Telling the Evolutionary Time: Molecular Clocks and the Fossil Record

Chapter 12

The fossil record and molecular clocks:

basal radiations within the Neornithes

Gareth J.Dyke

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 well-

preserved 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.

Introduction

The debate—molecules and fossils

The more than 10,000 species of modern birds are the living descendants of a large and
ancient radiation than can be traced back 150 million years to the earliest bird,
Archaeopteryx, from the Late Jurassic. The taxonomic diversity and genealogical
relationships of the early birds, the origin and refinement of flight, and the origin of avian
functional and physiological specialization are just some of the evolutionary issues that
have captured the interest of decades of palaeo-ornithological research (for reviews, see
Chatterjee 1997; Padian and Chiappe 1998; Feduccia 1999; Chiappe and Dyke 2002).
Recently, however, much research interest has been focused on addressing the question of