Aragonitic foraminifers: an unsuspected wall diversity

摘要

The wide range of wall microstructures observed in the test of calcitic Foraminifera is probably unequalled in nature, and contrasts strikingly with the rather homogeneously radial walls of hitherto examined aragonitic foraminifers, be they tubular or multichambered. But do we truly know the actual wall microstructural variability in aragonitic foraminifers? Aragonite is indeed extremely rarely preserved in the fossil record and seldom tested on modern forms. The present study introduces four new genera and species of planispiral to high trochospiral aragonitic multichambered Foraminifera (Incompositus involutus gen. et sp. nov., Krivajella krivaja gen. et sp. nov., Tenuiscolumna evoluta gen. et sp. nov. and Falsotrochosiphonia teres gen. et sp. nov.) discovered in a Berriasian level of the Krivaja Valley, Bosnia and Herzegovina. Their original, aragonitic wall is exceptionally well preserved and dominantly displays a singular microstructure, made of diversely oriented, inequigranular bundles of original aragonite crystals. Such a structure, reminiscent to some extent of that observed in various calcitic foraminifers (Miliolida, Fusulinida, granulate Rotaliida), had never been recognized as such or described in detail before. The existence of an unfamiliar wall type in morphologically distinct multichambered aragonitic foraminifers not only questions the monophyly of the order Robertinida but also evolutionary paths in the entire Phylum. We here demonstrate that the newly described forms are part of a lineage that experienced a long but concealed evolutionary history. It is time to look at the foraminiferal record with new eyes as several aragonitic groups may have eluded our scrutiny.