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. 

Study species.—Globigerinoidesella fistulosa is a short-ranging morphospecies with a distinct stratigraphic range (Fig. 3). Both its origination and extinction are used as markers in Neogene planktonic foraminiferal biostratigraphy (Wade et al. 2011). Globigerinoidesella fistulosa evolved from the Trilobatus sacculifer plexus in the late Pliocene at 3.33 Ma (King et al. 2020; Raffi et al. 2020) and went extinct in the early Pleistocene at 1.88 Ma, marking the Base of Zone PT1a (Raffi et al. 2020) (Fig. 3). The phylogenetic relationships are well-constrained, and the two morphospecies T. sacculifer sensu stricto (s.s.) and G. fistulosa s.s. are morphologically disparate (Poole and Wade 2019), but intermediate specimens bridge the morphological evolution (Fig. 2) and remain as common as G. fistulosa s.s. throughout the stratigraphic range of G. fistulosa (Poole and Wade 2019). Trilobatus sacculifer persists throughout the stratigraphic range of G. fistulosa  and is still alive today (Wade et al. 2011), occupying the mixed-layer in warm tropical oceans and harbouring algal photosymbionts (Kucera 2007). Geochemical data indicates that G. fistulosa had the same habitat as Trilobatus (Poole 2017).

Material. —Ocean Drilling Program (ODP) Site 1115 is located in the western Woodlark Basin, western Pacific (9º11.382’ S, 151º34.437’ E) at a water depth of 1149 m (Fig. 4). Site 1115 was chosen primarily because the G. fistulosa specimens are abundant and well-developed. Eleven late Pliocene samples containing solely T. sacculifer plexus, i.e., before the first occurrence of G. fistulosa sensu stricto (s.s.), and three pooled closely-spaced samples following the speciation (i.e., post-speciation samples) were analysed. Samples were dry sieved at the 250 μm size fraction and picked for all T. sacculifer plexus and G. fistulosa tests, resulting in over 900 specimens. Specimens were positioned in umbilical view and imaged using a camera mounted on a light microscope. Images were analysed using the Image Pro Premier software (version 9.3).  

A magnetostratigraphy was previously developed at Site 1115 (Takahashi et al. 2001) and provides independent age control via the most recent geomagnetic polarity timescale (Gradstein et al. 2020). The integrated stratigraphy for Site 1115 by Chuang et al. (2018) unfortunately ends slightly younger than our studied interval. We used biostratigraphy (this study) and magnetostratigraphy of Takahashi et al. (2001), to determine the age of the samples studied, extrapolating back in time from the tuned record of Chuang et al. (2018). 

 Analysis.—All specimens were analysed for total test area and curvature as measured from two-dimensional images (Fig. 5). Test curvature is calculated as follows:

C = (P²)/(4*pi*A)

Here P is test perimeter (μm) and A is test area (μm²). The equation divides specimen perimeter length by surface area, as measured from the umbilical side. Lengthier perimeters score higher curvature values, and thus specimens that are lobate with high numbers of protuberances such as G. fistulosa s.s. will have higher curvature values than specimens with a more rounded test periphery like T. sacculifer s.s.

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