Early Carboniferous ammonoids from Oued Temertasset in the Mouydir of Algeria frequently show conspicuous septal crowding at various stages in ontogeny, but particularly immediately at the end of the phragmocone. The analysis of this phenomenon suggests that closer septal spacing is probably not related to maturity in these specimens. Septal crowding occurs in different sizes, and interim septal crowding in juvenile growth stages with subsequent recovery to normal septum distances is common. Four groups of species with different modes of septal crowding can be separated. Close septal spacing is interpreted as a response to adverse ecological conditions such as depletion of oxygen, which caused deceleration of growth at the aperture.