To understand the developmental basis of a novel anatomical feature, we present a comparative developmental study of an ecologically significant novelty in fruit morphology. Most members of the tribe Brassiceae have heteroarthrocarpic fruits, in contrast to the unsegmented fruits of many Brassicaceae. Heteroarthrocarpy is characterized by a joint that bisects fruits into heteromorphic segments and by partial or complete indehiscence. In order to better understand the development of heteroarthrocarpic characteristics and their relationships to typical siliques, we studied carpel and fruit development in two closely related species of the Brassiceae, Erucaria erucarioides and Cakile lanceolata. Our results indicate that proximal segments of heteroarthrocarpic fruits correspond to valves of typical siliques, regardless of whether these segments are dehiscent. Indehiscent distal segments are composed of both stylar and ovary elements, although the ovary wall of this segment does not differentiate into valve tissue. The joint itself comprises the distal extent of the valve margin and an internal proliferation of the mesocarp. Additionally, Cakile fruits form a transverse dehiscence zone through the joint, allowing the segments to separate. Heteroarthrocarpy entails modifications of lignification patterns and alterations of relationships between valve, style, ovary, and mesocarp. Possible genetic mechanisms underlying these modifications are discussed in reference to what is known about silique development in Arabidopsis.