Dating cambrian rocks
Some of these are important in demonstrating that the biota of the Burgess Shale is unusual only in preservation and not in composition. So far as is known only one well, or a group of wells in a small area, encounters these rock types, which normally are structurally high. The beginning of this remarkable adaptive radiation has been used to divide the history of life on Earth into two unequal eons.
This suggests that Cambrian sponges were much more common and more diverse than is indicated by the known fossil record. These rocks contain the earliest record of abundant and varied life-forms.
These trilobites are small, rarely exceeding a few millimetres in length, and they have only two thoracic segments. Fossils from the Burgess Shale have been used to demonstrate the presence of a complex community as diverse in habit, structure, and adaptation as many modern communities.
Other new Cambrian phyla largely lack biomineralization and have a poor fossil record. It was outfitted with a pair of giant flexible appendages that could grasp and move prey toward a peculiar mouth structure consisting of armoured plates arranged in a circular pattern. Included in the paleogeographic reconstruction are cold and warm ocean currents. Two prominent trends may be noted, one in the position of the Rush Rib on the southwestern side of the uplift and the other along the Russell Rib.
The second phase of the Precambrian-Cambrian biotic transition is characterized by a marked increase in the diversity of its shelly fauna and a lack of trilobites. The recognition of small shelly fossils before the first trilobites, and Ediacara biota substantially earlier, led to calls for a more precisely defined base to the Cambrian period.
Brachiopod shells are present in many Cambrian continental-shelf deposits. The association of this rock type and positive structural features strongly suggests a genetic relationship.
They also evolved a complex symbiotic relationship with reef-building blue-green algae. They mainly represent deposits of floodplains and windblown sand. The lower boundary of the Cambrian was originally held to represent the first appearance of complex life, represented by trilobites.
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