Telling the Evolutionary Time: Molecular Clocks and the Fossil Record

analyses as scientific hypotheses for testing, and the inclusion of more sequences
representative of basal chordates and sister groups are likely to provide better constraints
on their time of origin. A more realistic attempt to assess errors on molecular clock
estimates is required and this can be developed in hand with more rigorous assessments of
the palaeontological data used in calibrating molecular clock analyses. However, unless
these errors can be reduced, molecular clock estimates will remain of low practical value;
the palaeontological record is imperfect but nevertheless provides the only firm constraint
on the timing of clade divergence.

Acknowledgements

Philippe Janvier (Museum National d’Histoire Naturelle, Paris) and Mike Coates
(University of Chicago) provided useful reviews of the manuscript. Donoghue was funded
through NERC Post Doctoral Research Fellowship GT5/99/ES/2; Smith was funded
through NERC Research Grant NER/B/S/2000/00284, and Smith and Sansom were
funded through NERC Research Grant GR3/10272.

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