Convergent evolution of spherical shells in Miocene planktonic foraminifera documents the parallel emergence of a complex character in response to environmental forcing
Peter Kiss, Natália Hudáčková, Jürgen Titschack, Michael G. R. Siccha, Zuzana Heřmanová, Lóránd Silye, Andrej Ruman, Samuel Rybár, and Michal Kučera
Summarized by Sofia Corsico-Sánchez, a junior double majoring in biology and environmental science with a minor in geology at Binghamton University. Her future plans include becoming an independent researcher whose work combines her interests in species conservation and the fossil record. When she is not studying, Sofia can be found reading, knitting, practicing ballet, or getting tackled in rugby.
What was the hypothesis being tested?: The hypothesis being tested in this paper was the potential evolutionary cause of spherical shells found in certain species of planktic foraminifera, which are marine zooplankton that float near the surface of the water. Researchers have been unsure of the specific factors that led to their fully spherical shell shape, which differs from the incrementally increasing chamber size shell shape seen in most other species of foraminifera. The scientists behind this study compared three foraminiferal genera: Praeorbulina, Orbulina, and Velapertina; Velapertina was endemic to, or only found in, the Paratethys, an ancient inland body of water that spanned across western Europe into central Asia. Praeorbulina is believed to be the ancestor of Orbulina. Even though Orbulina and Velapertina evolved at different times, they are both found with this unique spherical outer shell shape. The researchers wanted to determine if this shape is due to homoplasy, or a trait that develops in two species without a common ancestor, which could indicate similar environmental pressures that favored this shell shape. These scientists first needed to look at if Velapertina and Orbulina were different genera entirely or were just morphological variations of each other.
What data were used?: Scientists used specimens from three species: Praeorbulina glomerosa circularis (an extinct ancestor of Orbulina from the early Miocene), Orbulina suturalis, and Velapertina indigena. Because Velapertina was endemic to the Paratethys and the researchers were looking into the relationship between it and Orbulina, they used Velapertina and Orbulina specimens from this area. The Praeorbulina specimens were from the Pacific, and were compared to the Orbulina from the Paratethys to ensure that the Orbulina samples from Paratethys were morphologically similar to their ancestor, and were not diverging from the Orbulina lineage due to their location.
Methods: Using the specimens from the three species, scientists imaged their development using an X-ray (Fig. 1). Foraminifera grow by sequentially adding chambers. In the case of these species, their final chamber envelops their entire shell, which makes it difficult to see the growth pattern of the previous chambers. It is also difficult to determine the morphological differences among the three species, as differentiating features lie within the outer chamber. Thus, the usage of an X-ray is crucial to identify these differences.
Results: X-ray scans revealed the inner differences between the Orbulina and Velapertina specimens, indicating that they are, in fact, different species. These inner differences include a difference in growth patterns: both Praeorbulina and Orbulina chamber formation increased in size rapidly (showing their close evolutionary relationship), while Velapertina chambers were similarly sized throughout formation. Previous studies have shown that this spherical formation was evolutionarily advantageous because of its minimum surface-to-volume ratio, which is ideal for gas exchange mechanisms within the foraminifera. This spherical shape is indicative of an environment where oxygen did not deplete, which can tell researchers more about different environmental aspects of the habitats the organism lived in.
Why is this study important?: This study built on previous suggestions of foraminiferal genera differences between Orbulina and Velapertina by using technology to show their inner morphological differences. This shows that the development of certain traits between species that may not be directly related is possible under similar environmental conditions, as seen in the similar shell structure of these two genera. By looking at the evolutionary advantages of this structure, scientists can better understand how the environment of the past impacted the development of species, and what that might mean for the future.
Broader implications beyond this study: In a world where anthropogenic climate change is rapidly changing our environment, it feels as though we are stepping into the unknown. However, our geologic history preserved by our rock record shows periods of environmental similarities. Because these genera were found to be different (from the comparison of specimens from different species), scientists can better understand how the environmental pressures of the past impacted the developmental response of these species, and how modern-day species may react to similar (or even different!) environmental changes. By looking into the past, we can have a better understanding of our potential future.
Citation: Kiss, P., Hudáčková, N., Titschack, J., Siccha, M. G. R., Heřmanová, Z., Silye, L., Ruman, A., Rybár, S., & Kučera, M. (2023). Convergent evolution of spherical shells in Miocene planktonic foraminifera documents the parallel emergence of a complex character in response to environmental forcing. Paleobiology, 49(3), 454–470. https://doi.org/10.1017/pab.2022.48