Placement of a root
according to an evolutionary transformation series

    Morphological analysis of cardiac anatomy indicates that a two-chambered heart (single atrium leading into a ventricle) is structurally simpler than a three-chambered heart (two atria, single ventricle), which is in turn simpler than a four-chambered heart (separate ventricles, each with an atrium). The transformation series for the character "# of heart chambers" is 2  3 4. In a comparison of fish, lizards, crocodiles, and birds, the root will be placed on the fish lineage, which shows the ancestral condition. This character analysis is one of many that indicates a closer relationship of Crocodilia to birds than to other "reptiles".