The Biology of Trilobites

Trilobites are the earliest unambiguous arthropods found in the fossil record, evidently because they were the first arthropods to develop the mineralized skeleton necessary for frequent preservation. Modern arthropods have organic exoskeletons that are often strengthened with minerals such as calcite. In contrast, trilobites had an exoskeleton composed primarily of calcite, although it undoubtedly was modified with organic materials or other minerals. The appendages, legs, and antennae were probably an organic material like the chitin of extant arthropods, which is only preserved when replaced by minerals under special conditions (see Chapter 3). None of the organic skeletal materials have survived directly in the fossil record. The word arthropod means "jointed foot or leg," and the phylum Arthropoda includes trilobites, crabs, lobsters, spiders, horseshoe crabs, centipedes, millipedes, and the largest of all living animal groups, the insects. This large group includes animals with strong external skeletons that must be shed periodically to enable physical growth. Many also go through ontogenetic phases in their growth, which can include free-ranging near-microscopic larvae.

This chapter is divided into four parts: the external skeleton or exoskeleton, the growth phases or ontogeny, the appendages and internal anatomy (the unmineralized or soft body parts), and the mode of life of trilobites. This order roughly corresponds to the level of knowledge about the biology of trilobites, as most is known about the exoskeleton and least is known about the lifemode. The last part, life-mode, is inferred from knowledge of trilobites in the fossil record and observations of living arthropods. All of the illustrations were chosen from trilobite genera found in New York.

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