Phylum Arthropoda (1)
Arthropoda (see Figs. 4.27 and 4.28) possess an exoskeleton or carapace made of protein. In marine arthropods the carapace is reinforced with calcium carbonate or phosphate. The segmented body is divided into three major sections:
head, thorax, and tail. Each segment usually has a pair of jointed appendages, specialized for feeding, sensory, or locomotive functions. In marine arthropods, respiration occurs by gills; in terrestrial arthropods air enters by pores leading to internal tubes. In order to grow, arthropods must shed their exoskeleton and secrete larger ones (molting). Arthropods have a well-developed nervous system and an open circulatory system with a heart, but they have no extensive bloodvessel system. The four main classes of arthropods are Uniramia, Chelicerata, Crustacea, and Trilobita:
This class consists of onychophorans, centipedes, and insects. Most of the Uniramia have a poor fossil record, but occasional insect specimens are found preserved in amber.
Horseshoe crabs, eurypterids, scorpions, and spiders are chelicerates. Eurypterids (sea scorpions) lived in marine, brackish, and freshwater environments and can be found in rocks of lower Ordovician to Permian age. They are most common as fossils in the Silurian. The largest eurypterids attained a length of nearly nine feet. Other members of Chelicerata are poorly preserved as fossils.
This class includes ostracodes, barnacles, crabs, and lobsters. Only the ostracodes are common as fossils. Ostracodes are bivalved arthropods with shells of chitin and calcite. Their jointed legs can extend between the valves for feeding, swimming, and crawling. Individuals are very small, about I millimeter in size. They live in marine and freshwater environments, most commonly in shallow water. Ostracodes range in time from lower Cambrian to Recent. They are abundant in Paleozoic carbonate rocks and some species are useful as index fossils.