Pterosaur Darwinopterus (image) University of Southampton Share Print E-Mail Caption The image shows sexual dimorphism in the pterosaur Darwinopterus. Males had large head crests while females were crestless. This dimorphism is a clear indication that sexual selection pressure was shaping the evolution of these animals. Credit Mark Witton Usage Restrictions None Share Print E-Mail 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.