The evolutionary tree of dinosaurs contains a great deal of information. The horizontal axis represents time, and the vertical axis represents dinosaurian diversity. All the major families of dinosaurs are indicated with horizontal lines that record their known distribution in time, as based upon present fossil evidence. Future finds of dinosaur skeletons may extend the time ranges backwards or forwards in time.
There are two “fixed” time lines that do not seem to be breached, however. The dinosaurs arose from a single ancestor some 230 million years ago, in the Middle to Late Triassic, and it is unlikely that older dinosaur skeletons will be found. The second “fixed” time line corresponds to the extinction of the last dinosaurs 66 million years ago. Despite strenuous efforts to find post-Mesozoic dinosaur specimens, and many reports of supposed discoveries, no such remains have withstood close scrutiny. Most usually, post-mesozoic dinosaur bones have been reworked, that is, removed from the rock by natural erosion and redeposited in a younger sediment.
The most important aspect of this phylogenetic tree is the representation of the relationships between the different families. This is based on recent cladistic anlyses, carried out by a number of North American and European vertebrae paleontologists after 1985, and the pattern shown here is quite revolutionary in the sense that it is dramatically different from anything in popular books of this sort. It is also important because it shows a much higher degree of resolution than the earlier phylogenies; that is, the pattern of relationships is shown in a much more detailed way than was possible before.
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