When the male and female of a species have different features, like color, size, or shape, this condition is called sexual dimorphism. Many species of both plants and animals show sexual dimorphism, ranging from mild to very dramatic. A male turkey is not only larger than the female, he also has more colorful feathers and a long wiry feather “beard” coming out from a patch on his chest. In modern animals, even if a species has extreme sexual dimorphism, males and females can be identified as members of the same species based on DNA analysis, behavior studies, or looking at the soft tissue of the animal. In fossils, it gets more complicated.

Soft tissues rarely fossilize, most fossils are too old to extract DNA, and it is hard to study the behavior of an extinct species. Instead of using these clues, paleontologists compare extinct species to modern ones, and try to map the traits of as many individuals of a species as possible to determine if they have sexual dimorphism. For example, modern male and female ostracodes usually differ in size, shape, or both. When paleontologists look at fossil ostracodes, they can tell male from female using these same differences.