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Graptolithina
Graptolithina or graptolites belong to the phylum of the “Hemichordata”. They occur in the fossil record, since the middle Cambrian and became extinct at the beginning of the Carboniferous period. Many graptolites had a planktonic lifestyle, they floated in the upper waters of the oceans. Their exoskeleton (external skeleton) consisted of organic material (chitin) instead of calcium carbonate. Graptolites formed giant colonies by asexual reproduction. When they died, their skeletons sank to the bottom and in rare case, they were only preserved in very fine, and laminated clay, These clays, were formed in anoxic (oxygen low) environments. They are found as fossils in black shales.
Graptolites are common for (deep-sea) sediments from the Ordovician and Silurian periods, where they are also used as index fossils.
Example of a graptolite.
Photos or locations for Hemichordata on this site
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