Abiodun Musa
The major information needed to establish absolute timescales for timetrees comes from the fossil and geologic records. The fossil record is used to bracket divergence periods for the paleontological assessment of proposed timetree timescales and for node-based approaches for building timetrees. By using well-dated fossils that may be confidently assigned to lineages based on favourable morphological evidence, minimal brackets (minimum ages) can be established. Maximum brackets are far more challenging to establish, partly because it is challenging to provide conclusive proof that a taxon's absence from the fossil record is genuine and not only the result of incomplete fossil and rock records. The difficulty of determining maximum age brackets is made more difficult by the fact that a group's potential for fossilisation often declines the closer one gets to its time of inception. There are additional challenges:
1) Because fossil data actually bracket the time of origin of the first relevant fossilizable morphology (apomorphy), not the divergence time itself.
2) Because fossil placement is phylogenetically uncertain.
3) Because of peculiar temporal and geographic gaps in the rock and fossil records.
4) If a group's preservation potential changed significantly over time.