Environmentally controlled body size changes in the Upper Ordovician bivalve Ambonychia

Daley, G.M., 1993
GSA Abstracts with Program 25(3):15

Ambonychia, an epibyssate suspension feeding bivalve, is one of the most common mollusks of the Cincinnatian Series (Upper Ordovician). It is found in depositional environments ranging from low energy, muddy substrates in relatively deep water, to high energy hard grounds above wave base. While overall morphology of Ambonychia seems to change very little, a detailed analysis of the size of individuals within the various depositional environments throughout the Cincinnatian Series shows a significant relationship between changes in Ambonychia shell size and changes in sea level.

Samples of Ambonychia were collected from various facies in each sequence of the Cincinnatian Series. Shell size measurements show a clear correlation between changes in environment, brought about by changes in sea level, and changes in the mean body size of Ambonychia within each facies. Body size is significantly larger in onshore facies than offshore facies. For example, the mean shell length for Ambonychia collected from the Kope Formation (below wave base, low energy facies) is 2.05+/- .16 cm. In contrast the mean shell length in the Bellevue (high energy, above wave base facies in the same sequence) is 4.55 +/- .58 cm. This pattern of Ambonychia shell size increasing as sea level falls is repeated through four regressional sequences in the Cincinnatian Series, and matches published sea level curves for the Cincinnatian Series rather well. Various causes for facies controlled changes in size have been proposed.

In general, the observed range of Ambonychia body size in facies early in the series is smaller than the range of sizes in equivalent facies later in the series. This trend could have any number of causes. What have been termed equivalent facies/environments may actually be radically different in some aspect important to Ambonychia, such as nutrient supply, terrigenous input, or water temperature. Species within the genus Ambonychia could have evolved coping mechanisms that allowed it a wider size variation than previously, or it may simply be a result of the development of greater variability within and between the various species of Ambonychia.