Morphologic, biogeographic and ontogenetic investigation of Mid-Pliocene menardellids (planktonic foraminifera)

The definition of planktonic foraminifera species remains a major challenge, despite their central place in biostratigraphy and paleoceanography. During the last decade, combined outcomes of molecular and morphometric studies have brought to the light challenging aspects of their classification: within the formerly and traditionally defined morpho-species exists a considerable degree of genotypic variation that defines biogeographically and ecologically distinct sibling "cryptic" species. The recognition of sibling species requires techniques, which are based on other properties than the sole shell morphology.
The present work investigates the morphological variability in planktonic foraminifera of the sub-genus Menardella, a subset within the genus Globorotalia, in a time slice at 3.2Ma (Mid-Pliocene). This time was selected because it includes a major diversification of menardellids, during which 6 homeomorphic species evolved, leading to an intricate taxonomy. Here, a new population-based taxonomical approach is proposed, which relies on the combination of size frequency distributions (SFDs), geometric morphometry of the shells and sequential ontogenetic reconstructions of populations. A total of 7700 specimens collected from 19 localities in the tropical Atlantic, Pacific and Indian Oceans were investigated in great detail. The collection of morphometric data was achieved with the help of an automated device, the robot AMOR.
The analysis of Mid-Pliocene menardellid SFDs allowed the identification of 6 different sedimentary populations, which on the basis of morphometric and other properties - allowed the recognition of 8 distinct morphotypes.These morphotypes are differentiated by their size distribution, the shape of their test (given by the ratio of axial diameter (dY)/spiral height (dX)), their wall structure and reflectivity and their number of chambers in the final whorl. The relative abundance of these morphotypes permits the establishment of 5 distinct menardellid provinces at 3.2 Ma: morphotypes MA and MB are cosmopolitans, whereas the morphotypes MC3, SH1 and SH2 are restricted to the Atlantic Ocean, and the morphotype ME to the Pacific Warm Pool. These morphotypes are compared with the formally established menardellid morpho-species.
Both morphotypes MA and ME are interpreted as Globorotalia menardii. SFDs of these forms suggest the occurrence of two separated populations with distinct biogeographic ranges. Size is the most important parameter to distinguish these two morphotypes. Specific growth patterns derived from ontogenetic studies confirm that small specimens (<200µm) of morphotype MA are adult individuals that have undergone reproduced with a specific growth pattern.
The morphotype MB corresponds to the established species G. (M.) limbata. This morphotype shows an extended neanic stage with a high intra-specific variability, leading to larger shells. In comparison with morphotype MA, the morphotype MB show an early onset of chamber morphological evolution during growth, which can be explained by a delay of the reproduction age of the individual (hypermorphosis).
The traditional morpho-species G. (M.) multicamerata was found to split into the 3 morphotypes MC1, MC2 and MC3. Morphotypes MC1 and MC2 belong to the same population, and their biogeographies overlap significantly. Dominance of MC2 was found to be restricted to the West Atlantic and the Pacific Warm pool. In contrast, morphotype MC1 was typically observed to occur in the Indian Ocean and it also dominates the menardiform faunas in the eastern Atlantic locations. Morphotypes MC1 and MC2 are interpreted as two eco-variants of the same species. Investigation of ontogenetic sequence of these morphotypes revealed a strong extension of the adult stage in comparison to the entire ontogenetic life span of an individual, which is interpreted as a K-stragtegy behavior, possibly as a result of adaptation to their shallower adult habitat.
The morphotypes SH1 and SH2 are attributed to G. (M.) exilis and G. (M.) pertenuis, respectively. They are separated from each other by their densely perforated wall aspect and by a distinct allometric direction during growth.
The applied protocol of SFD analysis proved to be one possible way to describe and investigate diversity patterns in a complex morphological plexus of planktonic foraminiferal species. The identification of foraminiferal populations with the help of size frequency analysis could be confirmed by ontogenetic studies, i.e. the correspondence of a specific growth pattern within a population to the size SFD of that population.