Telling the Evolutionary Time: Molecular Clocks and the Fossil Record

Chapter 11

Bones, molecules, and crown-tetrapod

origins

Marcello Ruta and Michael I.Coates

ABSTRACT

The timing of major events in the evolutionary history of early tetrapods is

discussed in the light of a new cladistic analysis. The phylogenetic implications of this

are compared with those of the most widely discussed, recent hypotheses of basal

tetrapod interrelationships. Regardless of the sequence of cladogenetic events and

positions of various Early Carboniferous taxa, these fossil-based analyses imply that

the tetrapod crown-group had originated by the mid- to late Viséan. However,

such estimates of the lissamphibian-amniote divergence fall short of the date

implied by molecular studies. Uneven rates of molecular substitutions might be

held responsible for the mismatch between molecular and morphological

approaches, but the patchy quality of the fossil record also plays an important role.

Morphology-based estimates of evolutionary chronology are highly sensitive to new

fossil discoveries, the interpretation and dating of such material, and the impact on

tree topologies. Furthermore, the earliest and most primitive taxa are almost

always known from very few fossil localities, with the result that these are likely to

exert a disproportionate influence. Fossils and molecules should be treated as

complementary approaches, rather than as conflicting and irreconcilable methods.

Introduction

Modern tetrapods have a long evolutionary history dating back to the Late Devonian.
Their origins are rooted into a diverse, paraphyletic assemblage of lobe-finned bony fishes
known as the ‘osteolepiforms’ (Cloutier and Ahlberg 1996; Janvier 1996; Ahlberg and
Johanson 1998; Jeffery 2001; Johanson and Ahlberg 2001; Zhu and Schultze 2001). The
monophyletic status of the Tetrapoda and that of its major constituent clades—
lissamphibians and amniotes—is supported by a large number of morphological characters
and by a wide range of molecular data (e.g. Duellman and Trueb 1986; Panchen and
Smithson 1987, 1988; Duellman 1988; Gauthier et al. 1988a,b; Milner 1988, 1993;
Hedges et al. 1990; Carroll 1991; Trueb and Cloutier 1991; Hedges and Maxson 1993;
Hay et al. 1995; Laurin and Reisz 1997, 1999; Feller and Hedges 1998; Laurin 1998a–c;
Hedges and Poling 1999; Pough et al. 2000). The early evolutionary history of the
lissamphibian and amniote crown-groups has been the subject of detailed scrutiny (e.g.