Occurrences of velapertina indigena, orbulina suturalis, and praeorbulina glomerosa circularis within the central paratethys

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.