MICROFOSSILS/Chitinozoa 429 Figure Diagram of a chitinozoan vesicle with morphological terminology: (A) Lagenochitinidae; (B) Desmochitinidae Black, outer layer; blue, inner layer; orange and yellow, operculum and prosome Adapted from Paris F (1981) Les Chitinozoaires dans le Pale´ozoıque du sud ouest de l’Europe (cadre ge´ologique e´tude syste´matique biostratigraphie) Me´moire de la Socie´te´ Ge´ologique et Mine´ralogique de Bretagne 26: 496 of an inner and an outer layer The actual chemical composition and molecular structure of this wall, however, are still unknown The inner layer constitutes the frame of the vesicle (Figure 1) TEM observations have revealed neither ultrastructures, nor tiny pores The inner layer is usually thicker than the outer layer and extends even under the central pit Moreover, it survives longer than the outer layer when exposed to various physical (abrasion) and biological (e.g., fungi and/or bacterial attacks) degradations No structures are observed within the chamber or on the internal surface of the wall The outer layer gives rise to the ornamentation through its outgrowth (e.g., verrucae, cones, granules, spines), evagination (e.g., hollow spines or processes), or folding (e.g., carina, crests, wrinkles) Its ultrastructure is highly variable (e.g., spongy, microlamellar, or perforated with microcanals) Thinwalled taxa (e.g., Ancyrochitininae) have a very thin outer layer (less than mm), whose existence is mainly documented from scars on the hollow spines or processes In thick-walled taxa (e.g., Desmochitininae), the outer layer itself can be rather thick (up to 10 mm), but fragile and easily removed The aspect of the external surface of the vesicle is controlled by the texture of the outer layer (e.g., smooth, felt-like, spongy) The shape, size, density, and location and organization of the various ornaments of the vesicle are used as taxonomic criteria Intervesicle Adjustments Probably all the chitinozoans had linked vesicles, at least at one time in their development The less resistant connections were, however, destroyed during fossilization and/or processing The most common vesicle adjustments are linear or chain-like structures Planar aggregates are also known In linear catenary structures, the linkage between two successive vesicles occurs along the longitudinal axis The simplest linkage, but also the most fragile and therefore less commonly observed (e.g., Angochitina, Cyathochitina; vesicles are usually free in palynological residues), resulted from close contact of the flaring lip of one individual with the convex anti-apertural part of the preceding vesicle (Figure 2A) Such an apparatus, in all probability, required a covering with mucilage When the mucron, or the copula, is also in full contact with the operculum of the succeeding vesicle, the linkage is much more robust and the operculum frequently remains stuck to the preceding vesicle (e.g., Cingulochitina, Urnochitina) (Figure 2B) A more sophisticated stage is reinforced linkage, where important modifications of the vesicle are observed The apex of the chamber extends towards the anti-apertural pole and covers the operculum of the succeeding vesicle This is well illustrated in the genus Margachitina, where the operculum is fully part of the preceding vesicle (Figure 2C)