Abstract
Development plays an important part in shaping adult morphology and morphological disparity, yet its influence on evolutionary processes is seldom explored because of a lack of preservation of ontogenetic stages in the fossil record. By preserving their entire ontogenetic history within their test, and with the advent of high-resolution imaging techniques, planktic foraminifera allow us to investigate the influence of developmental constraints on disparity. Using Synchrotron radiation X-ray tomographic microscopy (SRXTM), we reconstruct the ontogenetic progression of seven species across several of the major morphotypic groups of planktic foraminifera, including morphotypes of a species exhibiting high phenotypic plasticity and closely related pseudo-cryptic sister-taxa. We show differences in growth patterns between the globigerinid species, which appear more tightly regulated within the framework of isometry from the neanic stage, and the globorotaliid species, whose adult stages present allometric trends. Morphological change through ontogeny results in a change in surface area to volume ratios. Different metabolic processes therefore dominate at different stages of ontogeny, changing the vulnerability of the organism to environmental influences over growth, from factors affecting diffusion rates in the juvenile to those affecting energy supply in the adult. These findings identify some of the parameters within which evolutionary mechanisms have to act.
Highlights
Studies of morphological disparity in fossil groups are often restricted to using adult forms, due to the fragmentary nature of the fossil record and the difficulty in obtaining complete ontogenetic sequences (e.g. Gerber et al 2008)
Planktic foraminifera provide excellent material to investigate the influence of development on morphological disparity, as they grow by the sequential accretion of chambers on to their test
Eight specimens representing seven species were chosen for imaging and analysis across the globigerine and globorotaliid groups, representing some of the main morphologies in the planktic foraminifera: Globigerina bulloides (d’Orbigny); Globigerinoides sacculifer (Brady) morphotypes with and without sac-like final chamber; pseudo-cryptic sister-taxa Globigerinella siphonifera (d’Orbigny) and Globigerinella radians (Egger), identified on the basis of symbionts respectively as Globigerinella siphonifera types II and I as per Huber et al (1997) and reassigned according to Weiner et al (2015); Globorotalia menardii (d’Orbigny); Globorotalia tumida (Brady); and Globorotalia truncatulinoides (d’Orbigny)
Summary
Studies of morphological disparity in fossil groups are often restricted to using adult forms, due to the fragmentary nature of the fossil record and the difficulty in obtaining complete ontogenetic sequences (e.g. Gerber et al 2008). The seemingly restricted variety of resources in the ecosystems and apparent lack of geographical or hydrographical barriers to gene flow (Hutchinson 1961; Palumbi 1994; Roy & Chattopadhyay 2007) make temporal and spatial effects likely drivers of isolation and evolutionary change through sympatric or parapatric speciation (Norris 2000; Roy & Chattopadhyay 2007) It is, essential to understand how the environment interacts with intrinsic constraints over the development of an organism to generate new forms. Huber 1994) to reveal ontogenetic changes in morphology Such methods, restrict our ability to resolve details of the morphology in three-dimensions and accurately quantify interior proportions, due to the overlapping nature of foraminiferal growth. We assess mechanisms of growth and morphological diversity within the group through ontogeny and aim to identify potential causes of variability and constraints thereon
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