Glaciation in the late Ordovician caused the second largest mass extinction in the Phanerozoic, with 26% of families made extinct. It is thought that atmospheric CO2, a major control of climate, was very high during the Ordovician. There are currently two hypotheses vying to explain how such extensive glaciers formed under these conditions.The productivity hypothesis suggests that increased phytoplankton growth drew down CO2 during the Ordovician, leading to cooling. The weathering hypothesis suggests that after a lesser drop in CO2, caused by silicate weathering, CO2 rose as the land was covered in ice, but that the increase in albedo was enough to maintain the cold temperatures. However, a deadlock remains due to the lack of proxy measures of CO2 over this period.Based on laboratory growth experiments, we report that the stable carbon isotope composition of mosses and liverworts is very sensitive to atmospheric CO2 levels. This finding indicates that fossil remains of Ordovician land plants can offer a means of reconstructing past CO2 levels, with the exciting potential of resolving the role of CO2 in the glacial event. Preliminary isotopic analysis of Ordovician mesofossils using an ion-microprobe mass spectrometer indicates high (10 x present) CO2 levels before the glaciation when calibrated against modern plants.