cyclostome monophyly (Stock and Whitt 1992; Mallatt and Sullivan 1998; Mallatt et al.
2001), although analysis of RNA datasets partitioned into small- and large-subunit
components provides conflicting support for both hypotheses (Zrzavý et al. 1998).
Resolution of the interrelationships of hagfishes and lampreys is critical to understanding
character evolution at the origin of vertebrates and gnathostomes. However, both groups
have a comparable fossil record, and there are no known intermediate taxa with a
fundamentally earlier or later first appearance in the fossil record than sister and ingroup
clades (the reality is quite the opposite). Thus, the difference between the two most likely
resolutions of hagfish-lamprey-jawed vertebrate interrelationships is not critical to our
understanding of the timing of early chordate diversification or the relationship between
the fossil record and molecular clocks.
Stratigraphic analysisAlthough stratigraphic range data are often taken at face value in attempts to calibrate
molecular clocks and phylogenetic trees, and to provide palaeontological estimates for the
divergence times, various techniques exist to provide confidence limits on the first and/or
Figure 10.2 The interrelationships of living and extinct groups of chordates and their nearest living
relatives.
200 THE ORIGIN AND EARLY EVOLUTION OF CHORDATES