the timing of divergence of the extant lineages of birds (Neornithes sensu Cracraft 1988;
alternatively ‘crown-clade’ Aves sensu Gauthier 1986; Figure 12.1).
The timing of the divergences of, and within, the Neornithes remains very
controversial. Debates have revolved around the question of the timing of the
appearance of these clades (i.e. the extant orders and families; Monroe and Sibley 1990),
and the extent to which clades of Neornithes had diversified by the end-Mesozoic, marked
by the Cretaceous-Tertiary (K-T) extinction event. The classical hypothesis, based on the
published fossil record (Unwin 1993; Hope 2002), and as outlined, for example, by
Olson (1985) and Feduccia (1999), states that a specific modern bird ‘morphotype’ was
present at low diversity during the Mesozoic—the so-called ‘transitional shorebirds’ or
‘waterbirds’ (Feduccia 1999)—and survived the end-Cretaceous extinction. All the non-
neornithine lineages were wiped out as a result of the K-T boundary event (Feduccia
1995, 1999; Figure 12.1). The second hypothesis, to a large extent based on lineage
divergence times estimated from molecular sequence data and making use of ‘molecular
clocks’ suggests that the majority, if not all, of the major clades of Neornithes diverged
during the Mesozoic (e.g. Hedges et al. 1996; Cooper and Penny 1997; Rambaut and
Bromham 1998; van Tuinen et al. 1998, 2000). By use of different datasets, these works
Figure 12.1 Consensus phylogeny (redrawn, with permission, from Chiappe 2001) depicting the
phylogenetic relationships within Aves and showing Neornithes at the crown (see text for details;
for details of competing hypotheses see Chiappe and Dyke 2002).
270 GARETH J.DYKE