Spira mirabilis in Heterostegina: evolutionary and ecological perspectives

ABSTRACT

Morphometric analysis of the marginal spiral outline of Heterostegina populations belonging to the ocalana-matteuccii lineage reveals that it fully fit with a logarithmic spiral identified by two variables: the initial radius (IRex) and the radius increase (Rinc). Spirals tend to become more looser through time as observable also in well-known Mediterranean lineages. Rinc tends to increase along a phyletic line leading to the definition of a trend of neanic acceleration in Heterostegina, defined as the tendency of the marginal spiral to become looser. Ecophenotypical variations, with high value of Rinc in respect to coeval populations, may occur as for other routinely employed biometric values for nummulitids, but the investigated populations can be well-constrained in a Rinc-X bivariate plot, where X indicates the number of undivided chambers. The spiral outline, going looser, seems to tend become a golden spiral, a spiral defined by value of Rinc of 0.306, leading to a recognition of golden selection in Heterostegininae.

KEYWORDS:

• Nummulitids
• Heterostegina
• biometry
• evolutionary trends
• golden selection

Aknowledgements

Thanks to A.I. Torres-Silva (University of Vienna) for sharing her data on the populations from Cuba, to G. Less (University of Miskolc, Hungary) for the specimens from Illats and Florida, to W. Eder for his unpublished conclusion about the marginal spiral in living Heterostegina, and finally to J. Pignatti (La Sapienza, University of Rome) and to all the friends of the Workgroup On Larger Foraminifera (W.O.L.F.), for their constructive criticism to preliminary data. I’m also grateful to the editor-in-chief G. Dyke for the careful editorial support and to A. Briguglio (University of Genova) an to an anonymous reviewer for the constructive comments. P. Young kindly revised the English.

Disclosure statement

No potential conflict of interest was reported by the author.