274 FOSSIL INVERTEBRATES/Arthropods FOSSIL INVERTEBRATES Contents Arthropods Trilobites Insects Brachiopods Bryozoans Corals and Other Cnidaria Echinoderms (Other Than Echinoids) Crinoids Echinoids Graptolites Molluscs Overview Bivalves Gastropods Cephalopods (Other Than Ammonites) Ammonites Porifera Arthropods L I Anderson, National Museums of Scotland, Edinburgh, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction Arthropods are the most diverse group of fossil and living invertebrate animals They have colonized land, sea, and air due to a relatively plastic and adaptable body plan organization Many different phyla of metazoans, including the arthropods, show repetition of structure along their long axis This is generally termed metamerism Arthropod bodies are characteristically segmented, and the different major groups are defined by various patterns of tagmosis (segment fusion) to form heads, bodies, and tails The phylum derives its name from the characteristic jointed appendages found in all members (Greek: aryros ‘arthros’, jointed; podos ‘podos’, foot) These jointed appendages, which consist of individual elements called podomeres, take the form of walking legs, gill branches, or antennae Broad divisions can be made within the arthropods depending on the exact nature of the leg appendages Uniramians, or arthropods with single-branched appendages, retain only a walking or swimming leg Biramians have an appendage which consists of two separate elements: a leg branch for movement and a gill branch used in respiration Arthropods all possess a rigid exoskeleton which provides support and protection, and acts as a buffer between the external environment and internal body processes The exoskeleton of arthropods is composed of a carbohydrate polymer called chitin In some arthropod groups, the chitin superstructure is reinforced with mineral salts, such as calcium carbonate or calcium phosphate Arthropods with this biomineralization show a higher preservation potential than non-mineralized forms, and consequently have a more complete fossil record through time For nonmineralized forms, we have to look to sites of exceptional preservation (Fossil Konservat-Lagerstaătten) to trace their evolutionary history and discover aspects of their palaeobiology Localities, such as the Middle Cambrian Burgess Shale, the Ordovician Soom Shale, the Devonian Rhynie chert, the Carboniferous Mazon Creek nodule hosted biota, the Jurassic Solnhofen Lithographic limestone, and Baltic, Dominican, and Mexican ambers, all provide windows onto the arthropod faunas at stratigraphical points