Lab 5: Trilobites

• Use Textbook Chapter 14, specifically pages 250-256.
• Fortey, R., 2000. Trilobite; eyewitness to evolution, Chapter 9, Time, p. 218-254.

Great trilobite information on the web:

• More than you would ever like to know about trilobites, great photographs, great graphics, animation; pictorial guides for classification
• Nice, short run through information on morphology and ecology

The soft body parts of the trilobite were for a large part under the front shield (cephalon), with the size of the stomach comparable to the width of the glabella (median ridge). A broad glabella indicates a large stomach (few, large meals), i.e., a scavenger or predator. A narrow glabella suggests many small meals, such as in a detritivore (mud or particle eater). Large eyes mean good vision; small/no eyes: life in dark, or floating as plankton (no swimming, no directionality).

Look at the following trilobites and try to recognize the body parts indicative of a specific lifestyle. Where applicable, try to see where the cephalon would have split in molting:

1. Agnostus: middle Cambrium, text book p. 252, fig. 14.6.A (use hand lens). Note absence of eyes; only two segment. Planktonic life style interpreted usually. There is disagreement because other trilobites with an inferred planktonic lifestyle have very large eyes. It has been suggested that such species as Agnostus, which usually are found in dark, organic-rich limestones in very large numbers, may have lived floating at greater depths (where it is dark), close the the oxic/anoxic boundary in the oceans, where there might have been chemosynthetic bacteria to feed upon.
2. Elrathia kingii (large and small specimen); middle Cambrium, text book p. 253, fig. 14.6.E. Use microscope to look at cephalon (head shield); look at number of segments, and note that most of the small and large specimens have the same number; both are holaspis stage. A few specimens show fewer segments and thus are meraspid stages). Small, holochroal eyes; usually not visible because damaged during molting. Sutures in cheeks well visible, with eye position on the suture; they are opisthoparian. Elrathia kingii occurs only in the middle Cambrian House Formation (Utah) and most commonly in the Wheeler Shale, but there they are amazingly abundant in dark grey to black (organic-rich) shales, commonly found together with Agnostid trilobites, few other organisms. It has been recently suggested that Elrathia kingii as well as other Ptychopariid trilobites, lived mainly in what has been called the 'exaerobic zone', close to the dysoxic/anoxic boundary (as was suggested for agnostids); trilobites with many broad segments have been proposed to have these broad segments in order to harbour symbiontic chemosynthetic bacteria.
3. Calymene (white) and Flexocalymene (dark grey, rolled up); Silurian. Text book p. 254, fig. 14.6J, K. Small holochroal eyes visible in some individuals (use microscope); cheek sutures visible in several specimens; gonatoparian.
4. Phacops (brownish in white rock: P. raymondi; dark grey in grey rock: P. rana); Devonian. Text book p. 254, fig. 14.6J, K. Schizochroal eyes very well visible. Cheek sutures more difficult to see; proparian.
5. Use microscope: look at skeletal elements of trilobite in various stages of life. Compare with Paradoxidus sp. , text book p. 256, fig. 14.7. Note small, roundish shield with spines for larval stage (row of pbjects on right side). Head shields of different life stages lined up on the left, tail shields in the middle. Note the presence of two hypostomes in the head-shield area. What type of cheek suture: proparian.
6. Rock with large accumulation of tail fragments of trilobites; also some head shields. From which trilobite were the head shields derived? The head shields are from the genus Trinucleus, a blind trilobite with very typically shaped headshiled (see textbook); these may have been filterfeeders. Under what circumstances does such a rock form? Note that trilobites must have benn broken up, probably by currents or waves, and the the tail ends sorted out on size/shape by these currents. This is rather strange in combination with the observation that the rocks are black-dark grey: usually the result of high organic matter content, and low oxygen at location of deposition. It is very unusual to have low oxygen/high organic carbon contents at a location of active currents. Probable location of deposition: similar to that of Burgess shale, where material from shallower oxygenated environments with active currents are moved down slope and deposited in oxygen-poor basin. Since there are ONLY trilobite tails, the age of this material is probably Cambrian (not many other organisms around).