FOSSIL INVERTEBRATES/Molluscs Overview 367 world When these approaches are used in concert, or in conjunction with information from other groups, such as conodonts, acritarchs, and chitinozoans, an extremely powerful corrective tool results See Also Biozones Fossil Invertebrates: Bryozoans; Corals and Other Cnidaria Palaeozoic: Ordovician; Silurian; Devonian Further Reading Bulman OMB (1970) Graptolithina with sections on Enteropneusta and Pterobranchia In: Teichert C (ed.) Treatise on Invertebrate Paleontology, Part V, 2nd edn, pp 163 Geological Society of America and University of Kansas Press, Kansas, USA Elles GL and Wood EMR (1901 1918) Monograph of British Graptolites Palaeontographical Society, London Fortey RA and Cooper RA (1986) A phylogenetic classification of the graptolids Palaeontology 29: 631 654 Kozlowski R (1949) Les graptolithes et quelques nouveaux groups d’animaux du Tremadoc de las Pologne Palaeon tooigca Polonica 3: 235 Palmer DC and Rickards RB (1991) Graptolites: Writing in the Rocks Woodbridge: The Boydell Press Rickards RB (1979) Early evolution of graptolites and related groups In: House MR (ed.) The Origin of Major Inverte brate Groups, pp 425 441 Special volume 12 Systemat ics Association, London, New York, San Francisco Rickards RB (1995) Utility and precision of Silurian graptolite biozones Lethaia 28: 129 137 Ruedemann R (1947) Graptolites of North America Memoir 19 Geological Society of America, Baltimore USA Urbanek A (1978) Significance of ultrastructural studies for graptolite research Acta Palaeontologica Polonica 23: 595 629 Molluscs Overview N J Morris, The Natural History Museum, London, UK Copyright 2005, Natural History Museum All Rights Reserved The Mollusca is a phylum of invertebrate Metazoa that is semi-coelomate, protostomatous, and triploblastic Molluscs inhabit marine, fresh water, and land habitats The Mollusca are recorded from the beginning of the Cambrian Period, spreading into non-marine aqueous habitats during the latest Silurian or Early Devonian and apparently onto the land during the Carboniferous Today they are only outnumbered, in terms of species, by the Arthropods (see Fossil Invertebrates: Arthropods) In most modern classifications nine classes of molluscs are generally recognised, of which eight are extant These are the Monoplacophora, the Cephalopoda (see Fossil Invertebrates: Ammonites; Cephalopods (Other Than Ammonites)), the Gastropoda (see Fossil Invertebrates: Gastropods), the Rostroconchia (now extinct), the Bivalvia (see Fossil Invertebrates: Bivalves) and the Scaphopoda making up the subphyllum Conchifera; and the Amphineura, the Solenogasters and the Caudofoveata making up the subphyllum Aculifera The molluscs are considered to be primitively bilaterally symmetrical, with an anterior head, a posterior anus, and a ventral foot They have a mucus-producing skin which needs to avoid dessication The dorsal surface of the body usually extends laterally to form an external cover (the mantle) which protects lateral pallial or mantle cavities Primitively the mantle encloses laterally placed appendages such as the labial palps and the gills The Mollusca need oxygen for their metabolism and usually use the blood pigment haemocyanin However, haemoglobin is sometimes used, particularly in areas of the oceans where the oxygen concentration is low A characteristic feature of the Mollusca is the presence of a calcareous shell The dorsal surface of the body and the mantle commonly secrete the shell, which has an outer organic layer, a periostracum, and an inner part made up of one or more layers consisting of complex microstructures of calcium carbonate with an organic matrix The periostracum is secreted at the mantle margins except in the earliest growth stages and at times of shell damage when repairs may be facilitated by other parts of the mantle surface The calcified shell is secreted across a space filled with extra-pallial fluid, by the dorsal surface and mantle margin on to the inner surface of the periostracum and or previous shell layers The calcium carbonate of the shell may be in the form of crystals of aragonite or calcite but which are modified by their organic matrix The Mollusca primitively have an anterior mouth and a posterior anus with a gut that passes through the heart They primitively have bilaterally paired kidneys and gonads The gut is commonly coiled making it longer than the total body length in many living taxa, including the Monoplacophora and Amphineura The gut is not coiled in the Scaphopoda and only slightly coiled in the Cephalopoda Natural casts of stomachs and intestines are sometimes fossilised in detritus