Pearson and Palmer (2000) use boron-isotope ratios of ancient plankton shells to reconstruct atmospheric CO2 concentrations for the past 60 million years. They conclude that atmospheric CO2 concentrations may have been as high as 2,000 ppm from about 60 to 52 million years ago. Concentrations in the most recent 24 million years have remained below 500 ppm and have been more stable than previous periods when outgassing from ocean ridges, volcanoes and metamorphic belts kept atmospheric levels high and erratic.
Reference
Pearson, P. N., and M. R. Palmer, 2000: Atmospheric carbon dioxide
concentrations over the past 60 million years. Nature, 406,
695-699