The Late Eocene Evolution of Nummulitid ForaminiferSpiroclypeusin the Western Tethys

Abstract

Megalospheric forms of Priabonian Spiroclypeus of the Western Tethys were morphometrically investigated. Based on the reduction of the average number of undivided, post-embryonic chambers, the investigated populations are grouped into two successive, phylogenetically linked species, S. sirottii sp. nov. and S. carpaticus. The evolution is also demonstrated by the increase of the number of secondary chamberlets in particular chambers, by the increase of the diameter of the first two whorls and by that of the size of the proloculus, although the latter turned out to be also ecologically controlled. This evolution is supported by the stratigraphic succession of populations in the Mossano section (N Italy) and by the change of accompanying fossils. Lacking in upper Bartonian beds, the first appearance of genus Spiroclypeus seems to be synchronous with the beginning of the late Eocene. The newly described S. sirottii is associated with Heterostegina reticulata mossanensis and orthophragmines containing forms o...