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Bristolia
''Bristolia'' is an extinct genus of trilobite, fossil marine arthropods, with eight or more small to average size species. It is common in and limited to the Lower Cambrian (Upper ''Olenellus''-zone) shelf deposits across the southwestern US, which constitutes part of the former paleocontinent of Laurentia.
== Taxonomy ==
''Bristolia'' can be separated into two distinct groups: one consisting of ''B. insolens'' and ''B. anteros'',〔 the other comprising a gradual spectrum of morphologies including ''B. mohavensis'', ''B. harringtoni'', and ''B. bristolensis'' morphotypes.
The second group reveals a dynamic morphological trend. From the oldest species ''B. mohavensis'', the lineage undergoes gradational increase in intergenal angle and advancement of the genal spines, progressing through ''B. harringtoni'', culminating in ''B. bristolensis''. Younger specimens show a trend back to more acute intergenal angles and less advanced genal spines typical of ''B. fragilis''.〔 This development reflects an initial deepening of the water, followed by a reversal to increasingly shallower water. ''Bristolia insolens'' represents an extreme extrapolation of the earlier trend and is restricted to a narrow stratigraphic interval at maximal flooding.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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