Ostracods are small organisms with hinged, two-part shells that are common in the fossil record. Ostracod remains can form huge deposits and are the main component of some shelly limestones (coquinas) that are used for building. They first appeared in the Cambrian more than 500 million years ago and are still abundant today. The evidence an ostracod leaves behind is its shell. Because they are widespread and well-preserved, ostracod shells serve as ecological indicators of past conditions. The location of fossil ostracods and chemistry of their shells provides paleobiologists information about water depth, temperature, salinity, and nutrients. Several indices of paleoclimates have been developed based on ostracod distributions. For example, the MOTR (Marine Ostracod Temperature Range) extrapolates from temperatures tolerances of modern ostracods to infer paleoclimates where fossil ostracods are found. The oil industry even uses fossil ostracods to find sites for exploration because ostracods are associated with particular types of rock.

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.

Even if several organisms are all members of the same species, there can be a lot of variation in how they look. These differences may be based on age or sex, but sometimes there is no clear explanation for why one individual looks different from another. All these differences are part of individual variation. Humans have a lot of individual variation in the shapes of our faces, color of our skin, and overall height, among other features. Animals, both modern and extinct, have a lot of individual variation within species, too.

It is important to consider individual variation when studying fossils. Without a living animal to look at, it is sometimes hard to tell whether a fossil looks a little different because of individual variation, or whether it is a member of a separate (but closely related) species. As is often the case in paleontology, the more fossils a researcher has access to, the better. With a lot of fossils to compare, it becomes easier to track the range of variations that can occur. There are some fossil sites with hundreds, or even thousands of individuals from the same species preserved. These sites are ideal for tracking the range of individual variation.