Reconstructing the anatomy of fossil mollusks
Although Hickman was first drawn to snails, clams, and other mollusks because of their beautiful shells, she developed an interest in the anatomy of their “soft parts” [muscles, internal organs, etc.] because they provide clues about the animals’ behavior and ecology.

Figuring out how muscles and other soft tissues looked and functioned can be difficult if the shell is the only part of the animal that remains. Hickman overcomes this obstacle by looking closely at marks these soft tissues have created on the inside surface of the shell, as illustrated below (click for an enlarged version).

For example, indentations on the inner surface reveal where muscles once attached to the shell. According to Hickman, the way that muscles attach to the shell tells us much about the range of movements that were possible for the living animal.

Hickman can also figure out how deeply a clam was digging into sand on the ancient sea floor by the length of its siphon, another soft organ that leaves revealing marks on the shell. The angle of the mark left by the siphon tells Hickman the position the animal assumed when buried in the sand, as illustrated at left (click for an enlarged version).

Investigating past interactions
Sometimes, the patterns Hickman observes can reveal how organisms might have interacted. She has traveled to localities such as Rottnest Island, in the Indian Ocean off Western Australia, and the Channel Islands, located off the coast of southern California, to study spatial relationships of shells in fossil beds.

The shells’ spatial arrangement and condition (complete or broken) provide clues about how they got there. Most shell accumulations are the result of transport and concentration by water. But Hickman is particularly interested in accumulations that have been created by predators. She believes that human predators created the shell piles (middens, such as the image above) she has encountered in the Channel Islands, but that some of the Rottnest Island snail operculum and shell piles, which had first been interpreted as middens, were actually made by sea gulls.

Hickman explains that in the case of the Rottnest Island shells and opercula, “These are accumulations in which two distinctive parts of the same animal have become separated into different areas. We are able to infer very strongly from the nature of the accumulation that sea birds were responsible for their creation. They were cracking shells over in one place and carrying the soft parts over to another place (with the opercula still attached) to eat it. Then the birds barfed up the undigested opercula, creating an accumulation separate from the shells.”

Visit Bioforum 2000 for more about what Hickman learns from fossils in the field, including sites such as this shell accumulation on Rottnest Island.