Paleontology dating methods
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.
Because fossil-based dates are constraints, and because molecular evolution is not perfectly clock-like, analysts should use more rather than fewer dates, but there has to be a balance between many genes and few dates versus many dates and few genes.
Some have even gone so far as to suggest that the fossil record is almost entirely an artifact of the rock record, with appearances and disappearances of fossil taxa controlled by the occurrence of suitable rock units for their preservation (Peters and Foote 2001, 2002), or the matching rock and fossil records controlled by a third common cause (Peters 2005).
However, the widespread congruence between the order of fossils in the rocks and the order of nodes in cladograms (Norell and Novacek 1992; Benton et al.
It is not our aim to determine the actual timing of divergence events as we do not believe that this is possible using paleontological data alone—though paleontological data can be used to test dates estimated using molecular clock methods (e.g., Foote et al. The most commonly used calibration node is the mammal–bird divergence, dated at 310 MYA and accepted in some 500 or more publications since 1990.
This date was based on the age of the oldest members of the synapsid and diapsid clades (Benton 1990), and yet these basal fossils have been debated, as has the dating of the rocks from which they come.
In doing this, we argue that paleontological data do not provide actual age estimates for divergence events, but they can provide rather precise minimum constraints on the calibration of molecular clocks, and much looser maximum constraints.