Phylogenies that are reconstructed without fossil material often contain approximate dates
for lineage splitting. For example, particular nodes on molecular phylogenies may be dated
by known geographic events that caused lineages to split, thereby calibrating a molecular
clock that is used to date other nodes. On the one hand, such phylogenies contain no
information about lineages that have become extinct. On the other hand, they do provide a
potentially useful testing ground for ideas about evolutionary processes. Here we first ask …