160-8 Change in Ostracod Sexual Dimorphism in Response to the Paleocene-Eocene Thermal Maximum (PETM)

Abstract:

Ostracods are highly sexually dimorphic microbenthos that are some of the few multicellular fossils common in samples ranging through early Cenozoic hyperthermals, such as the Paleocene Eocene Thermal Maximum (PETM) (Hunt et al, 2017). The record of the PETM in the Atlantic Coastal Plain has excellent geochemical proxies, but fewer studies on how these environmental changes affected community composition and evolutionary trajectory of organisms.  We measured length to height ratio amongst male and female adult ostracods to see if sexual dimorphism decreased with increased stress due to warming and acidification. Since shell dimension is linked to sexual fitness, changes in shell morphology can give us insight into evolutionary trade-offs between sexual selection and metabolic constraints. Shell formation is one of the most energy expensive maintenance processes in ostracods. However, shells provide protection from predators and display male sexual fitness (Fernandes et al. 2018). We found that sexual dimorphism lessened after the carbon isotope excursion (CIE). This may be due to a reduction in reproductive soft tissue structures, or possibly metabolic burden on making large tests. We found that this trend encompassed community change (in that species with less extreme sexual dimorphism made up a larger part of the community post-PETM than pre-PETM) and evolutionary change in single species.

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