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Comparative Study
. 2006 Sep 7;273(1598):2107-11.
doi: 10.1098/rspb.2006.3577.

Evolutionary patterns in early tetrapods. I. Rapid initial diversification followed by decrease in rates of character change

Marcello Ruta  1 , Peter J Wagner, Michael I Coates
Affiliations
Comparative Study

Evolutionary patterns in early tetrapods. I. Rapid initial diversification followed by decrease in rates of character change

Marcello Ruta et al. Proc Biol Sci. .

Abstract

Although numerous studies have examined morphological diversification during major radiations of marine taxa, much less attention has been paid to terrestrial radiations. Here, we examine rates of character change over phylogeny and over time for Palaeozoic limbed tetrapods. Palaeozoic tetrapods show significant decreases in rates of character change whether the rate is measured per sampled cladistic branch or per million years along phylogeny. Given changes per branch, rates decrease significantly from the Devonian through the Pennsylvanian, but not from the Pennsylvanian through the Permian. Given changes per million years, rates decrease significantly over each boundary, although the decrease is least significant over the Pennsylvanian-Permian boundary. Decreasing rates per million years through the Permian might be an artefact of the method being able to ascribe longer durations to Permian branches than to Carboniferous ones; however, it is difficult to ascribe the general pattern of decreasing rates of change over time to sampling biases or methodological biases. Thus, the results implicate biological explanations for this pattern.

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Figures

Figure 1
Figure 1
Example phylogeny. A–F represent observed taxa, g and h represent reconstructed ancestral morphotypes. ‘Unit’ refers to chronostratigraphic units.
Figure 2
Figure 2
Rates of character change (Δ). (a) Frequencies of character change per cladistic branch. Grey circles give only those branches that did not necessarily diverge prior to that interval; the corresponding τ and p are given in table 1. (b) Approximated frequencies of character change per million years (Myr) per branch. These are given on a log scale due to the apparent exponential decrease in rates. Partitions separate Devonian, Mississippian, Pennsylvanian and Permian branches. Dashed regression lines give the relationship between time and change for the entire study; solid lines give relationships between time and change over adjacent intervals. Time-scale from Gradstein et al. (2004).
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