There have been many mass die-offs of marine life in the last 600 million years, perhaps as many as twelve. About 250 million years ago, at the end of the so called Permian era, more than 90 percent of all species of fossil sea creatures vanished in a mass exinction that overshadows more recent ones. Another great mass extinction dates from 365 million years ago, at the end of the Devonian epoch. Do these mass extinctions have extraterrestrial causes too? Geologists have found excesses of iridium, the telltale sign of an asteroid or comet crash, and little glassy spherules in association with the latter two extinctions, but in neither case is the iridium excess as spectacular and widespread as in the case of the K-T catastrophe.

Smaller mass extinctions have also threatened life in the last 50 million years. For two of these catastrophes, which happened about 38 million and 12 million years ago, boundary layers rich in iridium were found, and glassy spherules too.

. . . . . . . Three Big Bangs, p. 31

At crucial times in the fossil record, large numbers of apparently thriving species became extinct, all within a relatively short interval. Different creatures replaced them, living in similar climatic conditions, in the same geographical areas. These successor species were not necessarily better adapted or more "fit"--they just appeared later in evolutionary history. Of the many extinctions known, five stand out as more globally devastiting than the others. Marking the boundaries of epochs in Earth's geological and biological history, these five great extinctions took place about 440, 365, 245, 210, and 65 million years ago. Of these, three extinctions have been linked with large Earth craters and three with boundary clays rich in iridium, suggesting an extraterrestial impact.

The last great extinction, the K-T catastrophe 65 illion years ago, saw the end of about 40 percent of all animal genera (genera is the plural for genus, the category in biologists' classification system between family and species) and about two-thirds of animal species. Marine reptiles, including the huge long-necked plesiousaurs, the mosasaurs with paddles instead of legs, and the sharklike icthyosaurs, died out completely. Every single species of terrestrial dinosaur vanished; the birds, considered by many biologists to be a form of dinosaur, survived. Most flowering plant species endured and made a comeback after ferns took over the continents.

... Two smaller extinctions, the ones dating from 35 million and 290 million years ago, have been tied to known craters.

.............The Three Big Bangs p. 56-57.