Abstract
The fossil and geologic records provide the primary data used to established absolute timescales for timetrees. For the paleontological evaluation of proposed timetree timescales, and for node-based methods for constructing timetrees, the fossil record is used to bracket divergence times. Minimum brackets (minimum ages) can be established robustly using well-dated fossils that can be reliably assigned to lineages based on positive morphological evidence. Maximum brackets are much harder to establish, largely because it is difficult to establish definitive evidence that the absence of a taxon in the fossil record is real and not just due to the incompleteness of the fossil and rock records. Five primary methods have been developed to estimate maximum age brackets, each of which is discussed. The fact that the fossilization potential of a group typically decreases the closer one approaches its time of origin increases the challenge of estimating maximum age brackets. Additional complications arise: 1) because fossil data actually bracket the time of origin of the first relevant fossilizable morphology (apomorphy), not the divergence time itself; 2) due to the phylogenetic uncertainty in the placement of fossils; 3) because of idiosyncratic temporal and geographic gaps in the rock and fossil records; and 4) if the preservation potential of a group changed significantly during its history. In contrast, uncertainties in the absolute ages of fossils are typically relatively unimportant, even though the vast majority of fossil cannot be dated directly. These issues and relevant quantitative methods are reviewed, and their relative magnitudes assessed, which typically correlate with the age of the group, its geographic range, and species richness.
Highlights
Developing rigorous methods for using paleontological and geological data to estimate divergence times between lineages has proven challenging
Quantifying how much older divergence times are than First Appearance Datum (FAD) is challenging because there is no positive evidence that a taxon existed a given temporal distance beyond its know temporal range; it is hard to establish whether the absence of the taxon is real or just due to the incompleteness of the fossil record
The stochastic nature of the fossil record means that the gap size between FADs and true divergence times will be heterogeneous in size, which becomes relevant when generating timetrees with methods that use uncorrelated rates of molecular evolution, and when contemplating the use of cross-validation approaches
Summary
Developing rigorous methods for using paleontological and geological data to estimate divergence times between lineages has proven challenging. Paleontological Evaluation of Timetree Timescales methods for constructing timetrees (see Donoghue and Yang, 2016 for a review), even though these do not require a priori maximum estimates of divergence times, for they still need to make assumptions about the rates of fossil recovery (Warnock et al, 2017) These methods include the Fossilized Birth Death (FBD) process (Heath et al, 2014; Stadler et al, 2018), total evidence methods that simultaneously estimate the phylogenetic position of the extant taxa and relevant fossils (Pyron, 2011; Ronquist et al, 2012), and integration of the FBD and total evidence methods (Zhang et al, 2016; Gavryushkina et al, 2017)
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