Hedges (1998; and Wang et al. 1998) that are based, in part, upon aldolase and TPI,
which are analysed in Hedges (2001). Given that the only palaeontological calibration
point used by Kumar and Hedges (1998) is the bird-mammal divergence date of 310 Ma,
Hedges (2001) has, in effect, calibrated multiple times, directly and indirectly, using a
single palaeontological datum.
No molecular estimates have been calculated for the divergence of invertebrate
craniates and vertebrates. This is because molecular phylogenies consistently resolve
hagfishes and lampreys as monophyletic and, hence, there is no distinction between
craniates and vertebrates within the field of molecular systematics. However, Hedges
(2001) used a subset of his data to provide an estimate for the divergence of hagfishes and
lampreys within the context of cyclostome monophyly and arrived at a date of 499±36.8
Ma, using the lamprey-gnathostome molecular estimate of 564 Ma (Kumar and Hedges
1998) as the calibration point. For once, palaeontological data indicate two older spot
dates (530 Ma on Myllokunmigia and Haikouichthys) with a slightly younger and better-
constrained date of 495 Ma bracketing the molecular estimates and falling well within the
standard error on the molecular estimate.
Overall, in consideration of the origin of chordates and divergence of the lower
chordate groups, there is poor correlation both between molecular and palaeontological
estimates, and between molecular estimates that are based upon different datasets. Thus,
in the absence of any independent constraint that the fossil record may otherwise afford, it
would be dangerous to conclude anything other than the fact that chordates, craniates, and
Figure 10.6 Observed stratigraphic range data including inferred ghost lineages and molecular
estimates on the divergence of various clades (from Kumar and Hedges 1998).
PHILIP C.J.DONOGHUE ET AL. 211