Bristolia is an extinct genus of trilobite, fossil marine arthropods, with eight or more small to average size species.[1] 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.[2]
Bristolia can be separated into two distinct groups: one consisting of B. insolens and B. anteros,[1] 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.[2] 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.[3]
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.
The genus Bristolia is derived from the species name of Mesonacis bristolensis, that was elevated to a separate genus.[4] The names of the species have the following derivations.
As with most early trilobites, Bristolia has an almost flat exoskeleton, that is only thinly calcified, and has crescent-shaped eye ridges. As part of the Olenellina suborder, Bristolia lacks dorsal sutures. Like all other members of the Olenelloidea superfamily, the eye-ridges spring from the back of the frontal lobe (L4) of the central area of the cephalon, that is called glabella. The headshield (or cephalon) of Bristolia carries conspicuous and curved spines (called genal spines) of approximately 8 thorax segments long (measured parallel to the midline). The genal spines are attached in front of the back of the headshield. The central raised portion that represents the axis in the cephalon (or glabella) touches the elevated ridge that borders the cephalon. The furrows that separate border, eye ridges, glabella and its lobes are distinct (unlike in the Biceratopsinae). The area outside of the axis (or pleural lobes) of the third segment of the thorax are greatly enlarged, and carrying large trailing spines. These extend further back than the rest of the body except for the axial spine that is carried by the most backward (15th) prothorax segment. The pleural lobes of the 1st, 2nd, 4th and 5th are consequently triangular in shape and edge forward and backward respectively, and do not carry spines. Prothorax segments further back carry slender spines that angle backwards. When the opistothorax is known it has at least 17 segments.
| 1 | Genal spines at their base directed outward and forward, before gradually bending more and more backwards. → 2 | |
|---|---|---|
| - | Genal spines at their base directed outward perpendicular to the midline or outward and backward, before gradually bending further backwards. → 3 | |
| 2 | Shape of the cephalon semi-circular, the genal spines attach approximately in front of the eyes. Eye ridges shaped like a pair of brackets, the tangent between the base and termination pointing backwards and slightly (10°) outwards. Small at adulthood (cephalon less than 1 cm measured along the midline). Lower half of the Bristolia insolens-zone. Picture. → Bristolia insolens | |
| - | Shape of the cephalon rectangular, the genal spines attached at the frontal "corners". Eye ridges shaped like a pair of strongly curved hooks (with a broad base and a narrow termination), the tangent between the base and termination pointing backwards and significantly (40°) outwards. Upper half of the Bristolia insolens-zone to the lower half of the Peachella iddingsi-zone. Picture → Bristolia anteros | |
| 3 | The angle the genal spine makes at its base compared to the midline is 50° or more. The border between the genal spine and the corner in the back edge of the cephalon (or intergenal angle) compared to the midline is 145° or more. → 4 | |
| - | The angle the genal spine makes at its base compared to the midline is 45° or less. The border between the genal spine and the corner in the back edge of the cephalon (or intergenal angle) compared to the midline is 135° or less. → 6 | |
| 4 | The angle the genal spine makes at its base compared to the midline is more than 65°. The border between the genal spine and the corner in the back edge of the cephalon (or intergenal angle) compared to the midline is 165° or more. → 5 | |
| - | The angle the genal spine makes at its base compared to the midline is between 50° and 60°. The border between the genal spine and the corner in the back edge of the cephalon (or intergenal angle) compared to the midline is between 145° and 160°. Middle part of the Bristolia mohavensis-zone. → Bristolia harringtoni | |
| 5 | The frontal lobe of the glabella (L4) is close to the anterior border, but does not touch it. The glabella between the outer points of the furrow between the second and third pairs of side lobes (S2) is about ¾× as wide as most backward lobe (L0). → Bristolia kurtzi | |
| - | L4 touches the anterior border. S2 is about ⅔× as wide as most backward lobe (L0). Upper half of the Bristolia mohavensis-zone to the lower half of the Bristolia insolens-zone. Picture. → Bristolia bristolensis | |
| 6 | The furrow between the second and third pairs of side lobes (S2) is convex (further back on the midline than to the sides of the glabella). Lower half of the Bristolia mohavensis-zone. Picture http://www.backtothepast.com.mx/gallerydosa.php?lang=eng#. → Bristolia mohavensis | |
| - | The furrow between the second and third pairs of side lobes (S2) is transverse. Upper half of the Bristolia insolens-zone to the lower half of the Bolbolenellus euryparia-zone. Picture. → Bristolia fragilis |
Bristolia was probably marine bottom dweller, like all Olenellina.
