons was the phenomenal di­versification of metazoan life. Paleontologists Stephen Jay Gould in Won­derful Life (1989) and Simon Conway Morris in Crucible of Creation (1998) provide detailed, popular accounts of the amazing evolution of Cambrian animals, though they reach somewhat different conclusions with regard to the implications for the subsequent history of life.

However, all paleontologists agree that virtually all of the modern in­vertebrate groups made their first definitive appearance in the early Cam­brian, including phylum Annelida (worms), phylum Mollusca (clams, snails, cephalopods), phylum Echinodermata (starfish, urchins, sea lilies), phylum Arthropoda (trilobites, crabs, lobsters, insects), and phylum Brachiopoda (lamp shells). Along with these familiar groups came more obscure animals such as the Archaeocyatha, which are an interesting assemblage of sponge­like fossils that presumably led a quiet existence on the Cambrian seafloor filtering food particles from the water column. The Archaeocyatha became extinct by the middle Cambrian. A significant geological phenomenon as­sociated with this great diversification of metazoan life is the enhanced record of bioturbation (the mixing of sediments by organisms seeking food and/or shelter) in rocks of Cambrian age. Prior to the Cambrian, most ma­rine sediments were relatively undisturbed by animal activity.

Marine life from the Cambrian period. Virtually all of the modern invertebrate groups made their first definitive appearance in the early Cambrian period.

Many scientists correlate the abrupt appearance of skeletons and the burst of biotic evolution in the early Cambrian with chemical changes in the world ocean, specifically an increase in the concentration of oxygen. Many scientists also point to the evolutionary first appearance of predatory lifestyles, with organisms adapting to this new ecological pressure with the construction of protective skeletons and the selection of burrowing habits. (An animal residing beneath the sediment surface is far less likely to be preyed upon.) Still others suggest that the relatively wide-open Cambrian oceans were an ideal setting for large-scale evolutionary ex­perimentation and the origin of Phyla. Regardless of the driving mecha­nisms, the Cambrian explosion will forever remain one of the defining episodes in the history of life on Earth.

References

Conway Morris, Simon. The Crucible of Creation. Oxford: Oxford University Press, 1998.

Cowan, Richard. History of Life. Boston: Blackwell Science Inc., 2000.

Gould, Stephen J. Wonderful Life. New York: W. W. Norton and Company, 1989.