The spherical encompassing final chamber of the planktonic foraminifera Orbulina universa is a prime example of a complex character whose evolution has been documented by a sequence of intermediate forms. However, the mechanism that induced evolution of the spherical chamber remain unclear. Here we show that shortly after the emergence of Orbulina, documented throughout the oceans, a convergent evolutionary transition occurred in the semi-isolated Paratethys, leading to the emergence of the endemic Velapertina, which occupied a similar niche to Orbulina in the surface waters. Using X-ray computed tomography scanning, we show that the evolution of the encompassing final chamber involved the same sequence of steps in both lineages, combining a progressively spherical shell shape with changes in the position, number, and size of apertures. The similarity in the sequence of character acquisitions suggests structural determinism in the way foraminifera shells are constructed and the presence of natural selection favouring a spherical morphology. Collectively, the emergence of spherical chambers in the two lineages at a similar time suggests that the evolution of this spectacular complex character occurred in response to a singular environmental driver.

In order to constrain the spatio-temporal distribution of Velapertina throughout the Paratethys, and to resolve the degree of its co-occurrence with the Orbulina lineage, we carried out an extensive literature review, compiling all localities where either lineage has been reported. Only localities that provide sufficient stratigraphic constraints to resolve the occurrences to the level of planktonic foraminifera Subzone M5b (Praeorbulina glomerosa Lowest-occurrence Subzone), Zone M5 (Praeorbulina sicana Lowest-occurrence Zone), Zone M6 (Orbulina suturalis Lowest-occurrence Zone) and Zone M7 (Fohsella peripheroacuta Lowest-occurrence Zone) were considered (Wade et al. 2011). The search was carried out by querying the literature through Google Scholar, using combinations of taxonomic (Velapertina, Orbulina, Praeorbulina, indigena, suturalis, glomerosa, circularis) and geographic (Paratethys) keywords. The taxonomy has been harmonised across the papers to the genus level and in each case the occurrence and biozone has been recorded. In all cases where Velapertina was recorded as co-occurring with one of the other taxa, we made sure that the co-occurrence was reported from within the same sample in the sediment sequences described in each paper.

Data used for the compilation of species distribution were collected from the literature listed here:

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