Biology : The Time of Ancient Life : Part Two
The Cambrian Explosion
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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