Brachiopod outline and the importance of the logarithmic spiral

Abstract To a first approximation, the logarithmic spiral is a close fit to the biconvex lateral outline of three terebratellid genera from Australasia. Investigation of spiral parameters confirms the value of the biconvex morphospace of McGhee (1980), but not the method for estimating the spiral itself. Deviations from the estimated spiral are episodic and consistent with Rudwick's (1959) observations of allometry in brachiopod growth. Morphospace location is postulated to be indicative of a species' substrate and lifestyle. The logarithmic spiral offers much opportunity for gaining better understanding of external shape and its cause in articulate brachiopods.