Oceanic reefs teemed with life during the Devonian (416–359 MYA). Various carbonate-secreting organisms coalesced into communities that built cities on the ocean floor which served all sorts of sea creatures.
Extinction at the end of the Devonian came as a rampant reign of invasive species in the oceans catapulted into catastrophe. The world’s watery realm was hard hit: loss of 95% of shallow-water species, 60% extinction of deep-water species, and 22% of all marine animal families lost.
Ocean life at the end of the Devonian declined because opportunities for new diversity did not exist. In a death spiral that lasted 10 million years (374–364 MYA), what invasive species started was finished off by environmental changes, as oceanic oxygen levels plunged.
Jawed fish fared fairly well, especially considering the devastation of the reefs; but then, these predators may well have been the invaders that were the extinction initiators. While their populations eventually plummeted, these generalists survived and thrived through coming eras.
In the early history of life, speciation by vicariance was 3 times more common than dispersal. That pattern was flipped upside down during the Devonian extinction event: speciation by vicariance happened only 28% of the time, with 72% by dispersal.
Life on land suffered a more modest decline at the Devonian–Carboniferous boundary, owing to tectonic activity and climate changes. The tumult was not only of species dying out but also a failure of new ones to form. Diversity plunged as opportunities for new life declined.
Recoveries from mass extinctions can be unpredictable. ~ English paleontologist Michael Benton