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Geometrical constraints in foraminiferal architecture : consequences of change from planispiral to annular growth

Hottinger L.

Studia Geologica Polonica

2005

Vol. 124

99-116

Large-sized, porcelaneous, planispiral-involute foraminifera react to the loss of alar chamber extensions in the transition from reniform to cyclical growth stages by compensating the abrupt loss of alar apertures with the production of meandrine lateral chambers fed from apertures in the ultimate alar prolongation. This is an analogy common to Late Cretaceous meandropsinids and Late Tertiary archaiasines. In genera with an early annular growth due to oblique-overcrossed stolon systems, the chamberlets of the main layer have oblique-retrovert stolons feeding lateral chamberlets disposed according to a chessboard pattern over the whole lateral surface of the discoďdal shell, as in so many orbitoďdiform genera. In lamellar foraminifera developing a median supplemental skeleton with radial canal systems, spiral marginal crests are laterally covered by a single, retrovert-spiral string of supplemental chamberlets per growth step, forming step by step a complete lateral cover as in Biplanispira. Early annular-concentric median supplemental skeletons are laterally covered by successive expanse chambers as in Vaughanina. The function of some, not all, lateral shell compar- tments as greenhouse for vegetal symbionts is briefly discussed.