During the Silurian Period, as shown here by data from Gotland, Sweden, and other regions of the Baltoscandian paleocontinent, fish faunas were struck hard by extinctions caused by the late Ludlow Lau Event, also known for being associated with the largest positive stable carbon isotope excursion of the Phanerozoic. This event had a profound impact on the early evolutionary history of vertebrates, wiping out two-thirds of the fish taxa and causing significant ecological reorganizations over an estimated time span of ~200 ka. Our data show that immediately prior to the event, jawed acanthodians dominated the fish faunas, whereas the event led to a diverse fauna and a brief but marked dominance of the jawless thelodonts. The stepwise changes observed mimic those of conodonts, suggesting a similar mode of life and response to atmospheric oceanic perturbations for these clades.

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