Telling the Evolutionary Time: Molecular Clocks and the Fossil Record

explosive evolutionary radiation of Neornithes in the aftermath of the K-T boundary
extinction is still a strong possibility.
As has been pointed out by Benton (1999) and Brochu (2001), even though techniques
for estimating divergence times on the basis of molecular sequence data have become
more and more sophisticated, these methods are still reliant on the fossil record to
provide a timescale, by use of taxa that are either internal or external with respect to the
clade in question. Although problems with the use of fossils for internal calibration are
clear (van Tuinen and Hedges 2001), this approach is clearly preferable to the use of
distant external events (such as the synapsid-diapsid divergence in the Carboniferous),
that is subject to even more sampling bias.

Conclusion

In this chapter I have discussed the phylogenetic evidence for the placement of a number of
well-preserved fossil modern birds from the early Tertiary within the most basal clades of
Neornithes. Although the fossil record of described taxa from the Cenozoic is vast, it is
clear that a degree of phylogenetic control is needed before such evidence can be
incorporated into hypotheses that discuss the timing and extent of the neornithine
radiation. Fossils that can be dated accurately are important for ‘molecular clock’
estimates of divergence but are useless in the absence of phylogenetic context. Work on
the systematic placements of early neornithines is ongoing and depends to a great extent
on improvements in knowledge of the phylogenetic relationships of extant taxa based on
morphological characters. This interval of the fossil record is not, however, entirely
depauperate—in contrast with described Mesozoic records, a large number of specimens
that are well enough preserved to be informatively included within cladistic analyses have
been described from the early Tertiary. Although much further work will be required to
combine morphological information from these taxa within analyses, some degree of
pattern is beginning to emerge.

Acknowledgements

I thank Philip Donoghue and Paul Smith for the invitation to participate in the symposium
‘Telling the evolutionary time: molecular clocks and the fossil record’ held as part of the 3rd
Biennial Meeting of the Systematics Association (London 2001). S.Chapman, L.Claessens,
G.Mayr, A.Milner and P.Sweet provided access to specimens, and P.Crab (Natural
History Museum Photographic Services) took the photographs used in Figure 12.4.
L.Chiappe and S.Magallón kindly allowed reproduction of figures used in their previous
papers. This work was funded by the Frank M.Chapman Memorial Fund, American
Museum of Natural History (Department of Ornithology).

References

Ballmann, P. (1969) ‘Les oiseaux Miocènes de la Grive-Saint-Alban (Isère)’, Geobios, 2: 157–204.
Benton, M.J. (1999) ‘Early origins of modern birds and mammals: molecules vs. morphology’,
BioEssays, 21:1043–51.

280 GARETH J.DYKE

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