Article: Silurian myodocopid ostracodes: their depositional environments and the origin of their shell microstructures
David J. Siveter, Jean M. C. Vannier and Douglas Palmer
European Silurian myodocopid ostracodes form part of an almost exclusively pelagic, recurring faunal association. In general, they are facies related to typically laminated mud-silt or carbonate sequences deposited in relatively quiet, low energy, probably outer shelf to shelf slope environments. Their palaeogeo-graphical and facies distributions may in part be due to depth control.Reticulation, corrugation, and tuberculation are recognized as the true, genetically controlled external shell ornament in Silurian bolbozoid and 'cypridinid' myodocopids. In contrast, enigmatic polygonal and associated radiate shell microstructures, described from a wide variety of Silurian myodocopids, are interpreted as the surface expression of calcium carbonate platelets and possibly the result of in vivo shell calcification and post-mortem 'diagenetic' changes; they cannot, therefore, similarly be used in taxonomy.A model based on analogues in Recent myodocopids and on possible variation in the pattern of nucleation centres in the shell is proposed to explain the mode, variety of styles, intraspecific, and ontogenetic variation of shell calcification in typical Silurian myodocopids. Pore canals may be the nucleation centres of calcium carbonate. If centres are numerous and densely arranged, this may account for the formation of closely packed, polygonal microstructural patterns on (and through) the shell. If nucleation centres are sparser and distant, calcification possibly terminates before all platelets can coalesce, thus accounting for observed patterns of isolated to clustered platelets of various larger sizes. Radiate ('rosette') type platelets are possibly produced when post-mortem acicular recrystallization affects various sized platelets. Smooth valves may represent newly moulted individuals; patches of platelets might indicate poor or an intermediate stage of calcification; specimens with densely packed platelets may have completed cuticle calcification.