Elephant Shark (image) Washington University School of Medicine Caption Byrappa Venkatesh, Ph.D., holds an elephant shark, one of the world's oldest-living jawed vertebrates. Sequencing its genome offers new clues to why the skeleton of this fish is made of cartilage rather than bone and how the immune system evolved in higher organisms. Credit Byrappa Venkatesh, Ph.D. Usage Restrictions Credit to Byrappa Venkatesh, Ph.D. 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.