Are the Late Triassic to Early Jurassic aragonitic Oberhauserellidae (Robertinina) the ancestors of planktonic Foraminifera?
von Hillebrandt, Axel
published: Dec 1, 2012
ArtNo. ESP155026603005, Price: 29.00 €
The attempt was made to prove if the Oberhauserellidae (Robertinina) are the ancestors of the planktonic foraminifera. The Robertinina described here (Oberhauserellidae and Reinholdeinae) are from strata of late Rhaetian to early Hettangian age of the Northern Calcareous Alps. Specimens are especially well preserved with their original aragonitic tests and these are compared here with similarly preserved Reinholdella and in all probability planktonic Conoglobigerina from the Bathonian of Poland. A detailed systematic description is given for different genera and species. The outer and inner characters could both be studied and compared in detail. Described and figured are types of chambering, chamber shape, sutures between chambers, wall structure and surfaces, primary and secondary built apertures. Secondary thickening of the multilayered wall is typical for the Oberhauserellidae and can be seen in SEM photographs of wall surfaces and broken specimens. Pore diameter is similar in the investigated genera. Thin-walled, aragonitic genera such as Globuligerina are hyaline, whereas thick-walled, aragonitic genera of Oberhauserellidae and Reinholdellinae are whitish to pale honey yellow. Only in Oberhauserellidae an arcus is found. The most important differences exist in the formation and appearance of apertures. A secondary built foramen is not known in Conoglobigerinidae and also the primary apertures differ clearly. A bulla is only present in Conoglobigerinidae. Only a few and relatively unimportant characters coincide in Oberhauserellidae and Conoglobigerinidae. No transitional forms exist between both families, in contrast to the Oberhauserellidae and Reinholdellinae which have intermediate forms in the early Hettangian of the Northern Calcareous Alps. The youngest Oberhauserellidae are described from the Toarcian of Switzerland. The oldest Conoglobigerinidae known are probably the Globigerina-like cross sections described from Turkish Toarcian limestones. Conoglobigerinidae can be rock building in limestones, in contrast to Oberhauserellidae which often preferred poorly oxygenated clayey to marly sediments. The search for the earliest Conoglobigerinidae should be concentrated on pelagic limestones and marls of the Tethys margin. It can not be proved a phylogenetic descendance of planktonic foraminifera from Oberhauserellidae.