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.

The most common type of microscope, a light microscope, illuminates small objects with beams of light and magnifies their image with a series of lenses. When an object is too small to be seen using a light microscope or greater detail is needed, electron microscopes are used. There are two varieties of electron microscopes: Scanning electron microscopes (SEM) and transmission electron microscopes (TEM). Both types shoot beams of electrons at objects to get their images, though the methods differ.

SEM imagery creates a highly detailed picture of the surface of a specimen. To perform scanning electron microscopy, the specimen is first firmly mounted to a holder. Often the specimen will be coated in a conductive metal like gold or chromium. Then, specimens are placed into the microscope and scanned using a beam of electrons. The electron beam hits the specimen and these electrons, or new electrons emitted by material in the specimen, travel away from the specimen. A special detector registers the electrons coming from the sample and uses the information to construct an image of the specimen’s surface.

Not all fossils are giant dinosaur bones, and SEM imagery can be used to get a closer look at the very small ones, like microscopic grains of pollen or tiny ostracode shells.