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.2011 May 17;108(20):8339-44.
doi: 10.1073/pnas.1018959108. Epub 2011 May 2.

Resetting the evolution of marine reptiles at the Triassic-Jurassic boundary

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Resetting the evolution of marine reptiles at the Triassic-Jurassic boundary

Philippa M Thorne et al. Proc Natl Acad Sci U S A..

Abstract

Ichthyosaurs were important marine predators in the Early Jurassic, and an abundant and diverse component of Mesozoic marine ecosystems. Despite their ecological importance, however, the Early Jurassic species represent a reduced remnant of their former significance in the Triassic. Ichthyosaurs passed through an evolutionary bottleneck at, or close to, the Triassic-Jurassic boundary, which reduced their diversity to as few as three or four lineages. Diversity bounced back to some extent in the aftermath of the end-Triassic mass extinction, but disparity remained at less than one-tenth of pre-extinction levels, and never recovered. The group remained at low diversity and disparity for its final 100 Myr. The end-Triassic mass extinction had a previously unsuspected profound effect in resetting the evolution of apex marine predators of the Mesozoic.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Phylogenetic tree of ichthyosaurs plotted against geological time. Times runs from left to right, showing known ranges (dark gray), ghost ranges (minimal implied phylogenetic gap; light gray), and Lazarus ranges (missing within-range representation; diagonal bars). Silhouette outlines (Ca.,Californosaurus;Ch.,Chaohusaurus;Mi.,Mixosaurus;Pl.,Platypterygius;Sh,Shonisaurus;Te.,Temnodontosaurus;Ut.,Utatsusaurus) indicate major body morphologies in Triassic (red), Early Jurassic (blue), and Middle Jurassic–Cretaceous (green). Relative sizes of the various major ichthyosaur morphotypes are indicated by scale drawings at top right, with a 1-m scale bar indicated in the blue cloud. Phylogeny is based on the majority-rule LE50 tree (SI Appendix, Fig. 3), similar to earlier findings (7, 13), and stratigraphic ranges are from a review of the literature (SI Appendix, Fig. 4). Ichthyosaur silhouettes are based on various sources (7).
Fig. 2.
Fig. 2.
Morphospace occupation by ichthyosaurs. Plots are based on the first two axes from the principal coordinates analysis for each dataset: (A) whole-body characters; (B) cranial characters; and (C) postcranial characters. Taxa from the four time bins are distinguished by symbols, and minimal morphospace occupations by ichthyosaurs of each time bin are indicated by polygon outlines, differently colored for each time bin (red, Early–Middle Triassic; black, Late Triassic; blue, Early Jurassic; green, Middle Jurassic–Cretaceous).
Fig. 3.
Fig. 3.
Disparity and diversity of ichthyosaurs through the Triassic and Jurassic. The global disparity values are based on the sum-of-ranges metric, and diversity is the number of species. Error bars are 95% confidence intervals. (A) Disparity change is shown for all characters, cranial characters, and postcranial characters. (B) All-character disparity compared with genus-level diversity change, showing plots for study genera (those included in the disparity study) and for all genera (addition of incompletely known taxa that were excluded from the disparity study but are close relatives of included taxa). The Tr-J boundary is marked by a heavy dashed vertical line.
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References

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