| Not everything big and dead is a dinosaur. 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. Dimetrodon is neither a reptile nor a mammal, but a basal synapsid -- that is, an early relative of the ancestors of mammals. |
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