Was there an EB radiation at the base of the Ichthyosauria?

Moon and Stubbs 2020 bring us a discussion on
the EB (early burst) radiation of ichthyosaurs shortly after their genesis (Fig. 1).

the authors did not know which taxon was the last common ancestor and which taxa formed the proximal outgroups.

Figure 1. Cladogram from Moon and Stubbs 2020.

Figure 1. Cladogram from Moon and Stubbs 2020.

The authors
chose as their outgroup, Hupehsuchus nanchangensis, a derived ingroup member of the ichthyosaur radiation according to the large reptile tree (LRT, 1647+ taxa; Fig. 2).

Verified outgroups not included in the cladogram
include the clades Thalattosauria, Mesosauria and Pachypleurosauria (Fig. 2).

The authors mistakenly included
Cartorhynchus and Sclerocormus as ichthyosaurs. The LRT recovered these two taxa as ichthyosaur mimics at the base of the Sauropterygia, alongside Qianxisaurus (not in the taxon list).

Other ichthyosaur basal in-groups recovered by the LRT
and not mentioned by Moon and Stubbs include Wumengosaurus and Thaisaurus.

Figure 3. Aquatic younginiform subset of the LRT demonstrating relationships within the Enaliosauria (=Sauropterygia + Ichthyosauria)

Figure 2. Aquatic younginiform subset of the LRT demonstrating relationships within the Enaliosauria (=Sauropterygia + Ichthyosauria).

The LRT recovered
Wumengosaurus as a basal ichthyosaur in 2011. It nested Cartorhynchus with the pachypleurosaur, Qianxisaurus in 2014. These have been available with a quick Google search for the last several years. You can say the LRT is not a recognized resource, but then you run the risk of excluding appropriate taxa and including inappropriate taxa, as Moon and Stubbs did. Their taxon list fell short of optimal and thereby opened up their study for criticism on those grounds.

The referees for this paper were more generous in their appraisal:

  1. This is an excellent, well-written, thoroughly executed and smartly illustrated study that rigorously analyses patterns of disparity and diversity throughout the evolution of ichthyosaurs. It will be of great interest to those working on marine reptiles, as well as all those interested in clade dynamics. I have nothing of any significance to criticise, and recommend publication more or less as the manuscript stands – in many ways it is a model.
  2. This is a well written, well executed and very scholarly contribution to macroevolutionary studies of ichthyosaurs…Their paper is a detailed tour-de-force through the analysis of ichthyosaur morphological diversification in a three-pronged approach to the study of evolutionary rates, disparity, and morphological space occupation… It is well written and provides ample documentation for data and result reproducibility. I have no qualms with this paper.
  3. This is a great paper with a solid and well used methodology… The authors did a great job with sharing the code and the data to reproduce this paper. Although this should be a standard in science, it is unfortunately done well and in depth way to rarely. Well done! 

Since this paper deals with comparative measurements 
the Moon and Stubbs data represent a resource that can be used for future workers, who should consider expanding their taxon set a wee bit and get rid of interlopers.

Bottom line: Was there an EB radiation at the base of the Ichthyosauria?
In the LRT it is not apparent that ichthyosaurs had an early burst (EB) radiation and disparity when Cartorhynchus and Sclerocomus are excluded, as they should be. Rather, when appropriate taxa are included and inappropriate taxa are excluded, only microevolution is apparent (i.e. a gradual accumulation of derived traits).

Moon BC and Stubbs TL 2020. Early high rates and disparity in the evolution of ichthyosaurs. Communications Biology 3, Article number: 68
doi: https://doi.org/10.1038/s42003-020-0779-6




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