Bivalves (Phylum Mollusca, Class Bivalvia)
Bivalves are invertebrates belonging to the Phylum Mollusca that possess a mineralized shell made of calcium carbonate consisting of two hinged halves (valves) that enclose the soft body of the organism. Superficially, the shells resemble those of brachiopods (see above) but there are fundamental differences in shell symmetry and structure as well as soft part anatomy, and the two groups are not closely related. Included among the bivalves are familiar organisms such as clams, mussels, oysters, and scallops. Bivalves can be found in both freshwater and marine environments and display a diversity of life habits. Most bivalves are active suspension feeders, generating currents and filtering small particles of food from the water using specialized comb-like structures known as ctenidia (“gills”) that are also used in respiration. Other bivalves are deposit feeders, and a few are parasitic or even carnivorous. Most bivalves burrow into soft sediment using a well-developed foot, but there are forms that lie either attached or free-living on the bottom of seas and lakes; still others live attached or cemented to rocks and other hard surfaces (e.g., oysters, many mussels). Some groups even have the ability to bore into wood or rock. Although most bivalves are sedentary or have limited mobility, there are some forms (e.g., some scallops) that have the ability to swim short distances using jet propulsion that is achieved by rapidly opening and closing the two valves to direct a stream of water which generates thrust. Bivalves are highly successful and diverse and have a long and rich fossil record that extends back to the Cambrian Period. Over 9,000 living species are known, about 8,000 of which are marine, and over 12,000 fossil species (mostly marine) have been described. Following the great extinction that marked the end of the Paleozoic Era some 250 mya, marine bivalves diversified and expanded their geographic distribution into environments that, in many cases, had previously been occupied by brachiopods. They continued to diversify during the Mesozoic and Cenozoic Eras and today are very important components of most marine bottom communities.
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Gastropods (Phylum Mollusca, Class Gastropoda)
Including forms commonly known as snails and slugs, the gastropods are the most diverse taxonomic class within the phylum Mollusca, with an estimated 60,000 – 80,000 known living species, accounting for over 80% of all known living mollusks. Among invertebrates, gastropods are second only to insects (Phylum Arthropoda) in overall diversity. Gastropods live in marine and freshwater habitats and are the only mollusks to have invaded terrestrial environments. Most gastropods have a calcareous shell, often helicospiral in form, which protects the soft body. In some groups, however, the shell is absent or greatly reduced, and many shell-less forms are commonly referred to as slugs (the shell-less marine nudibranchs are commonly known as sea-slugs). Gastropods have a well-developed head and eyes and typically have a large, muscular foot that is used for locomotion via crawling, burrowing, or swimming. Gastropods display a broad array of forms and ecologies with representatives that are filter feeders, deposit feeders, herbivores/grazers, scavengers, active predators, and parasites. Most gastropods possess an organic rasp-like structure known as a radula that is used in feeding. The oldest gastropod dates from the Cambrian Period, and they have a rich fossil record, with over 15,000 fossil taxa described.
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Cephalopods (Phylum Mollusca, Class Cephalopoda)
Cephalopods are an exclusively marine group of mollusks that today include squids, cuttlefish, octopi, and the chambered nautilus. Two other important cephalopod groups, the ammonoids and the belemnoids, went extinct at the end of the Cretaceous Period (~65 mya). Cephalopods have a well-developed head with relatively complex eyes and many arms that are used for capturing and handling food. Cephalopods are good swimmers and can locomote using jet propulsion. Many are active predators that exhibit complex behavior, and they successfully compete with fish in many food webs. Most living cephalopods do not have an external shell, with the exception of the shelled nautilus, which has a multi-chambered shell composed of calcium carbonate. There are approximately 800 living cephalopod species, and they have a rich fossil record with over 11,000 described species, the oldest of which dates back to the Ordovician Period (years). Although cephalopods that lack a shell or mineralized internal structures are only rarely preserved as fossils, shelled cephalopods such as the nautiloids and ammonoids have an excellent fossil record. The extinct belemnoids, which resembled squids, had hard internal parts that are also commonly preserved. The fossil record of some cephalopods, particularly the ammonoids, is marked by bursts of rapid evolution and diversification followed by intervals of extinction. As a result, many species existed for only relatively brief intervals of geologic time. The chambered shells of cephalopods can sometimes float after the organism dies and be widely distributed by ocean currents before settling to the bottom. The combination of short temporal duration and wide geographic distribution makes the cephalopods especially useful to paleontologists for relative age dating and the correlation of sedimentary rock units over long distances.
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