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A timetree is a phylogenetic tree scaled to time. It shows the evolutionary relationships of a group of organisms in a temporal framework. Therefore, if living organisms are represented, the branch length between the base of the tree and all leafs (e.g., species) is identical because the same time has elapsed, although extinct organisms can be shown in a timetree. As with a phylogenetic tree, timetrees can be drawn in different shapes: rectangular, circular, or even spiral. The only figure in Darwin's On the Origin of Species, one of the earliest printed evolutionary trees, is a hypothetical timetree. Because the fossil record has always been tightly linked to the geologic record, evolutionary trees of extinct organisms are typically illustrated as timetrees.