Phenomenon of morphological differences (besides primary sexual characters) that distinguish the males from the females of a species. For example, male deer often have larger antlers than females, and the males of many birds have differing plumage (often more brightly coloured). Sexual dimorphism is known to have been common in ammonites (Ammonoidea), and many fossils originally thought to have represented separate species are now recognized as dimorphs within one species (e.g. the Jurassic Kosmoceras jason and K. gulielmi are dimorphs within K. jason). See also dimorphism.