(Chemical Equation Presented) Four new alkaloids have been isolated during chemical investigations into a southern Australian marine sponge, Stelletta p. Detailed spectroscopic analysis, supported by chemical degradation and partial synthesis, permitted structure elucidation of bistellettazines A-C (1-3), the first reported examples of terpenyl-pyrrolizidine conjugates, and bistellettazole A (4), a unique cyclic terpenyl-imidazole conjugate. The alkaloids 1-4 feature unprecedented carbon skeletons that are proposed to share a common convergent biosynthetic origin, arising via the biogenic equivalent of a Diels-Alder addition between two hypothetical polyenyl norsesquiterpene precursors. © 2008 American Chemical Society.