Biology : Semester II


IntroductionSection 1 | Section 2 | Section 3 | Section 4 | Section 5

  Section Three:

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8 | Part 9 | Part 10 | Part 11 | Part 12 | Part 13 | Part 14

Biology : The Time of Ancient Life : Part Two

The Cambrian Explosion

The Cambrian period, the first in the Paleozoic era, spans the time between 544 and 505 million years ago. During what is termed the Cambrian Explosion, living things developed hard structures, such as shells and exoskeletons. The presence of these structures greatly enhanced the chance of an organism becoming a fossil. Prior to the Cambrian, fossils were quite rare and inconspicuous since the organisms were small. This scarcity of the precambrian fossil record is also a consequence of the soft-bodied forms of life needing exceptionally rare circumstances to form fossils.

The sudden appearance of relatively complex fossils in the Cambrian was a troubling issue to Charles Darwin. His ideas on evolution included descent with modification and the slow, gradual development of more complex creatures from simpler ones. The lack of a precambrian fossil record was compounded by the wild diversity seen to suddenly appear during the Cambrian. Darwin's dilemma would not be solved until the 1960s when scientists got a better handle on the actual diversity of precambrian life.

The early Cambrian was a wild time for animal evolution. During a time as short as 5 million years numerous body plans developed, including the three animal body plans that exist today. All major animal phyla, except the Bryozoans, have their first appearance during the Cambrian. One of the dominant animal groups that appeared was the trilobites. These now-extinct members of the Phylum Arthropoda became the dominant animals in many Cambrian marine environments. Another major change was the evolution and spread of the archaeocyathids, a group of sponges (Phylum Porifera) that became extinct before the end of the Cambrian. Archaeocyathids were important reef-building organisms, along with some calcareous algae. Some scientists speculate that archaeocyathids existed symbiotically with cyanobacteria, much the was modern coral do with zooxanthellae (symbiotic dinoflagellates).

Archaeocyathids, an extinct group of sponges, dominated the reefs of the Cambrian seas. The image on the left is a cross-section of the animal, while the right image shows the external morphology of several of these cone-shaped animals.

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