ABSTRACT
The fossil record of the extant clades of birds (Neornithes) is critical to understanding both the timing and pattern of the evolutionary divergences within this major vertebrate group. Interpretations of the fossil record have indicated that this radiation occurred in the aftermath of the Cretaceous-Tertiary (K-T) extinction event. However, the use of ‘molecular clocks’ to estimate the timing of lineage divergences on the basis of sequence data have instead led to proposals that most of the major lineages of modern birds originated deep in the Cretaceous. Use of the neornithine record to address estimates founded on a molecular timescale is necessary, but remains problematic because of uncertainties surrounding the placement of most fossil taxa within existing phylogenetic hypotheses for extant clades. Although variance in the relative position of a fossil within a clade will impact on divergence estimates, few attempts have actually been made to distinguish the placements of such taxa with respect to stem-or crown-groups. Here, I present osteological evidence for the phylogenetic placement of some wellpreserved fossil neornithines from the early Tertiary and discuss the implications of constraining such taxa to the development of ‘molecular clock’ hypotheses for the timing of the divergence of modern birds.
