Palaeontological dating definition
Fossil evidence is used to provide initial estimates for node ages in phylogenetic trees, which are then updated in light of molecular evidence.Yang and Rannala (2006) found that, for divergence time estimates based on sequence data, the largest source of uncertainty came from the fossil calibration dates used.The age of the oldest fossil may be the single most informative piece of data, but other parts of the record can help reduce the uncertainty in any estimate and provide a more nuanced picture.By considering more of the fossil record, we can move from using prior distributions based on the oldest fossil to distributions based on a wider appreciation of the fossil record and its underlying processes.Fossil evidence is the only direct source of information about long-extinct species and their evolution.Morphological similarities between extant species and fossil remains are used to infer the existence of ancestral species during the geological period to which the fossil is allocated.They chose prior distributions by fitting a parametric form (a gamma distribution) to estimated maximum and minimum bounds on the node age, allowing for 5% uncertainty outside this range.
Sequence data from extant species provide an indirect source of information about evolutionary history.
This information is then used to calibrate the molecular clock and thus has consequences for estimation of other node ages.
Early analyses using the molecular clock to date divergence times typically treated the oldest fossil representative as if it estimated the age of the node without error (Graur and Martin 2004, al 2004).
Yang and Rannala (2006) moved beyond the use of uniform distributions for calibration dates to allow general prior distributions, which can incorporate more information about the calibration age.
These allow for most of the probability to be placed on dates near to the minimum bound, but with long tails to account for the a priori unlikely event that the node age is actually much older than the fossil evidence suggested.