Size and shape data of Globigerinoidesella fistulosa, Trilobatus sacculifer and intermediate specimens from ODP Site 1115

Planktonic foraminifera are extremely well-suited to study evolutionary processes 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 can be overcome, we study morphological change leading up to the origination of the unusually ornate species Globigerinoidesella fistulosa. Our results show that the origination of G. fistulosa from the Trilobatus sacculifer plexus involved an amalgamation of three different heterochronic expressions: addition of chambers (hypermorphosis), earlier onset of protuberances (pre-displacement), and steeper allometric slope (acceleration) as compared to its ancestor. We argue that the protuberances unique to G. fistulosa were necessary to sustain a surface-area: volume ratio that could host sufficient numbers of photosymbionts. Our work provides a case study of the complex combination of processes required to produce unusual shell shapes and highlights the importance of developmental processes in evolutionary origination.