Chapter 26 phylogeny and the tree of life. Defining species in terms of measurable anatomical criteria. Example: birds and crocodiles share a number of anatomical features such as mandibular fenestra (a hole in the lower jaw). This along with molecular features were use to combine birds and crocodiles into the classification archosauria. Since most forms of life are extinct, fossil records are used to determine the evolutionary relationships between species who are extinct (such as dinosaurs) The structure of the fossil is studied to determine the relationships between species. Fossils show how forms adapted from one species to another. Some of the most convincing evidence linking birds and dinosaurs come from the discovery of feathered dinosaur fossil specimens. Fossils show that different eras are dominated by different groups of organisms. The classification of organisms is now heavily depended on this. Computer programs are used to determine similarities between molecular characteristics.