Reef building corals of Lower Bajocian of France.: Part 1.

Lower Bajocian (Propinquans and Humphriesianum zones) reef-building corals of France are rather poorly diversified: 16 to 19 genera (including morphogenera) and 17 to 26 species. The new genus Atelophyllia is created and the poorly known genus Ebrayia is revised. Synonymies of species are well understood, owing to a statistical analysis of populations sampled in the field (1410 samples, 900 thin sections). The taxonomic list and statistics offer the first paleoecological tool to study the trends of diversity. Because this diversity increases southwards (South Jura, Maconnais), it suggests that a climatic control underlies the diversity gradient. Functional morphologic analysis of the various taxa offers a second method to evaluate ancient light intensities and sedimentation rates. For instance, a comparison with Montastrea, a recent zooxanthellate coral, shows that the variations of the colonial shape of Isastrea are light-dependant. An interpretation of Bajocian bioherms as lithoherms built by nonzooxanthellate corals must be rejected. The average annual growth rates of Bajocian corals are very low. The new findings suggest evolutionary and climatic explanations.