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Life in the Palaeozoic
Life in the Palaeozoic

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4.1.1 More on trilobites

Many thousands of trilobite species are known, mostly from Cambrian to Silurian rocks, and all were confined to the Palaeozoic Era. By the time trilobites became extinct in the late Permian, their diversity had dwindled to a small number of species, and the group was long past its peak. The variation in trilobite form is enormous, but the basic three-lobed division of the exoskeleton is always present. The number of trunk segments varies from 2 to 40. Not all have eyes. Most are about 2-10 cm in length but some are 1-2 mm and a few species grew to nearly 1 m long. The majority of trilobites lived on or near the floor of shallow seas, but some trilobites swam in the surface waters of the open ocean, and some were adapted to low concentrations of oxygen in water hundreds of metres deep. How they reproduced is not clear. Figure 6 shows a range of trilobites from the Ordovician Period.

Figure 6
Figure 6 A selection of Ordovician trilobites from central Wales. (a) Oygiocarella (7.5 cm). (b) Cnemidopyge (3 cm). (c) Telaeomarrolithus (2 cm). Trilobites like this with a pitted fringe around the head are called trinucleids. The main function of the fringe, which has tiny holes at the centre of each little pit, may have been to allow water to pass out from a feeding chamber underneath the head after the animal had strained off small food particles. This specimen is enrolled - the tailpiece, along which the specimen has partly split, is tucked up underneath the headshield. (d) Segmentagnostus. This trilobite, about 1 cm long, belongs to a group of small trilobites called agnostids, which have only two or three trunk segments and a headshield and tailpiece of similar size and shape