Bristolia
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Bristolia Temporal range: late Lower Cambrian (Lower Olenellus-zone) 522–516 Ma |
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| Bristolia mohavensis, cephalon superimposed on a roled-up specimen with cephalon and thorax, pygidium hidden or absent | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | Trilobita |
| Order: | Redlichiida |
| Suborder: | Olenellina |
| Superfamily: | Olenelloidea |
| Family: | Biceratopsidae |
| Subfamily: | Bristoliinae |
| Genus: |
Bristolia Harrington, 1956 |
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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.
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.
Bristolia is most related to Fremontella halli and slightly further removed from Lochmanolenellus mexicana. These three genera together comprise the subfamily Bristoliinae. The sister group Biceratopsinae can be distinguished by their strongly effaced cephalic features. Basic to both these subfamilies are the two species of the genus Laudonia. In Laudonia the anterior cephalic border is developed as a flattened ledge, not as an elevated ridge as in the Bristolinae. Also the furrow (S3) between the front lobe (L4) and the bordering side lobes (L3) is deepest at midline, while in the Bristolinae the depth is the same in the middle as to the sides. Fremontella, Lochmanolenellus and Laudonia have shorter genal spines (comparable to 4-5 thorax segments) than Bristolia (8 segments). Lochmanolenellus and Laudonia both have intergenal spines, while Bristolia and Fremontella lack intergenal spines in adults.
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Wikipedia