Ecophenotypic plasticity versus evolutionary trends – Morphological variability in Upper Jurassic bivalve shells from Portugal

Upper Jurassic marginal marine strata of the Lusitanian Basin (central Portugal) yield a rich benthic macrofauna from which three bivalve target taxa, i.e., Arcomytilus, Isognomon, and Eomiodon, were chosen for morphometric studies, because of their abundance both in space and time and their variability in shell shape. The shells have been analysed with regard to outline shape (Fourier shape analysis), dimensions, ornamentation (Arcomytilus) and ligament arrangement (Isognomon). Additionally, data on co−occurring fauna and palaeotemperatures calculated from δ18O values have been recorded. The results of the morphometric analyses have been interpreted with regard to phylogeny and palaeoecology. In all target taxa, a distinct, rapid size increase at around the Early/Late Kimmeridgian boundary is evident. Potential causes for this process are discussed, and an increase in food availability is regarded the most likely scenario. In Isognomon rugosus, a distinct change in resilifer arrangement co−occurs with size increase, resulting in the evolution of an endemic species in the Lusitanian Basin, for which the name Isognomon lusitanicus is re−established. Like in several extant Mytilidae, morphological species characterisation in Arcomytilus turns out unsatisfactory, due to high intra−specific variability. However, Arcomytilus morrisii is still regarded as a valid species that evolved in the Lusitanian Basin. Despite high shape variability, Eomiodon securiformis is also considered to be a clearly distinguished species. For all target taxa morphologic variability is discussed with regard to environment, and variation between populations is delineated. The data suggest a weak correlation of facies and shell shape in Arcomytilus, while Isognomon lusitanicus seems to develop local varieties in different subbasins. Finally, the great morphologic plasticity of bivalves from rather distinct systematic entities is shown to result from different causes, thus demonstrating that careful studies of the involved species are a prerequisite to draw correct palaeoecological conclusions.