Early high rates and disparity in the evolution of ichthyosaurs

Feb 2020 Communications Biology

Moon, B.C. & Stubbs, T.L. 2020 Early high rates and disparity in the evolution of ichthyosaurs . Communications Biology 3: 68 doi:10.1038/s42003-020-0779-6

Rates of evolution in ichthyosaurs have been considered a few times previously (Fischer et al. 2016, Motani et al. 2017), but not at the level of the whole group and for the 160 million years of their existence. We aimed to combine discrete and continuous characters for all valid ichthyosaur species to examine their evolutionary rates throughout the Mesozoic.

We used cladistic data from a previous phylogeny of ichthyosaurs that I published in 2019 (doi:10.1080/14772019.2017.1394922) and collected additional skull length for 64 ichthyosaur species. Then we put these data through an extensive set of analyses to calculate rates of change from a range of trees produced by Moon (2019).

Rates of cladistic character evolution in ichthyosaurs. Significantly high rates of evolution occur early in ichthyosaur evolution, but are distinctly reduced after the Triassic–Jurassic boundary. (From Moon and Stubbs 2020.)

Ichthyosaurs had an archetypal ‘early burst’ of evolution following their first appearance: high rates of evolution early on in their history. This manifests itself as a rapid diversification in taxonomy, morphology, and ecology during the Early–Middle Triassic followed by relatively slower rates.

Rates of skull length evolution in ichthyosaurs. The most rapid changes in skull length occur early in ichthyosaur evolution before decreasing in the later Triassic and again in the Late Jurassic. (From Moon and Stubbs 2020.)

The deceleration of evolution seems to be particularly impactful following the bottleneck that ichthyosaurs went through in the latest Triassic. Jurassic and Cretaceous ichthyosaurs were significantly less disparate and evolved less quickly than Triassic species.

We caveat these results because we used only valid taxon referrals, and so did not incorporate the portly preserved remains of the Rhaetian that have recently been reported (Fischer et al. 2014; Lomax et al. 2018), or the ecological variety of the Cretaceous (Fischer et al. 2016). However, we suggest our results are robust as we did not focus solely on ecology, and also included a selection of tree topologies and dating methods, attempting to remove the effects of fossil record uncertainty and find a representative range of evolutionary rates.

References

Fischer, V., Cappetta, H., Vincent, P., Garcia, G., Goolaerts, S., Martin, J.E., Roggero, D. and Valentin, X. 2014. Ichthyosaurs from the French Rhaetian indicate a severe turnover across the Triassic–Jurassic boundary. Naturwissenschaften 101: 1–14.

Fischer, V., Bardet, N., Benson, R.B.J., Arkhangelsky, M.S. and Friedman, M. 2016. Extinction of fish-shaped marine reptiles associated with reduced evolutionary rates and global environmental volatility. Nature Communications 7: 10825.

Lomax, D.R., De la Salle, P., Massare, J.A. and Gallois, R. 2018. A giant Late Triassic ichthyosaur from the UK and a reinterpretation of the Aust Cliff dinosaurian bones. PLoS ONE 13: e0194742.

Moon, B.C. 2019. A new phylogeny of ichthyosaurs (Reptilia: Diapsida). Journal of Systematic Palaeontology 17: 129–155.

Moon, B.C. and Stubbs, T.L. 2020. Early high rates and disparity in the evolution of ichthyosaurs. Communications Biology 3: 68.

Motani, R., Jiang, D., Tintori, A., Ji, C. and Huang, J.-D. 2017. Pre- versus post-mass extinction divergence of Mesozoic marine reptiles dictated by time-scale dependence of evolutionary rates. Proceedings of the Royal Society B: Biological Sciences 284: 20170241.

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