Their data base is almost entirely morphological, and many cladists explicitly reject information from the stratigraphic record. Trees are hypotheses of phylogeny, and morphological and stratigraphic data are both vitally important in their formulation. Both of these styles of hypothesis seek to answer questions about what happened in evolution. They are descriptive, narrative hypotheses based on data that contain as little interpretation as possible. They are testable as hypotheses when new pieces of comparable data become available.
Scenarios are different kinds of hypotheses.
By necessity they are interpretative, and will normally include as a central theme a suggestion for a mechanism of selection, radiation, or extinction, of a clade or within a clade. The data base for a scenario will include not only morphology and stratigraphy, but also biological, ecological, and environmental reconstruction. Because the hypothesis seeks to explain rather than describe, it can rarely be tested directly, but only by testing its implications and correlates. For this reason, and because a scenario is explicitly interpretative, it has often been regarded as less rigorous than a cladogram or a tree. Critics have suggested that a scenario should not be attempted until there is general agreement on the cladogram or tree that describes an evolutionary event.