Other kinds of invertebrates that are found, but not well represented in the fossil record include sponges, cirripids, annelid worms and, rarely, crustaceans. This map shows the extent of the seaway about 90 million years ago. Although the Smoky Hill Chalk has been a excellent source of well preserved vertebrate remains, the preservation of invertebrates, except Inoceramid clams and rudists, is generally poor. Pearls are occasionally found pressed into the Inoceramid shell . A common clam found in the lower third of the chalk.
Shells are thick and generally bowl shaped.
In many areas, the surface of the chalk is littered with thousands of fragments of this shell, some of which may resemble bone in outward appearance. The edge of a large Cladoceramus undulatoplicatus eroding from the edge of a gully in the lower Smoky Hill chalk. A very large clam shell that occurs throughout the chalk, sometimes reaching more than four feet in diameter. As the name implies, these shells are flat and often very thin. A portion of a large Platyceramus platinus eroding from the Smoky Hill chalk.