Trilobite mode of life

A great deal of work has been done on determining the mode of life of trilobites. Evidence for these deductions comes from the distribution of a particular species oftrilobites, modifications to their shape, and physical experiments conducted on model trilobites. The result is that many trilobites can be confidently located in terms of the depth of water in which they lived and the ecological role that they occupied. Several elements of trilobite morphology are useful in helping to infer life habit, especially the hypostome.

The earliest lifestyle adopted by trilobites was probably a predatory one. These animals ate worms and other soft invertebrates and had a rigidly attached hypostome and spiny bases to their limbs (as in Fig. 8.3c). Deposit feeders, extracting food from soft sediment, and filter feeders, extracting food from the water, evolved from these carnivorous ancestors. In deposit feeders the hypostome tends to be detached from the cephalon. Filter-feeding trilobites functioned by suspending sediment in water using their legs, and then drawing the water into their partly enrolled body and extracting food from it. These trilobites changed the shape of their cephalon, and sometimes of their whole body, to form a large chamber under the carapace within which sediment could be suspended. The hypostome is often set back into the cephalon, at an unusual angle.

All trilobites were mobile and most seem to have crawled on the sea bed. A small number of species became burrowers or active swimmers. Most extremely, some trilobites became pelagic, with a permanently swimming, active lifestyle above the sea bed. These forms had eyes with up to 360° vision and a streamlined body. The most bizarre lifestyle adopted by trilobites is seen in a group of genera exemplified by Olenus. These trilobites had flattened bodies and many thoracic segments. They often occur in black shales, indicative of low oxygen availability above the sea bed, and had a great abundance. Each thoracic segment would have had a gill, allowing maximum extraction of oxygen, and it may be that they farmed sulfate-reducing bacteria, as do modern animals living in similar conditions near black smoker vents. Most of these lifestyles are shown in Fig. 8.4.

Trilobite Lifestyles

Bergamia: an Ordovician blind trilobite characteristic of deep water, where it probably fed on suspended food particles

Ampyx: an Ordovician filter feeder. Water was drawn through the partly enrolled body and food removed with the limbs

Cybeloides: an Ordovician burrower, with an elevated eye designed to emerge from the sediment when the rest of the animal was buried

Olenus: a Cambrian trilobite that may have been adapted to farming sulfate-reducing bacteria in low oxygen conditions, a lifestyle seen in modern organisms living near deep sea vents and black smokers

Bergamia: an Ordovician blind trilobite characteristic of deep water, where it probably fed on suspended food particles

Ampyx: an Ordovician filter feeder. Water was drawn through the partly enrolled body and food removed with the limbs

Cybeloides: an Ordovician burrower, with an elevated eye designed to emerge from the sediment when the rest of the animal was buried

Olenus: a Cambrian trilobite that may have been adapted to farming sulfate-reducing bacteria in low oxygen conditions, a lifestyle seen in modern organisms living near deep sea vents and black smokers

Fig. 8.4 A reconstruction of a range of late Cambrian to Silurian trilobites showing their probable life habits and positions. The reconstruction deepens from right to left, and there is an oxygen minimum zone above the sea bed in the extreme left of the image. All trilobites were marine.

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  • yorda
    What is mode of life for a fossil?
    6 years ago

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