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.

An organism can change a lot from the time it first forms to when it reaches adulthood. This process of growth and development is called ontogeny. Some animals are born looking like small versions of adults, and their main ontogenetic change is growing in size. Others look similar to adults, but their body parts change in proportion as they get older. In humans, for example, infants have proportionally larger heads and shorter limbs than adults. Some animals change even more drastically from birth to adulthood, like insects that undergo metamorphosis. No matter how small or large the changes from birth to adulthood are, those changes are all part of ontogeny.

Paleontologists study ontogeny in fossils, too. If they have a lot of fossils of the same species, they can compare their sizes and shapes to understand their growth. If they only have a few fossils, the structure of the bone or shell can tell them how old the animal was when it died.

Comparing extinct species to modern ones is also very useful in studying ontogeny. Both extinct and modern ostracodes have two shells that enclose their body for protection. Some features of their shells are different between adult and juvenile ostracodes. For example, ridges are often larger and more pronounced in adults. By looking at these and other features, researchers can tell which ostracode remains are from fully-grown individuals.

Most animals, like humans, have bilateral symmetry – they can be divided with a line into left and right sides. These two sides usually look like mirrored images of each other, but there can still be differences. For example, male fiddler crabs have a claw on one side that is much larger than the other. This larger claw, which can be on either the left or right side, is used by males in a behavioral display as they court females.

Knowing the difference between the left and right sides of animals is very useful when studying fossils, as often paleontologists will find only a single bone or shell and need to determine what part of the animal it came from. Small crustaceans called ostracodes are common fossils and have mirrored left and right sides, but with subtle changes between the two. Their bodies are protected by a two-sided, hinged shell. One side of the shell is always a little larger, so they can pull in their limbs and close the shell tightly when they are threatened. The part of the shell with the hinge also often has small differences between the left and right sides.