Telling the Evolutionary Time: Molecular Clocks and the Fossil Record

analyses based on morphological characters and synopses of phylogenetic relationships
within basal Neornithes (e.g. Bledsoe 1988; Cracraft 1988; Livezey 1986, 1997, 1998;
Ericson 1997; Lee et al. 1997; Cracraft and Clarke 2001; Livezey and Zusi 2001).
References to avian taxonomic groups follow the standard checklist of Monroe and Sibley
(1990). I will then discuss the implications of these fossil records for addressing the timing
of basal neornithine divergences within the context of ‘molecular clock’ analyses.

Institutional abbreviations

BMNH PAL—The Natural History Museum, London, England (Palaeontology
Department Collection); GM—Geiselthal Museum, Halle, Germany.

Fossils and the basal clades of the Neornithes

Although little is known about the phylogenetic interrelationships of the Neornithes based
on morphological characters (especially those relevant to the inclusion of fossils), at least
the basal clade relationships of extant birds are relatively uncontroversial. A basal split
within the group has consistently been recovered through analysis of both molecular and
morphological datasets, forming the two major clades Palaeognathae (palaeognaths,
comprising the living ratites and tinamou) and Neognathae (the remaining clades of extant
birds). Within the more diverse of these two clades, the Neognathae, a second division in
the phylogeny is well-supported dividing the sister-taxa Galli-formes and Anseriformes
(collectively termed Galloanserae; Sibley et al. 1988; Groth and Barrowclough 1999) from
all other modern birds (this remaining clade is commonly referred to as Neoaves; Sibley et
al. 1988; Cracraft and Clarke 2001) (Figure 12.2).

Figure 12.3 The phylogenetic placement of fossils in either the stem-group or crown-group of a
particular clade (re-drawn with permission from Magallón and Sanderson 2001, figure 1). The
relative phylogenetic placements of fossils imply minimum ages for divergences within large clades—
the position of fossil taxa relative to either the stem- or crown-group cannot be known except
through phylogenetic analysis.

BASAL RADIATIONS WITHIN THE NEORNITHES 273