Growth Rate Biometric Quantification by X-ray Microtomography on Larger Benthic Foraminifera: Three-dimensional Measurements Push Nummulitids into the Fourth Dimension

This work demonstrates the potential of three-dimensional biometric quantification using microtomography on larger benthic foraminifera. We compare traditional linear and area measures used for calculating three-dimensional characters with actual 3D measurements made from volume images obtained using X-ray microtomography (microCT). Two specimens of recent larger benthic foraminifera, i.e., Palaeonummulites venosus and Operculina ammonoides, were imaged with a high-resolution microCT scanner. This method enables three-dimensional imaging and calculation of measurements like 3D distances, surfaces and volumes. The quantitative high-resolution images enabled the extraction of the lumina from the proloculus to the last complete scanned chamber and of the canal system spreading into marginal chord and septa. External surfaces and volumes were calculated on the extracted parts. These measurements allowed the calculation of porosity and microporosity to obtain the test density, which is the basis for many inferences about foraminifera, e. g., reconstructions of transport and deposition. Volume and surface measurements of the proloculus allow the calculation of sphericity deviation, which is useful for determining evolutionary trends in species based on individuals resulting from asexual reproduction (A forms). The three-dimensional data presented here show the actual growth of the foraminiferal cell and the development of the test. Measurements made on an equatorial section cannot be considered representative of a three-dimensional test, unless a correspondence between 2D data with 3D data shows significant correlation. Chamber height, septal distance, spiral growth and chamber area were measured on the equatorial section and correlated with the volume measurements from 3D images to determine the predictive value of the 1D and 2D measures for estimating the 3D morphological parameters. In particular, we show that the equatorial section area of chambers correlates significantly with the chamber volume and can be used to differentiate between nummulitid genera according to their different growth patterns.