Paleontology dating methods
Recently, authors have suggested an age range from 330 to 288 MYA at most (Lee 1999; Reisz and Muller 2004; van Tuinen and Hadly 2004).
Fossils can provide good “minimum” age estimates for branches in the tree, but “maximum” constraints on those ages are poorer.
The second step is to date the geological formation in which the oldest fossil, or fossils, occurs, or occur.
The identity of that geological formation is clear in all cases—the earliest members of the Zebrafish (ostariophysean) and Pufferfish (euteleost) lineages, for example, both date from the lithographic limestones of the Obere Solnhofener Schichten of southern Germany.
Deviations from the molecular clock may occur because of changes in selective pressures and mutation rates, and this requires that molecular clock analyses rely upon a law of large numbers in which an average rate may be derived from a data set that is sufficiently large (Rodríguez-Trelles et al. It is still debated whether an analysis based on many genes and few dates or few genes and many dates is preferable.
However, multiple calibration points are particularly helpful in relaxed-clock methods where the rate is allowed to vary among branches in the tree; multiple calibrations throughout the tree act as anchor points, allowing the method to estimate the patterns and degree of rate variation more accurately.