Palaeoenvironmental vs. evolutionary control on size variation of coccoliths across the Lower-Middle Jurassic
Résumé
Size is an easily quantifiable trait probing the respective influence of evolution and palaeoenvironment on organisms through time. After the stressed period that spanned across western Tethys epicontinental seas during the early Toarcian, the following rebuilding of a restored water column ecosystem is coeval with the size increase of the Lotharingius genus coccoliths, one of the main pelagic producers at that time. In order to quantitatively assess the coccolith size evolution during a time span of ~10 myr, measurements of seven morphospecies of Lotharingius across the Toarcian and early Aalenian were performed. This study was carried out on a total of 5500 specimens from samples retrieved from two distinctive palaeoenvironmental settings in the western Mediterranean Tethys: three sections from the Lusitanian Basin in Portugal and one section from the Causses Basin in southern France. For each specimen, coccolith length, width and central area length and width were measured using specific measurement software. Statistical and data analysis tools were used in order to evaluate significant resemblances and/or differences between different size groups and samples. Two main clusters are acknowledged from this study. Small-sized Lotharingius is b4 μm and large-sized Lotharingius is N 4 μm. In addition to common evolutionary processes, changes in palaeoenvironmental conditions were taken into account to explain such a size distribution. Although background evolution pushed the Lotharingius genus toward larger sizes, palaeoenvironmental conditions seemed to play an important role in the Toarcian-early Aalenian morphological evolution of this taxon. The production of larger coccoliths profited from stable environmental conditions, whereas smaller morphotypes dominated during periods marked by stress. The transition from small to large Lotharingius was synchronous in the two investigated western Tethys basins at the Bifrons-Gradata (or Variabilis) ammonite zone boundary (middle Toarcian), thus representing an important event for the Lower Jurassic calcareous nannofossil biostratigraphy.