MICROFOSSILS/Conodonts 443 Figure Reconstruction of a conodont animal, based on the specimens from Scotland and South Africa comparable with those of the Scottish animals, then the entire length of this individual may have been as much as 400 mm Several tens of specimens from the Soom Shale preserve just the apparatus and the eyes From these it can be determined that the eye capsules formed outwardly expanding cups, not much larger than those of the Scottish specimens In a few specimens, including the most complete, the position of the eyes is occupied by fibrous tissue, which probably represents extrinsic eye musculature The evidence from these rare fossils permits a broad reconstruction of the morphology of a living conodont (Figure 4) A single, very poorly preserved, specimen with soft tissue is also known from the Silurian strata of Wisconsin, USA This individual is from an order of conodonts, the Panderodontida, that possessed an apparatus of coniform elements, and the indistinct remains hint at a rather broader, flatter morphology for this type of animal Morphology and Internal Structure of the Elements The more primitive conodonts possessed oro/pharyngeal skeletal apparatuses that consisted of an array of coniform elements These elements vary within and between apparatuses in characteristics such as the degree of curvature, presence or absence of surface striations, and the development of costae More derived conodont groups possessed elements of more complex morphology, with greater morphological distinction within each apparatus Animals belonging to the most derived conodont order, the Ozarkodinida, bore a bilateral apparatus of 15 elements, which have been differentiated into two sets: a rostral array of nine denticulate, ramiform ‘S’ elements flanked by a pair of pick-shaped ‘M’ elements; behind these are two pairs of robust ‘P’ elements (Figure 5) The prioniodontid apparatuses from the Soom Shale differ in having four pairs of P elements that are situated above Figure Architecture of an ozarkodinid conodont apparatus Reproduced with permission from Aldridge RJ and Purnell MA (1996) The conodont controversies Trends in Ecology and Evo lution 11: 463 468 the S elements and behind the M elements, but the ozarkodinid 15-element pattern seems to apply to the vast majority of complex conodont apparatuses Conodont elements are composed of two structures: the crown and the basal body The crown is heavily mineralized and relatively coarsely crystalline, and its upper surface may be elaborated into complex denticulated processes or ornate platforms The basal body is finely crystalline with a higher content of organic material The two components grew by the external apposition of layers of calcium phosphate In many derived conodont elements, the basal body is either loosely attached or unmineralized and the crown has an open basal cavity The tip of the basal cavity marks the origin of growth of the element In categorizing the morphology of individual conodont elements, the curvature of the cusp is taken conventionally as pointing towards the ‘posterior’, although this does not always correspond to the true biological orientation of the element The cusp is defined as the denticle immediately above the tip of the basal cavity, and it is often the most prominent denticle of an element Ramiform, or bar-like, elements are categorized by the disposition of processes (Figure 6), which radiate from the cusp and are commonly denticulate, but may be adenticulate: alate elements are bilaterally symmetrical, with a posterior process and a lateral process on each side; tertiopedate elements are asymmetrical with a long posterior process and a lateral process on each side; digyrate elements are asymmetrical, with a lateral process on each side, but normally with a very weakly developed