Westphalian macrofaunas in Nova Scotia : palaeoecology and correlation
Westphalian non-marine bivalves in Nova Scotia inhabited fresh to brackish temporary lakes. A series of lacustrine facies are identified on the basis of the faunas. Myalinid genera including the probably pseudoplanktonic Curvirimula, and Naiadites dominate the middle Westphalian A to early Westphalian B. Carbonicola occurs rarely in the late Namurian to early Westphalian A and is the only reported Anthracosiid. Anthraconauta dominates the upper Westphalian C to early Stephanian occurring in a wide range of sediments. The elongate Anthraconaia arenacea group range through the Westphalian D and occur in coarser lithologies. Small-shell Anthraconaia inhabited ephemeral lake margins. Cluster and principal components analysis are used to classify the faunas. The method requires the measurement of a small number of linear and relatively uncorrelated morphological variables and allows the construction of reproducable pictographs. Three new morphospecies are described. Principal components analysis is used to study morphological variation with respect to palaeoenvironment. Consistent morphological trends occur in Naiadites and Anthraconauta and are considered to have been the product of spat selection and phenotypic changes. With increased turbulence and sedimentation rates, the shell's centre of gravity is shifted towards the anter-umbonal region and the bivalves adopt a semi-infaunal mode of life thus, the morphological changes offer increased stability. Evolution was inhibited by the absence of marine incursions and progressive environmental change. Such palaeoenvironmental changes that did occur were local. Competition may have been of evolutionary importance in younger communities of Anthraconaia. The bivalves are of limited stratigraphical use as compared to the British faunas. In conjuction with micro- and macrofloral work, the Namurian/Westphalian A and Westphalian C/D boundaries are lowered and a possible Cantabrian stage recognised. A correlation with the British Chronozones is attempted.