Morphological variability of menardiform globorotalids in the Atlantic Ocean during Mid-Pliocene

The morphological variation of the planktonic foraminifera plexus of Globorotalia (Menardella) (Bandy, 1972) has been studied in a Pliocene time-slice at 3.2 Ma. Using a combination of size, linear shell measurements and shape analysis, an extended morphological protocol is explored in order to define morphological subgroups within the Menardella subgenus (Bandy, 1972). Isochronous samples at 3.2 Ma have been selected at five ODP/IODP Sites in the low latitude Atlantic Ocean, in which up to 600 specimens per sample have been oriented, imaged and analyzed using a new automated prototype for morphological analysis called AMOR. Multimodal size frequency distribution is related to the occurrence of several distinct populations. Three main ubiquitous populations of such menardellids are isolated, next to two additional biogeographically limited subgroups. These populations strongly differ in abundance and size. Using morphological classifiers, subpopulations are distinguished among these populations, leading to the establishment of seven different morphotypes informally named: MA, MB, MC1, MC2, MC3, SH1 and SH2. These morphotypes are assigned to formal species, i.e., MA corresponds to Globorotalia (Menardella) menardii, MB to G. (M.) limbata, SH1 to G. (M.) exilis, and SH2 to G. (M.) pertenuis. In contrast, the species G. (M.) multicamerata is interpreted as being composed of three distinct morphotypes, sharing a similar size range, but differing in shell morphology. type MC1 shows thin and elongated chambers, whereas morphotype MC2 is characterized by a thick and robust test. MC3 is inflated with a distinct flexure in the final chamber. Size differences are linked to variations in habitat temperature and oxygenation, with the exception of G. (M.) multicamerata morphotypes, which are probably adapted to a productivity gradient.