Early Palaeocene Parvularugoglobigerina and late Eocene Praetenuitella: does evolutionary convergence imply similar habitat?

Abstract. The species of the earliest Palaeocene planktonic foraminiferal genus Parvularugoglobigerina are characterized by a small test with a smooth, microperforate wall and an elongate aperture. Pore-mound structures may occur on some specimens, while the coiling mode varies from high- to low-trochospiral. Four morphotypes are distinct enough to be recognized as species: P. eugubina (Luterbacher & Premoli Silva) is low spiral and multichambered and P. perexigua n. sp. is four-chambered; P. fodina (Blow) has a Globigerina bulloides-type morphology and P. alticonusa n. sp. has a high trochospire. With the exception of the high spire, this general morphology is repeated in late Eocene forms classified as Praetenuitella: P. insolita (Jenkins), P. patefacta Li, and other associated forms.

The two groups acquired their strong similarities – highly distinctive, peculiarly elongated aperture and microperforate walls – through evolutionary convergence, not by being directly related. Stratigraphically they each have a brief occurrence in a geologically significant slice of Palaeogene time: the one during the recovery of ecosystems in the earliest Palaeocene; the other immediately prior to the great cooling and in the time of rapid change known as the Terminal Eocene Event. This study shows that Praetenuitella flourished in eutrophic conditions and that their evolution might have been nutrient-driven. We suggest by morphological pattern analogy that forms of Praetenuitella and Parvulorugoglobigerina reflect similar habitat in their similar form and especially in their aperture.