Fossil Slab (image) Ohio University Caption This slab of rock contains fossils of invasive species that populated the continent of Laurentia 450 million years ago after a major ecological shift occurred. Ohio University geologists found that rising mountains and cooling oceans prompted the spread of these invasive species. Credit Alycia Stigall Usage Restrictions Please credit Alycia Stigall. 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.