Evolutionary prospection of globorotalia menardii: an investigation of tempo and mode of evolution since the late miocene

Deciphering tempo and mode of evolution from the fossil record is a challenge for evolutionary studies. The observed pattern in the fossil record often results in ambiguous interpretations due to lack of temporal and spatial information of distribution as well as first appearance and disappearance or extinction of species.
Evolutionary prospection, which is the tracking of morphological changes of a species or group of organisms through time and different locations, may serve as a strategy to decipher tempo and mode of evolution from the fossil record.
In this thesis, this strategy is used with the aim to unravel the processes which influenced the evolution of the planktonic foraminifer Globorotalia menardii the Atlantic Ocean since the late Miocene.
The initial point is the observation of a rapid test-size increase in the Atlantic Ocean, which took place in the early Pleistocene between 2.3 Ma and 1.95 Ma. Within this time interval, the maximum test size of G. menardii more than doubled from ca. 550 µm to ca. 1150 µm in the axial length. This size had never been reached in the Atlantic Ocean before, so that Knappertsbsuch (2016) postulated the occurrence of a new, giant G. menardii morphotype. The evolutionary history of the proposed new type remained unsolved, as this event happened after the closure of the Central American Seaway. Therefore, the giant morphotype could not have derived from the eastern Pacific Ocean, where specimens with a size of more than 1000 µm had already been observed before.
Two hypotheses are tested, which could explain the appearance of this giant type: The Agulhas leakage hypothesis proposes the dispersal of the giant morphotype from the Indian Ocean via the Agulhas leakage around the southern tip of Africa. Alternatively, the giant type evolved in the Atlantic Ocean by a punctuated evolutionary event.
Following the idea of evolutionary prospection, quantitative measurements of morphological characteristics of the test of menardiform globorotaliids in keel view were made on samples from ODP Hole 667A in the eastern equatorial Atlantic Ocean for the last 7.96 Myr and IODP Hole U1476A in the Mozambique Channel from 6.49 Ma to the present. A comparison of these two locations allows to track the first occurrence of the giant morphotype and thus if it could spatially originate in the Indian Ocean. In the eastern tropical Atlantic Ocean ODP Hole 667A, 6719 menardiform globorotaliids specimens, including 4482 specimens of G. menardii, were picked from 33 stratigraphic levels. 7823 menardiform globorotaliids of which 4978 specimens belong to G. menardii, were collected from 31 stratigraphic level at Hole U1476A.
The use of the “Automated Measurement system for shell mORphology” (AMOR; Knappertsbusch et al., 2009; Knappertsbusch and Eisenecker, 2022) guarantee the objectivity and comparability of the performed morphological measurements. This system is capable of processing a huge quantity of objects, as it automatically orientates and images tests of menardiform globorotaliids in keel view.
The results are in accordance with the Agulhas leakage hypothesis. In the eastern tropical Atlantic Ocean, the doubling of the test size is observed between 2.58 Ma and 2.057 Ma. It coincides with a change in the predominant coiling direction of G. menardii populations from dextral to sinistral. If the giant morphotype is defined as exhibiting a sinistral coiling direction and having an axial length of more than 1000 µm, it already appeared at 2.95 Ma in the tropical Indian Ocean. Thus, a precondition for the Agulhas Leakage hypothesis is fulfilled with the earlier, diachronic occurrence in the Indian Ocean.
The evolution of form and shape of G. menardii, which are two further parameters derived from the performed morphological measurements, through time do not allow to distinguish the giant morphotype from the ancestral, small type.
Yet, the possibility that the giant morphotype evolved in a regional, punctuated evolutionary event in the Atlantic Ocean between 2.3 Ma and 2.057 Ma cannot be excluded based on the gathered data.
Patterns in the test-size evolution of G. menardii show a coinciding trend to proxies for the strength of the Atlantic Meridional Overturning Circulation (AMOC) in the Atlantic Ocean and the Indian Monsoon in the Indian Ocean. It suggests that changes in the strength of these major oceanographic drivers influence the long-term test-size evolution of G. menardii in the investigated time interval, probably due to their influence on the hydrography of the upper water column, which is the habitat of G. menardii.
A further objective of the thesis is the observation of interspecific trends within the G. menardii-Globorotalia limbata-Globorotalia multicamerata lineage. Continuing morphological splitting of the descendants G. limbata and G. multicamerata from its ancestor G. menardii could point to an ongoing diverging evolution of those morphospecies. Diverging trends in mean population test size are observed at the investigated sites: at Hole 667A in the eastern tropical Atlantic Ocean, G. limbata and G. multicamerata increase in size in comparison to G. menardii from 5.268 Ma until 4.14 Ma, while in the Indian Ocean a similar trend is observed from 5.78 Ma until 3.44 Ma. However, those time intervals of divergent test-size evolution are followed by intervals of converging trends. This probably indicates that those trends were rather caused by changes in the environmental conditions than being based on ongoing genetic differentiation.
Evolutionary prospection of demonstrates a high potential as a strategy to investigate the tempo and mode of evolution in menardiform globorotaliids.