Changes in morphology during ontogeny can have profound impacts on the physiology and biology of a species. Studies of ontogenetic disparity through time are rare because of the lack of preservation of developmental stages in the fossil record. This leaves important processes difficult to address such as changes in ontogenetic disparity through the evolution of a higher taxon. As they grow by incremental chamber accretion, and retain evidence of growth in their shell, planktic foraminifera are an ideal group for the study of this process. Here, we show how different developmental stages in Jurassic foraminifers can be used to decipher the ecology and therefore infer the evolutionary implications of shape of these earliest representatives of the group. Using a Zeiss XRadia micro CT-scanner, the development of Globuligerina bathoniana and Globuligerina oxfordiana from the Bathonian sediments of Gnaszyn, Poland, and Globuligerina balakhmatovae and Globuligerina tojeiraensis from the Kimmeridgian Tojeira Formation of Portugal was reconstructed. Disparity is low through the early evolution of planktic foraminifers. The number of chambers and range in surface area per unit volume is lower than in modern specimens which we interpret as an indication of opportunistic behaviour. Strong ontogenetic constraints indicated by low plasticity during the juvenile stage noted in the modern ocean are already present in Jurassic specimens. The high surface area per unit volume points towards the need to satisfy a higher metabolic demand than is found in the adult specimens. The short life cycles and potentially rapid reproduction may have allowed these species to exploit the warm, shallow and nutrient rich waters of the Jurassic Tethys Ocean.