Passage I

Scientists examined a continuous 40,000-year fossil record of a snail species that lives today on Bonin Island in the Pacific Ocean. Fossil snail shells, along with living snails' shells, were studied to determine how the snails have changed over time.

Twenty complete fossil snail shells were randomly extracted from each rock layer on the island. The shells were cleaned, photographed, and measured. A small piece of each shell was used to determine the shell's age by radiocarbon dating.

Study 1

Living snails had dark-colored stripes on their light-colored shells. The stripes on the fossil shells were not vis­ible to the naked eye, but could be seen when the shells were viewed under ultraviolet light. The pattern of stripes on each shell was recorded, starting at the top of the shell. Any stripe having a width less than the height of the shell was called a line (L) and any stripe exactly the height of the shell or wider was called a band (B). The ages of the shells and the percent of the shells of each age with a given pattern of stripes are shown in Table 1.

Study 2

The number of whorls (turns of the shell as viewed from above), the inner lip length, and the diameter of the shell were measured for each shell (see Figure 1).

The average of each of the 3 geometric characteristics for shells of a given age was calculated. These results are shown in Figure 2.

Table and figures adapted from Satoshi Chiba, "A 40,000-year Record of Discontinuous Evolution of Island Snails." ©1996 by The Paleontological Society.