Steps of Morphogenesis and Iterative Evolution of Imperforate Larger Foraminifera in Shallow Carbonate Shelves During Mesozoic Times: Possible Relations to Symbiotic and Abiotic Factors

Abstract

1. The microgranular/agglutinated imperforate larger foraminifera (ILF, chiefly “lituolids”) of Mesozoic shallow marine carbonate shelves are a polyphyletic group of K-strategists, ecologically homogeneous inhabitants of the photic zone (nutrient poor) and hosting symbionts as their larger porcelaneous recent equivalents. They contrast with deeper water “lituolid” taxa and other deeper marine dwellers (hyaline perforate and planktonics r-strategists). They are narrowly linked to the carbonate platform history and evolution, birth and demise (correlated to paleotectonics or sea-level variations), and global climatic or volcanic events, OAE, etc. 2. General principles used in evolutionary sciences can be applied to decipher some morphological pathways among foraminifera through time to reconstruct phylogenetical arborescences, e.g., embryology (heterochronies of development), recapitulation law (Haeckel), size increase (Depéret), parallel evolution, and biostratigraphy. 3. Iterative evolution is the main mode of morphological variation through time and biostratigraphy helps to reconstruct the time repartition of diverging advanced homeomorphic taxa and their distinction. The new characters are issued from a constant, more tolerant stock of smaller and simpler primitive atavic forms by the transformation of previous structures or the addition of new ones. Advanced homeomorphic taxa are temporarily preserved and their morphological innovations linked probably to (epi)genetic constraints and symbiotic history increasing their energy efficiency in a rather stable (but cyclical) environment. 4. Morphological innovations appear slowly during the Jurassic and more rapidly (and frequently) during the Cretaceous following a probable saltative and allo-patric mode of evolution during favorable periods after main environmental crisis, anoxia, and sea-level changes. The Cretaceous is a time of great diversification (adaptive radiation of microgranular lituolids and larger porcelaneous tests). 5. Morphological changes at a scale of one or two stages in one evolutive bioseries (general trend: size and internal complications increase) cannot be linked to global events or other abiotic factors (e.g., the orbitopsellids in the Pliensbachian) but are rather driven by internal epigenetic factors related to algal symbiosis and coevolution. The constancy of the model (text Fig. 8.2) through time in variable clades could be explained that way. The structural incompatibility between pseudokeriotheca/alveolae within the wall and marginal microstructures (radial partitions or hypodermic network) could be explained by a biological incompatibility of bacterial symbionts to live aside with larger microalgae as dinoflagellates. Local discontinuous events like cyclic (metric) parasequences are common on carbonate platforms but have no apparent influence on morphogenesis or population composition. 6. Conversely, global environmental crises affect deeply the larger foraminifera (chiefly advanced larger tests) and population compositions at a higher taxonomic rank (families, etc.), replacing in the aftermath the larger solid tests by smaller forms with a fragile (thin) test during anoxic crisis (T/J-Late Pliensbachian-Late Cenomanian, etc.). Larger forms are too specialized and complicated in their architecture and are particularly sensitive to environmental changes (Hallock, Paleobiology 11(2):195-208, 1985). They cannot adapt their test morphology (“retrograde evolution, " Guex, Retrograde Evolution During Major Extinction Crisis, 2016) back to some kind of more tolerant simpler form because their low reproduction rates impede a rapid reaction to environmental stress, even for minor adjustments. Major sea-level and facies changes (deeper water conditions or large-scale emersions) cause the brutal disappearing of this very sensitive group of foraminifera. However, some large discoid taxa and other smaller siphovalvulinids-valvulinids cross the K/T environmental crisis, interpreted as survivors in the Paleocene. 7. Moreover, gigantism is probably a disadvantage against physical perturbations like tidal currents and storms in an otherwise stable carbonate environment. In a first evolutionary step, increasing of test size and flattening of chambers are selective advantages in terms of energy (increasing of chemical exchanges with more symbionts) and stability on flat surfaces; but near the end of the evolutive bioseries, the same traits become a drawback concerning test anchorage and risk of displacement in unfavorable microenvironments.