resolved as more closely related to gnathostomes than lampreys and represent part of the
lineage leading to gnathostomes after its divergence from that last common ancestor
shared with lampreys. However, since they lack jaws, osteostracans cannot be considered
part of the group ‘Gnathostomata’ as perceived by most biologists. Current hypotheses of
early vertebrate relationships indicate that there are many such groups that are more closely
related to gnathostomes than lampreys. Systematists have devised a means of obviating
this state of taxonomic purgatory by allying such lineages with their nearest living group
to form an inclusive ‘total-group’ version of the existing taxonomic concept
(Figure 10.1). Thus, the extinct members of the gnathostome lineage that fall outside of
crown-group Gnathostomata become part of the total-group Gnathostomata, composed
of the paraphyletic ‘stem’-lineage leading to a ‘crown’-group circumscribed by the living
representatives of the Gnathostomata, and including all of their descendants, fossil and
extant. The difference between the time of origin of the total-group and that of the crown-
group (approximately equivalent to the original concept ‘Gnathostomata’) reflects the time
gap between the point of divergence from the lamprey lineage, and the origin of the living
clade, palaeontological estimates for the timing of which differ by as much as 60 million
years. Both events are approachable through molecular clock theory (because the time of
origin of one crown group is also the time of origin of its two constituent total groups (see
figure 10.1)), although they are often confused and/or conflated, and it is usually only the
origin of the total-group that is calculated owing to the ease of data collection and the
reduced reliance upon subsidiary hypotheses of intrarelationships of constituent crown-
group taxa.
Figure 10.1 The relationships between stem-, crown-, and total-groups. TØ is time zero (present
day). * denotes the time of origin of a crown-group and its two constituent total-groups. After
Jeffries (1986).
196 THE ORIGIN AND EARLY EVOLUTION OF CHORDATES