Aragonitic foraminifers: an unsuspected wall diversity

Abstract

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.

http://zoobank.org/urn:lsid:zoobank.org:pub:C6E18A63-7407-42BF-A267-F5FA991655FB

Keywords:

• wall microstructure
• aragonitic Foraminifera
• high-rank taxonomy
• macroevolution
• aragonite nanograins

Acknowledgements

This manuscript is dedicated to Professor Roland Wernli (Université de Genève) who passed away on 12 November 2019, a very thorough and passionate micropalaeontologist and professor. We thank Jean-Pierre Debenay (Université d’Angers) who kindly provided modern robertinid specimens to SR. Jeffrey Oalmann (Nanyang Technological University) helped in acquiring EBSD data and Nicolò del Piero (Université de Genève) helped to acquire pictures of some microgranulate foraminifers (BG’s thin sections) that are not illustrated here but were used for taxonomic purposes. Daniel Vachard (Université de Genève) and an anonymous reviewer are thanked for their detailed reviews.

Associate Editor: Stephen Stukins