If the paralic domain is seen as an autonomous entity, in terms of the Earth and its history, it takes on a dimension other than that of a more contact (or exchange) zone between the land and the sea.
The sediment associated with the first traces of life on Earth (“sensu lato” stromatoliths) along with various considerations (concentration of nutritive elements, indications of shallow or intermittent water levels) imply that it was truly in a paralic milieu that life first appeared on our planet, about 3 000 million years ago (Cloud, 1968). In this respect, it is indeed the paralic rather than the marine domain which appears as the original one (see inset, Fig. 34).
The paralic domain is an obligatory transit channel for anadromous and catadromous species. Moreover, because of its high productivity (Fig. 34), it represents a place of growth for many species which breed at sea, and is probably indispensable for the completion of the reproductive cycles of many marine species.
A remarkable property of the strictly paralic stocks is their slow almost non-existent evolution. This is the case of the “langunar” molluscs whose shape has hardly changed since the Tertiary, or certain Foraminifera, which have remained identical since the Cretaceous (Ammonia tepida). Lingula are an example of a paralic “panchronic” form (Appendix 3). By comparison, the rapid evolution of marine or continental stocks is clearly revealed by stratigraphy (cephalopods, echinoderms, branchiopods, mammals, etc.). There is an obvious reason for the stability of paralic stocks: the species are adapted to a certain physical and chemical variability of the milieu and, if confinement is the essential parameter of the distribution of the species, there is always a zone in the paralic domain where an adequate degree of confinement allows them to survive. Thus, the physical or chemical variations of the milieu exercise o only a low or indeed non-existent “selective pressure” on the paralic stocks, and the genetic shift is extremely weak. Conversely, in the marine or continental domains, the stability of the abiotic parameters of the milieu is greater and their variations slower: there, the species are much more specialized and unable to adapt to sudden changes should they occur: these stocks are therefore condemned to evolve or disappear in case of “crisis” in the domain (variation of the salinity, temperature, currentology, etc.).
One may even wonder whether the paralic domain does not constitute some sort of potential reservoir for stocks capable of replacing the marine or continental stocks in case of a crisis leading to the disappearance of the latter.
The problem arises of the species that hitherto referred to as “mixed” i.e., species which colonize both the marine and the paralic domain, in spite of the obvious differences in the biological dynamics of these domains. It should be noted that many mixed “species” have an ambiguous taxonomic status, with duplications of specific nomenclature or an adventitious nomenclature (a variety of the form referred to, or “endemic”).
Moreover, recent research in population genetics (electrophoretic spectral analyses of the inner milieu) detects notable differences within a same mixed species, between the marine and the paralic populations (Worms and Pasteur, 1982; Buroker et al., 1979; Kerambrun, personal communication).
This would tend to show that in any case, at the borderline between the paralic domain and the marine domain, the notion of species is to say the least ambiguous - as regards mixed species, naturally.
This could well be taken as an indication of the colonization of marine species by stocks from the paralic domain. Could there be a symmetrical phenomenon as regards continental species?
Thus, it seems that far from being a marginal or secondary domain its permanence and its biological stability cause the paralic domain to be a fundamental element in the history of the biosphere.
The various types of sedimentary deposits in the paralic domain have been considered, and also the importance of the biogenic carbonates in the Near paralic, that evaporites (Appendix 3) and of the “deltaic” structures in the Far paralic. The main characteristics of these deposits is their high speed of sedimentation (the highest known in present-day natural conditions) by comparison with the oceanic domain where the rate of sedimentation is minimal, and with the continent where erosion predominates. The paralic milieux make a leading contribution to the accumulation of sediments on the continental shelves.
The importance of the sedimentation of organic matter should also be remembered. The high intrinsic productivity 1 (Fig. 35) of the paralic domain is combined with its ability to conserve organic matter owing to the prevalence of reducing zones, hyperhaline or otherwise. The coincidence or the frequent proximity of mother rocks, detrital or carbonated deposits (possibly dolomitized under the influence of magnesian brines) liable to constitute reservoirs, and muddy or evaporitic sediments able to produce excellent covering, explain the presence of a great number of hydrocarbon deposits in the paralic series. Coal beds, for a large part, also belong to the paralic domain (Barrabe and Feys, 1965). However, whereas with petroleum and natural gas the organic matter is essentially autochthonous, the continent makes a large contribution in the case of coal.
The observation of present-day nature offers only an incomplete idea of the geological importance of the paralic domain (Perthuisot, 1980)1. The study of the sedimentary series of the great plateux of the past shows that at certain periods in time, particularly outside the “orogenic phases”, the paralic milieu could spread considerably, over a region several thousands of kilometres wide (Busson, 1972) 2.
These few considerations show the fundamental role, too often ill-known, played by the paralic domain in the geological and biological history of our planet.
