Body size trends in the Ordovician to earliest Silurian of the Oslo Region
Abstract Body size is an important ecological parameter that can be understood in both evolutionary and environmental terms. We present a database of changes in overall body size in brachiopods and trilobites through the Ordovician and earliest Silurian of the Oslo Region, Norway. In contrast with global studies, the limited geographical extent of our data allows correlation with environmental parameters such as climate and water depth, and simplifies taxonomical standardization and stratigraphical correlation. In our data set there is a clear indication of increasing size both for brachiopods and trilobites during most of the Ordovician, although this could result from an unbiased random walk. The size increase is followed by a reduction in size during the latest Ordovician. Trilobites show a strong increase followed by a decrease in body size during the Middle Ordovician, while brachiopod body size peaks later, in the Late Ordovician. These changes are partly correlated with the changes in species diversity in the Oslo Region. We discuss the results in terms of two models for change in body size — Cope's rule and the Lilliput effect. Cope's rule is the theory of an increasing body size through geological time, while the Lilliput effect describes a decrease in body size in the aftermath of a mass extinction or a severe environmental disturbance. We find no clear correlations between body size and sea level.