Heterochrony in the evolution of the planktonic foraminifer Globigerinoidesella fistulosa from the Trilobatus sacculifer plexus

Abstract

Planktonic foraminifera are extremely well-suited to study evolutionary change in the fossil record due to their high-resolution deposits and global distribution. Species are typically conservative in their shell morphology with the same geometric shapes appearing repeatedly through iterative evolution, but the mechanisms behind the architectural limits on foraminiferal shell shape are still not well understood. To understand when and how these developmental constraints evolve, we study morphological change leading up to the origination of the unusually ornate species Globigerinoidesella fistulosa. We measured the size and circularity of over 900 specimens of G. fistulosa, its ancestor Trilobatus sacculifer and intermediate forms from a site in the Western Equatorial Pacific. Our results show that the origination of G. fistulosa from the Trilobatus sacculifer plexus involved a combination of two heterochronic expressions: earlier onset of protuberances (pre-displacement) and steeper allometric slope (acceleration) as compared to its ancestor. Our work provides a case study of the complex morphological and developmental changes required to produce unusual shell shapes and highlights the importance of developmental changes in evolutionary origination.

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ORCID for Anieke Brombacher: orcid.org/0000-0003-2310-047X

ORCID for Thomas H.G. Ezard: orcid.org/0000-0001-8305-6605

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