Dinosaurs are animals that evolved into many sizes and shapes.
All too often books written for a popular audience include animals such as mammoths, mastodons, plesiosaurs, ichthyosaurs, and the sail-backed Dimetrodon . Dinosaurs are a specific subgroup of the archosaurs , a group that includes crocodiles and birds, whereas mammoths and mastodons are mammals . Other archosaurs included the pterosaurs , relatives of dinosaurs but not true dinosaurs. More distantly related to true dinosaurs were the marine plesiosaurs and ichthyosaurs . These were marine reptiles, not dinosaurs or even close relatives of them.