Ontogenetic disparity in early planktic foraminifers

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.

个体发育过程中的形态变化，可对物种的生理特性与生物学特征产生深远影响。受限于化石记录中发育阶段的保存缺失，针对地质历史尺度上个体发育差异度的研究相对匮乏，这使得诸如高级分类单元演化过程中个体发育差异度的变化这类核心科学问题难以得到有效解答。浮游有孔虫（planktic foraminifera）通过逐级房室增生完成生长，且壳体留存有完整的生长痕迹，因此是开展此类研究的理想类群。本研究阐明了侏罗纪有孔虫的不同发育阶段如何被用于解析其生态特征，进而推断该类群最早代表类群的形态演化意义。研究团队使用蔡司XRadia显微CT扫描仪（Zeiss XRadia micro CT-scanner），对四类有孔虫的发育过程进行了三维重建：采自波兰格纳什恩巴通阶沉积物的巴通球盖虫（Globuligerina bathoniana）与牛津球盖虫（Globuligerina oxfordiana），以及采自葡萄牙启莫里阶托热拉组的巴拉赫玛托夫球盖虫（Globuligerina balakhmatovae）与托热拉球盖虫（Globuligerina tojeiraensis）。浮游有孔虫早期演化阶段的形态差异度整体偏低，其房室数量与单位体积表面积的变化范围均低于现代标本，我们据此推断这些早期类群采取了机会主义生存策略。现代海洋中观察到的幼体阶段低可塑性所体现的强烈个体发育约束，在侏罗纪标本中已普遍存在。较高的单位体积表面积特征表明，这些早期类群需要满足比成体标本更高的代谢需求。较短的生命周期与潜在的快速繁殖能力，或许使得这些物种能够充分利用侏罗纪特提斯洋温暖、浅海且营养丰富的水域环境。