Bivalves Before Low O2 (IMAGE) University of Texas at Austin Caption Before the low oxygen period, bivalves were larger and more numerous. Credit The University of Texas at Austin/Rowan Martindale Usage Restrictions Credit image 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.