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.