Fig. 4

a Simplified phylogeny of living terrestrial vertebrates (or tetrapods). Note that the three living mammal groups (monotreme mammals, marsupial mammals, and placental mammals) all share a more recent common ancestor with each other than any does with the reptiles and that mammals and reptiles represent separate lines of descent from a common ancestor. Also note how patterns of shared ancestry are used to delimit the groups Mammalia, Reptilia, Amniota, and Tetrapoda. The position of caecilians on this tree is uncertain; they may belong on the branch with frogs and salamanders or they may belong on the branch leading to the Amniota [e.g., compare the phylogenies of Ruta and Coates (2007) to that of Anderson et al. (2008)]. b Simplified phylogeny of tetrapods to which a number of extinct amniotes have been added. Groups with living members are shown in bold. Note that birds are most closely related to non-avian saurischian dinosaurs on this tree, reflecting that birds are descendants of a dinosaur ancestor. Also note the large number of extinct groups of non-mammalian synapsids, which are descendants of the common ancestor of all synapsids, and thus are more closely related to living mammals than they are to any reptiles. In turn, this fact means that terms like mammal-like reptile do not accurately describe these animals because they are not part of the reptile line of descent. Dimetrodon is a sphenacodontid