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, 107 (22), 10131-5

End-Devonian Extinction and a Bottleneck in the Early Evolution of Modern Jawed Vertebrates

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End-Devonian Extinction and a Bottleneck in the Early Evolution of Modern Jawed Vertebrates

Lauren Cole Sallan et al. Proc Natl Acad Sci U S A.

Abstract

The Devonian marks a critical stage in the early evolution of vertebrates: It opens with an unprecedented diversity of fishes and closes with the earliest evidence of limbed tetrapods. However, the latter part of the Devonian has also been characterized as a period of global biotic crisis marked by two large extinction pulses: a "Big Five" mass extinction event at the Frasnian-Famennian stage boundary (374 Ma) and the less well-documented Hangenberg event some 15 million years later at the Devonian-Carboniferous boundary (359 Ma). Here, we report the results of a wide-ranging analysis of the impact of these events on early vertebrate evolution, which was obtained from a database of vertebrate occurrences sampling over 1,250 taxa from 66 localities spanning Givetian to Serpukhovian stages (391 to 318 Ma). We show that major vertebrate clades suffered acute and systematic effects centered on the Hangenberg extinction involving long-term losses of over 50% of diversity and the restructuring of vertebrate ecosystems worldwide. Marine and nonmarine faunas were equally affected, precluding the existence of environmental refugia. The subsequent recovery of previously diverse groups (including placoderms, sarcopterygian fish, and acanthodians) was minimal. Tetrapods, actinopterygians, and chondrichthyans, all scarce within the Devonian, undergo large diversification events in the aftermath of the extinction, dominating all subsequent faunas. The Hangenberg event represents a previously unrecognized bottleneck in the evolutionary history of vertebrates as a whole and a historical contingency that shaped the roots of modern biodiversity.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Gnathostome genus level diversity curves for the Givetian to Serpukhovian (n = 1,018) (SI Appendix). Tetrapoda is defined here as all taxa closer to crown Tetrapoda than Rhizodontida based on the tree in the article by Coates et al. (31). This includes elpistostegalians. All other sarcopterygians are referred to Sarcopterygii.
Fig. 2.
Fig. 2.
Histogram of species-level faunal composition for 66 well-sampled macrofossil localities (n = 1,267) (Dataset S1). Localities are arranged temporally, although some are concurrent. Restricted intervals used in some analyses are noted above the time scale. End-Devonian sites are from formations conformable to Hangenberg sediments and are treated as contemporaneous. The Hangenberg and Kellwasser events are represented by black lines at the relevant stage boundaries.
Fig. 3.
Fig. 3.
Faunal comparison using CCA of the later Devonian and Mississippian sites and taxa. Data were not grouped a priori by either stage or environment. (A) Ordination of sites along the first two correspondence axes based on raw diversity of taxa (n = 66). Polygon colors adhere to the RGB color code of the Commission for the Geological Map of the World. (B) Ordination of taxonomic groups (n = 22) and representation of gradients determining placement of sites along the axes. The first correspondence axis represents a temporal gradient between the Devonian and Mississippian, whereas the second axis corresponds to relative salinity.
Fig. 4.
Fig. 4.
Faunal comparisons. (A) NMDS for all sites based on raw diversity and Bray–Curtis distances (n = 66). (B) FA plot for all sites based on raw diversity (n = 66). Factor 1 (39.68% of variance) is positively correlated with the Devonian nonmarine fauna presented in the CCA results (Fig. 2B and SI Appendix). Factor 2 (22.10% of variance) is negatively correlated with members of Mississippian fauna (SI Appendix). (C) NMDS of Frasnian and Famennian sites based on relative diversity and Bray–Curtis distance (n = 32). (D) NMDS of Famennian and Tournaisian-Viséan sites based on relative diversity and Bray–Curtis distance (n = 34).

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