Bathymetry and biostratigraphy

Articulated brachiopods and paleobathymetry

Recent Articulata live in very variable depths, from the littoral zone to several thousands of meters, in abyssal zone. External morphology of their shells seems to have been a little standardized.

During the Jurassic, on the other hand, the diversity of the forms was much more significant. This diversification is probably in direct relationship to the various ecosystems, in which the species evolved. The depth of water, its oxygen and salt content, the kind of the sediments and the nutritive contributions, thus conditioned the shape, the size and the ornamentation of the shell. (Boullier, 1993)

The dating

Stratigraphic use of fossil brachiopods is from now on quite admitted for Paleozoic strata. This use is less obvious for the mesozoïc deposits. A zonation based on associations of brachiopods allows nevertheless, in certain cases, to draw a parallel with the ammonitic biostratigraphy.

Drawing Christophe Souchet Natural History Museum Bourges / France

Abyss

A particular fauna, characterized by total lacking of ornamentation, often sulcate or perforated, developed in the Alpine Mediterranean region. The smooth representatives (Securithyris, Linguithyris, Apringia...) much contrasted with "ordinary looking" European brachiopods.

However, these western tethysian forms are more similar to recent species, living, confined in large basins, in the depth (200-4500m) of the Pacific Ocean. Extrapolating from this pattern, it is possible to put forward the hypothesis, that from the Triassic onwards, a special fauna might have survived in protected environment, avoiting in this way biological crises of shelf sea.

(VÖRÖS, A. 2002)

Index species

Certain species with limited vertical distribution, added to a significant horizontal expansion, become fossils markers.

Rhynchonelloidella alemannica (Rollier) Small species of the external shelf		Postepithyris cincta (Cotteau) Huge Terebratula of surroundings of coral reefs

Biostratigraphy : dating of geological

deposits by fossils.

Paleobathymetry : measurement of the ancient sea depths.

Ecosystem : ecological unit formed by the natural habitat and the living organisms which live there.