Trilobites
These fossils (a–d) belong to trilobites, extinct arthropods that appeared in the early Cambrian period 525 million years ago and disappeared from the fossil record during a mass extinction at the end of the Permian period about 250 million years ago.
Source
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"OpenStax College, Biology. November 14, 2013."
http://cnx.org/content/m44660/latest/?collection=col11448/latest
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