MICROFOSSILS/Chitinozoa 439 shallow-water deposits as well as in shelf and slope sediments; (2) chitinozoans are present and often very abundant (up to several thousand specimens per gram of rock) in marine anoxic or epoxic deposits devoid of benthic or endofaunas (e.g., Silurian black shales); (3) distal deposits (outer shelf and slope) yield abundant chitinozoans (several hundred to several thousand specimens per gram of rock; Figure 9), whereas acritarchs and/or miospores are rare or virtually lacking; (4) the geographical distribution of many chitinozoan species is frequently much wider than that of benthic or neritic organisms; and (5) some chitinozoan species extended throughout all the palaeoclimatic belts Thus, the environmental control prevailing for pelagic organisms should also affect the chitinozoan vesicles, as well as the ‘chitinozoan animals’, if the ‘egg theory’ is accepted This control is assessed through a variety of parameters, such as: (1) chitinozoan abundance (expressed as the number of specimens per gram of rock); (2) chitinozoan diversity (number of species and genera recorded per sample); (3) the relative frequency of the represented taxa; and (4) the chitinozoan ratio in palynological residues containing marine phytoplankton (acritarchs) and to land-derived elements (miospores, tracheid fragments) All of these numerical data are closely related to the production of the chitinozoans (nutrients, temperature), predation pressure, oxidation of organic matter, hydrodynamic constraints, and the fossilization processes provincialism of the chitinozoans at a generic level, as illustrated by the restricted distributions of Eremochitina, Velatachitina, Pogonochitina, Siphonochitina, and Sagenachitina These genera have been considered as typical northern Gondwana taxa but, probably due to the southern hemisphere gyre, some of these genera are now also reported in south China Hercochitina was long believed to be an endemic genus from Laurentia, but recent investigations have revealed its occurrence in the Middle Ordovician of the northern Gondwana regions As the record is being completed, the number of endemic genera amongst the Ordovician chitinozoans is decreasing At species level, the geographical control is more effective This has led to three different chitinozoan biozonations for low (Laurentia), medium (Baltica), and high latitudes (northern Gondwana), respectively A few species only, usually corresponding to long-ranging and poorly discriminated taxa, display a more or less global distribution (e.g., Cyathochitina campanulaeformis, Desmochitina minor, Belonechitina micracantha, and Lagenochitina baltica) It is worth noting that the specific diversity of the Ordovician chitinozoan is slightly higher in the subequatorial assemblages (about ten species per sample) than in the high-latitude ones The Silurian and Devonian were times of convergence of the main palaeoplates This is reflected by the distribution of the chitinozoan species The weaker geographical control during these epochs allowed the appearance of chitinozoan biozones with global value However, some regions displayed sporadic endemism at species level The most obvious case concerns the assemblages recorded in the very large marine gulf which extended temporarily over westernmost Africa, Brazil, and Bolivia These assemblages include a number of endemic species in the Early Silurian (e.g., Pogonochitina djalmai), Middle Devonian (e.g., Ancyrochitina biconstricta), and Late Devonian (e.g., Sommerochitina langei) These endemic species, however, coexisted with more widespread taxa, indicating communication with the global ocean Palaeobiogeography Miscellaneous Chitinozoans display a rather discrete provincialism compared with benthic Palaeozoic faunas Most of the chitinozoan genera and many species have a worldwide distribution, comparable with the palaeogeographical distribution of graptolites or conodonts However, the peculiar palaeogeographical context prevailing during the Ordovician (i.e., wide dispersion and contrasting latitudinal positions of the main palaeoplates; a very large ocean occupying most of the northern hemisphere) favoured a weak Chitinozoans registered certain chemical parameters of the environment The d13C values measured on carefully sorted chitinozoan vesicles show that the carbon of their wall is in equilibrium with the carbon reservoir of the ocean Therefore, chitinozoans can be used for an accurate record of the d13C variations through time At high latitudes, where carbonates are missing, they represent an alternative carbon source as they have an almost continuous record Figure Diagram of the abundance of chitinozoans along a continent basin transect