A minimum cellular automaton, carrying precise biophysical significance in each rule, is presented to model pigmentation patterns on molluscan shells. We find the following types of modes: self-organisation into stationary (Turing) structures, travelling waves, chaos, and so-called class IV behaviour. The latter consists of a disordered spatio-temporal distribution of periodic and chaotic patches; it differs from chaos in that it has no well-defined error propagation rate. The calculations of the modes agree well with observations in natural shells. In particular, our results suggest evidence in nature for class IV behaviour, a mode that had so far been reported only as the result of simulations. Moreover, we show that patchiness results in a class IV mode from the same algorithm that renders chaos and periodicity; thus, there is no need to invoke two competing pattern generators, as in previous approaches.