Biometric analyses as a tool for the differentiation of two coccolith species of the genus Crepidolithus (Pliensbachian, Lower Jurassic) in the Basque-Cantabrian Basin (Northern Spain)

Biometric analyses were performed on 20 Pliensbachian samples from the Santotis section (Basque-Cantabrian Basin, N Spain), in order to separate the species Crepidolithus crassus and Crepidolithus crucifer based on their size, and to evaluate the role, if any, of the paleoenvironmental conditions on C. crassus and C. crucifer size changes. In each sample, 100 specimens of both C. crassus and C. crucifer were randomly selected and imaged for taking measurements of their length and width. The length/width ratio or eccentricity was calculated for each specimen. Our results show that C. crucifer is around 2 μm longer and 1.3 μm wider than C. crassus. Based on length measurements, two groups were statistically differentiated. Crepidolithus crucifer coccoliths are longer than 8.2 μm. However, an overlap has been observed for C. crassus and C. crucifer in the 8–8.5 μm range; in these cases, central area structure and crystallographic properties are essential for species separation. A trend towards a progressive increase in the size of both taxa has been observed throughout the studied interval and no obvious relationships have been recognized between the lithology, TOC values and coccolith size. However, the size increment in both C. crassus and C. crucifer coincides with a cooling period, suggesting a potential environmental control on coccolith size.Size shifts have been recorded first in C. crassus and later in C. crucifer. Based on the sizes of both species, we speculate that the large C. crucifer, perhaps inhabited a slightly deeper portion of the photic zone and, that the diachronous size variations in the two investigated taxa could be related to the transgressive phase recorded during the Late Pliensbachian. The discrepancy between size variations of C. crassus and C. crucifer specimens in the uppermost part of the studied section might be indicative of modifications of specific portions of the photic zone. We propose that the stratification of water masses probably reduced the ecologic niche of C. crucifer that was forced to reach shallower depths by reducing its size.