Leaf Pores Clue to Ancient Carbon Dioxide (IMAGE) Southern Methodist University Caption Researchers measured and counted the leaf pores and epidermal cells on well-preserved fossil leaves from the Highlands of Ethiopia to determine the concentration of carbon dioxide present in the Earth's atmosphere 22 million years ago and 27 million years ago. (Credit SMU) Credit SMU Usage Restrictions Please credit License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.