Developmental plasticity, where traits change state in response to environmental cues, is well studied in modern populations. It is also suspected to play a role in macroevolutionary dynamics, but due to a lack of long-term records, the frequency of plasticity-led evolution in deep time remains unknown. Populations are dynamic entities, yet their representation in the fossil record is a static snapshot of often isolated individuals. Here, we apply for the first time contemporary integral projection models (IPMs) to fossil data to link individual development with expected population variation. IPMs describe the effects of individual growth in discrete steps on long-term population dynamics. We parameterize the models using modern and fossil data of the planktonic foraminifer Trilobatus sacculifer. Foraminifera grow by adding chambers in discrete stages and die at reproduction, making them excellent case studies for IPMs. Our results predict that somatic growth rates have almost twice as much influence on population dynamics than survival and more than eight times more influence than reproduction, suggesting that selection would primarily target somatic growth as the major determinant of fitness. As numerous paleobiological systems record growth rate increments in single genetic individuals and imaging technologies are increasingly available, our results open up the possibility of evidence-based inference of developmental plasticity spanning macroevolutionary dynamics. Given the centrality of ecology in paleobiological thinking, our model is one approach to help bridge eco-evolutionary scales while directing attention toward the most relevant life-history traits to measure.