Multispiral growth in Nummulites: Paleobiological implications

An analysis of multispiral growth in Eocene nummulitids was performed. The ontogeny of some multispiral specimens was reconstructed, quantified and modeled, and the occurrence of multispiral growth in the different Nummulites and Assilina species has been reviewed. The results showed that all larger species display multispiral growth. In Nummulites, multispiral growth appears independently in each group when a critical size of about 15 mm as the minimum test diameter of B-Forms is reached. In multispiral tests, up to 20 different spirals may grow simultaneously, so that several chamberlets are formed in each growth step. Multispiral growth thus produces a considerable increase in the growth rate, in terms of volume added in each growth step, of up to 500% in N. gizehensis and 570% in N. millecaput, yielding a test near seven times larger than a single-spiralled test with the same number of growth steps. The gigantic sizes of 5 to 15 cm reached by Nummulites are produced mainly by the increase in growth rate provided by multispiral growth, and only secondarily by an increase in longevity that is thought to be of less than 6 years. The current view of larger foraminifera as slow growing, long-living, extreme K-strategists is questioned and discussed.