Abstract
Studies of size trends have increased in recent times. This work continues a growing number of studies of the Ordovician and the Ordovician-Silurian transition. The Ordovician is a period of Earth`s history with severe environmental changes, large scale plate movements and one of the five largest mass extinctions in Earth’s history. Four central problems in evolution are addressed here. First, Cope´s Rule is the theory of an increasing body size through geological time. Second, the Lilliput Effect is an interruption of Cope´s Rule and describes a decrease in body sizes in the aftermath of a mass extinction or a severe environmental disturbance. Third, the distinction between passive and driven evolutionary trends is discussed. Finally, the distribution of species sizes within a clade has long been debated. Here the distribution of species body size in mm2 and log(mm2) is presented for Baltic stages through the Ordovician and the first stage in Silurian. Overall body size trends are presented for brachiopods and trilobites. There is a clear indication of an increasing size trend for brachiopods during the Ordovician towards the end Ordovician mass extinction. The body size trend in trilobites indicates an increase during the Early and Middle Ordovician. The data presented here show strong support for a Lilliput effect in both brachiopods and trilobites. This may be a result of a locally severe ecological effect of the environmental changes that follow the end Ordovician ice age.