8th International Nannoplankton Association Conference
Emanuela Mattioli, Jeremy R. Young, Paul R. Bown, Bernard Pittet:
Morphometric analyses on different species of the Family Biscutaceae are used in the present work with two main objectives: 1) to examine intra- and interspecific variations; and 2) to investigate the possible relationships between palaeoenvironment and coccolith size. Coccolith size (major and minor axes of the ellipse, ellipticity, central-area dimensions) of 90 specimens/sample was measured in 17 samples, coming from different sections of various ages, for a total of 1530 measured specimens. The studied sections are: DSDP Hole 547B, offshore Morocco (Late Sinemurian-Pliensbachian), the Santuario section in NW Sicily (Pliensbachian), the Somma section in C Italy (Early Toarcian), the Dotternhausen section in S Germany (Early Toarcian). The samples have been selected according to the following criteria: a relatively good preservation of nannofossils; an age continuously spanning from the Pliensbachian to the Toarcian, period of important evolutionary changes within the Family Biscutaceae; sections distributed along a N-S transect.
Frequency classes of coccolith lengths and central opening ((central-area length/distal shield length)*100) show an unimodal Gaussian distribution for the coccoliths measured in the oldest samples of Hole 547B (see histogram, Figure 1). In these samples, the Biscutaceae are represented by different species of the Genus Similiscutum (namely S. cruciulus and S. orbiculus), that biometric analyses performed right now fail to discriminate. The topmost studied samples in Hole 547B (14/2-12; 14/1-95) display a weakly bimodal distribution of coccolith lengths (Figure 1a, arrows); the histogram of central-opening distribution is unimodal but there is a marked increase in central-opening size compared to the previous samples (Figure 1b, arrows). Despite the observed morphometric variations, neither coccolith diagnostic characters nor preservation change through the section. These coccolith size and shape variations may be either evolutionary or palaeoenvironmental in origin. A palaeoenvironmental interpretation is arguably supported by the observed change in the sedimentary pattern: the lowermost studied samples are micritic nodular limestones, the uppermost samples detrital limestones. The isolated class of frequencies of the sample 14/1-95 (7.25-7.50µm) in Figure 1a corresponds to the first occurrence of Biscutum grande.
The assemblage composition in the Sicilian studied samples is not ostensibly different from that in Hole 547B. The majority of measured coccoliths shows an unimodal distribution of length and central opening (Figure 1) and is formed by S. cruciulus plus S. orbiculus. The few coccoliths showing very small dimensions (length <3.25µm) are referred to Biscutum dubium, the rare coccoliths larger than 6.50µm are specimens of B. grande.
The significant change of the assemblage composition in the Lower Toarcian samples (Somma and Dotternhausen sections), characterised by the dominance of Biscutum novum instead of the Similiscutum species, is not strongly evidenced by the length distribution, whose mean values are quite similar for the Pliensbachian and Toarcian measured coccoliths (Figure 1a). But the mean dimensions of central opening are shifted towards some 10% higher values (Figure 1b).
The main differences between populations of the same age, measured in C Italy and S Germany are: 1) absence of the largest specimens (corresponding to B. grande) in S Germany; 2) an unimodal length distribution of the main group of coccoliths (mainly B. novum and subordinated B. finchii, S. cruciulus, S. orbiculus) in the Somma section versus a bimodal distribution in the Dotternhausen section; and 3) a slight shift towards larger sizes of the main group in S Germany with respect to C Italy. All these differences might be interpreted in terms of a palaeoenvironmental control on coccolith size in different palaeolatitudinal settings. The preliminary results of this work seem to indicate that morphometric analyses are not sufficient to confidently differentiate the species of the Family Biscutaceae, although they constitute a potential tool to recognise size variations as a response to palaeoenvironmental changes in the Early Jurassic. A further development of this study consists of the application of morphometric analyses to each species of the Biscutaceae, to test if they statistically display significant differences in morphometry.
Copyright © 2000, most recent revision July 25, 2000Tania Hildebrand-Habel (firstname.lastname@example.org)