Planktonic foraminifera are single-celled marine protists, roughly the size of a grain of sand. They have an extremely detailed fossil record: a teaspoonful of seafloor sediment contains more than 1000 fossil shells. Some species have unusually ornamental shapes, but it is not clear why or how these shapes evolved. Here, we study the developmental changes that led to the speciation of one particularly ornate species: Globigerinoidesella fistulosa. We find that changes in the developmental timing of adult life stages likely account for the complex morphology. This study highlights the complex morphological and developmental changes required to produce unusual shell shapes and highlights the importance of developmental changes in evolutionar origination.

Planktonic foraminifera are extremely well suited to studying evolutionary change in the fossil record due to their abundant 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 determine how these developmental constraints arise, we study morphological change leading up to the origination of the unusually ornate species Globigerinoidesella fistulosa. We measured the size and circularity of more than 900 specimens of G. fistulosa, its ancestor the Trilobatus sacculifer plexus, and intermediate forms from a site in the western equatorial Pacific. Our results show that the origination of G. fistulosa from the T. sacculifer plexus involved a combination of two heterochronic expressions: earlier onset of protuberances (pre-displacement) and a steeper allometric slope (acceleration) as compared with 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 deviations in evolutionary origination.