A new Eocene lineage of reticulate Nummulites (Foraminifera) from Kilwa district, Tanzania; a place for Nummulites ptukhiani?

Abstract

Reticulate Nummulites are a widespread and distinctive group of Nummulites, frequently used in biostratigraphy, but their evolution is poorly understood. Studies from the Western Tethys suggest they form a single lineage, the Nummulites fabianii lineage, with an increasing proloculus size over time. This has led to their use as one of the diagnostic taxa for larger benthic foraminiferal biostratigraphy. However, outside of this region, additional taxa have been recorded. The most widely discussed example is Nummulites ptukhiani, which was described from Armenia, whose morphology does not fit with the N. fabianii lineage. This raises the question whether reticulate Nummulites are monophyletic, or the result of multiple independent convergent evolutionary lineages. Here we present data from three newly identified populations of Lutetian to Bartonian reticulate Nummulites from the stratigraphically well-constrained Tanzania Drilling Project records, which shed new light on the ancestry of these aberrant forms. These populations are characterized by extremely large proloculi and unusual morphology. We demonstrate that the populations are consistent with an evolutionary lineage that is morphologically distinct but contemporaneous with the N. fabianii lineage of the Tethys. These forms are remarkably similar in external and internal morphology to the Armenian Nummulites ptukhiani. We therefore refer to them as the N. ptukhiani lineage. The existence of a second lineage of reticulate Nummulites indicates that their evolution is more complex than previously thought and raises questions as to whether they evolved from a common ancestor, or independently. It also underlines the importance of carrying out thorough studies of larger benthic foraminifera with independent stratigraphical control from outside of the Tethyan region to more fully understand their evolution and to enable accurate biostratigraphy.