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.2013 Aug 28;280(1769):20131865.
doi: 10.1098/rspb.2013.1865. Print 2013 Oct 22.

The radiation of cynodonts and the ground plan of mammalian morphological diversity

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The radiation of cynodonts and the ground plan of mammalian morphological diversity

Marcello Ruta et al. Proc Biol Sci..

Abstract

Cynodont therapsids diversified extensively after the Permo-Triassic mass extinction event, and gave rise to mammals in the Jurassic. We use an enlarged and revised dataset of discrete skeletal characters to build a new phylogeny for all main cynodont clades from the Late Permian to the Early Jurassic, and we analyse models of morphological diversification in the group. Basal taxa and epicynodonts are paraphyletic relative to eucynodonts, and the latter are divided into cynognathians and probainognathians, with tritylodonts and mammals forming sister groups. Disparity analyses reveal a heterogeneous distribution of cynodonts in a morphospace derived from cladistic characters. Pairwise morphological distances are weakly correlated with phylogenetic distances. Comparisons of disparity by groups and through time are non-significant, especially after the data are rarefied. A disparity peak occurs in the Early/Middle Triassic, after which period the mean disparity fluctuates little. Cynognathians were characterized by high evolutionary rates and high diversity early in their history, whereas probainognathian rates were low. Community structure may have been instrumental in imposing different rates on the two clades.

Keywords: cynodonts; disparity; diversity; morphospace; phylogeny; rates.

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Figures

Figure 1.
Figure 1.
Cynodont tree plotted onto a stratigraphic scale; rectangular bars or dots show the known observed ranges of taxa; e, early; m, middle; l, late; each taxon is identified by a number, for ease of comparisons with the plots in figure 2a–c. For stage abbreviations, see §2d.
Figure 2.
Figure 2.
Cynodont morphospace, with taxon identification numbers as in figure 1. (a–c) Phylomorphospace two-dimensional plots using the PCo1–3 axes. (d)K function (solid line) for taxon distribution in the three-dimensional space delimited by axes PCo1–3; the dashed line is the theoreticalK function for a random 54-point distribution; the grey area is the confidence envelope; ther distances on the horizontal axis are dimensionless. (Online version in colour.)
Figure 3.
Figure 3.
Unrarefied mean disparity with 95% CIs for four indices. (a–d) Group disparity (B, basal taxa; C, cynognathians; P, probainognathians). (e–h) Temporal disparity in intervalst1–t11 (see main text).
Figure 4.
Figure 4.
Plots of rates versus time, including regression line and equation, and the strength and significance of Kendall's correlation. (a) ACCTRAN and (b) DELTRAN rates for all taxa. (c,d) ACCTRAN rates for (c) probainognathians and (d) cynognathians. (e,f) DELTRAN rates for (e) probainognathians and (f) cynognathians.
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