Stratigraphic Range Chart for some of the taxa discussed in this lab. You will note that some of the taxa (living cycad genera, Cycas and Zamia) are represented by dashed lines. These are not actual, but inferred, ranges. In other words, we assume that these lineages must have existed because their sister taxa are found in the fossil record. However, we do not have actual fossil examples of the dashed taxa. Taxa in this situation are sometimes called "ghost taxa" because we beleive that the lineages must have existed, but we have no physical substance (fossil record) for them.
An important issue to keep in mind when thinking about ghost taxa and their distributions: Although we believe the lineages must have been independent from the earlier time because we have fossil evidence of their sister taxa, this doesn't necessarily mean that the early representatives of the lineages in question looked like their modern representatives. This point is particularly important as we begin to study the origin of the angiosperms. The sister group of the angiosperms -- the Gnetales -- dates to at least the Triassic, but we do not see clear evidence of flowering plants until the Early Cretaceous. By the ghost taxa argument, the angiosperm lineage must have been present during the Triassic and Jurassic. However, the early representatives might not have accumulated all the synapomorphies that would allow us to recognize them as angiosperms in the fossil record.