taceans, such as crabs and lobsters, in addition to a range of more obscure forms. If estimates of10 million living species of insect are correct, then this phylum outnumbers vertebrates, for example, by a hundred times.
Arthropods all have a segmented body and many, jointed limbs (this is what gives the group its name). They have a well-developed head and sensory system and probably evolved from an ancestral worm during the late Precambrian. The skeleton is external and is shed and replaced as the animal grows. This process, known as molting or ecdysis, is a major controlling
factor in the success of the group. On the positive side it allows a single individual to assume multiple body plans through life; for example it allows caterpillars to turn into butterflies. On the negative side, it uses up costly resources and makes the animal helpless and prone to predation each time it occurs.
Trilobites radiated early in arthropod history, and were most abundant during the early Palaeozoic. However, recent work suggests that they were highly adapted and were not a primitive root stock for other arthropods.
Although trilobites are well known, there are frustrating gaps in our knowledge. The main reason for this is that the exoskeleton on the upper body was mineralized, by the addition of calcite to an organic template, and so preserves well, but the lower body exoskeleton was not. Limbs and appendages were rarely mineralized, and then only very lightly. As a result, few trilobites have been preserved with legs, gills, or antennae, and the hypostome, a mineralized plate under the mouth, usually fell away as the animal decayed. The range of variation in the morphology of the underside of trilobites is largely unknown and most reconstructions rely on one of a handful of exceptional specimens.
Trilobites were mainly bottom dwellers, and tended to develop marked provincialism in their faunas. This makes them useful paleogeographic indicators, especially in the Cambrian and early Ordovician. For rocks of this time period, trilobites are also useful for biostratigraphy, although in younger rocks the typical duration of a species is too long to be of much value.
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