Ostracod Body Size Change Across Space and Time

Abstract:

Many factors drive evolution, although it is not always clear which factors are more influential. Miller et al. (2009) found that there is a change in geographic disparity in diversity in marine biotas over time. We tested if there was also geographic disparity in body size during different epochs.

We used marine ostracods, which are tiny crustaceans, as a study group for this analysis. We also studied which factor is more influential in body size change: distance or time. We compared the mean body size from different geologic time intervals as well as the mean body size from different locations for each epoch. We grouped ostracod occurrences from the Paleobiology Database into 10º x 10º grid cells on a paleogeographic map. Then we calculated the difference in mean size and the distance between the grid cells containing specimens. Our size data came from the Ellis & Messina“Catalogue of Ostracod” as well as the“Treatise on Invertebrate Paleontology”. Sizes were calculated by applying the formula for the volume of an ellipsoid to three linear dimensions of the ostracod carapace (anteroposterior, dorsoventral, and right-left lengths).

Throughout this analysis we have come to the realization that there is a trend in ostracods towards smaller size over time. Therefore there is also a trend through time of decreasing difference in size between occurrences in different grid cells. However, if time is not taken into account, there is no correlation between size and geographic distance. This may be attributed to the fact that one might not expect a big size difference between locations that are far apart but still at a similar latitude (for example, at the equator). This analysis suggests that distance alone is not the main factor in driving changes in ostracod size over time.