Aside from the lumbering relics and casts of dinosaurs, perhaps the most famous lineage of fossilized organisms is that of the trilobites (Figure 3.6). These rather ovoid, marine creatures have fascinated both professional and amateur paleontologists for centuries. The group is well documented in the fossil record and was present from the Cambrian until the

Permian, having become extinct during the crisis marking the end of the Paleozoic. The group was most abundant during the Cambrian and Ordovician periods, apparently experiencing declines through the later Paleozoic. Trilobites were probably benthic feeders, although a few may have been predatory. Some bore elaborate ornamentations, perhaps to prevent other marine animals from preying upon them. True

3.6. Trilobite from the Devonian of Morocco. Trilobites were the most abundant and diverse marine arthropods in the Paleozoic (there are nearly 4,000 species known). They succumbed to extinction in the Permian. Length 26 mm.

trilobites are monophyletic and are supported by the rounded terminal segment (which bears the anus), the structure of the eye, and a unique tagmosis of the pygidium composed of a series of fused segments (Fortey and Whittington, 1989; Ramskold and Edgecombe, 1991; Wills et al., 1998; Edgecombe and Ramskold 1999; Fortey, 2001).

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