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.2016 Mar 8:7:10825.
doi: 10.1038/ncomms10825.

Extinction of fish-shaped marine reptiles associated with reduced evolutionary rates and global environmental volatility

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Extinction of fish-shaped marine reptiles associated with reduced evolutionary rates and global environmental volatility

Valentin Fischer et al. Nat Commun..

Abstract

Despite their profound adaptations to the aquatic realm and their apparent success throughout the Triassic and the Jurassic, ichthyosaurs became extinct roughly 30 million years before the end-Cretaceous mass extinction. Current hypotheses for this early demise involve relatively minor biotic events, but are at odds with recent understanding of the ichthyosaur fossil record. Here, we show that ichthyosaurs maintained high but diminishing richness and disparity throughout the Early Cretaceous. The last ichthyosaurs are characterized by reduced rates of origination and phenotypic evolution and their elevated extinction rates correlate with increased environmental volatility. In addition, we find that ichthyosaurs suffered from a profound Early Cenomanian extinction that reduced their ecological diversity, likely contributing to their final extinction at the end of the Cenomanian. Our results support a growing body of evidence revealing that global environmental change resulted in a major, temporally staggered turnover event that profoundly reorganized marine ecosystems during the Cenomanian.

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Figures

Figure 1
Figure 1. Phylogeny and ecological diversity of parvipelvian ichthyosaurs.
(a) Time scaled strict consensus tree arising from equal weight maximum parsimony analysis. Numbers denote >1 Bremer decay indices. Grey bars denote range extensions by specimens identified at the generic level. Colour coding of taxa refers to the results ofb. (b) Cluster dendrogram based on the ecological data set, with gut-content data and the general features of each guild. (c) Teeth representative of each guild across the Late Albian–Cenomanian interval, illustrating the ecological extinction at the beginning of the Cenomanian. ‘Platypterygius campylodon' and ‘Platypterygius' sp. from the US are early Cenomanian in age,Pervushovisaurus bannovkensis is Middle Cenomanian in age and ‘Platypterygius' sp. from Germany is Late Cenomanian in age. *denotes taxa from the Stoilensky/Kursk fauna. Scale bar, 50 mm.
Figure 2
Figure 2. Ichthyosaur diversity through the Cretaceous.
(a) Taxonomic/lineage richness. The orange thick line is the mean value per bin, while the light orange outline represents the range of values, encompassing all most-parsimonious trees, under both the ‘basic' and ‘equal' methods of branch length reconstruction (PADE, phylogeny-adjusted diversity estimate). The long-term sea-level is taken from Haq. (b) Weighted mean observed pairwise dissimilarity compared with the Jurassic–Early Cretaceous average value. Light grey outline represents the 95% confidence interval. Bins are: Berriasian–Valanginian, Hauterivian–Barremian, Aptian, Albian, Cenomanian and Turonian. Important events and factors explaining the shape of the curve are indicated. Note the all-time disparity peak for Parvipelvia during the Hauterivian–Barremian. The average value for the Jurassic only is 0.24. (c) Sum of variances from the phylogenetically reconstructed data set, compared with the Jurassic–Early Cretaceous average values. The light orange and light grey outline represent the 95% confidence intervals. Again, an all-time disparity peak for Parvipelvia is recorded during the early Early Cretaceous. The average values for the Jurassic only are 7.53 (basic) and 9.38 (equal). (d) Ecological niches occupied per bin. *denotes data obtained from the Stoilensky/Kursk fauna.
Figure 3
Figure 3. Evolution and extinction rates for parvipelvian ichthyosaurs.
(a) Median rate of morphological evolution (morphological clock) arising from the constrained Bayesian inference. (b) Median rate of morphological evolution (morphological clock) arising from the unconstrained Bayesian inference. Both analyses indicate high rates in the early evolution of Parvipelvia, confined in the Triassic (c). (d) Cladogenesis rate using the time scaled trees arising from the constrained Bayesian inference. (e) Cladogenesis rate using the time scaled trees arising from the maximum parsimony analysis and extinction rate. The light grey outline represents the range of values, encompassing all most-parsimonious trees. (f) Number of marine reptile-bearing and ichthyosaur-bearing formations throughout the Cretaceous. (g) Proportion of marine reptile-bearing formations containing ichthyosaurs throughout the Cretaceous, with calculation of a 95% confidence interval. (f,g) Indicate ichthyosaurs disappeared in a two-phase extinction during the Cenomanian, and that this extinction is not biased by the fossil record: ichthyosaurs rarefy and disappear during a time of excellent recovery potential.
Figure 4
Figure 4. A two-phase extinction for ichthyosaurs.
(a) Biostratigraphic ranges of the last ichthyosaur taxa. Questions marks indicate uncertainty of the stratigraphic range of the material from Stoilensky quarry (western Russia). Thin lines indicate uncertain but probable occurrence of taxa, based on the presence of compatible remains. See Supplementary Note 1 for the details and discussion on the specimens considered in the bracketed numbers. (b) Evolution of worldwide ichthyosaur diversity (at the species level in black and at the lineage level in orange) for each bin considered (Late Albian, earliest Cenomanian, Early Cenomanian, Mid-Cenomanian, Late Cenomanian, Turonian. The lighter colour indicates how the curve would look inPlatypterygius campylodon is not regarded as a valid entity. (c) Evolution of the number of feedings guilds colonized, based on the results from the cluster analysis of ecological data. Note the sharp reduction after the earliest Cenomanian. (d) Extinction rate at the boundaries of each bin. Per-lineage extinction rates≥40% are recorded in the two phases of ichthyosaur extinction.
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References

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