Extinctions

The greatest mass extinction of the last 500 million years or Phanerozoic Eon happened 250 million years ago, ending the Permian Period and beginning the Triassic Period. More than nine-tenths of all species disappeared, far exceeding the toll of the later, more familiar Cretaceous-Tertiary extinction. For many years not much was known about the Permian-Triassic (or P-Tr) extinction. But starting in the 1990s, modern studies have stirred the pot, and now the P-Tr is a field of ferment and controversy.

The fossil record shows that many lines of life went extinct both before and at the P-Tr boundary, especially in the sea. Most notable were the trilobites, the graptolites, and the tabulate and rugose corals. Almost completely exterminated were the radiolarians, brachiopods, ammonoids, crinoids, ostracodes and conodonts. Floating species (plankton) and swimming species (nekton) suffered more extinctions than bottom-dwelling species (benthos). Species that had calcified shells (of calcium carbonate) were penalized; creatures with chitin shells or no shells did better. Among the calcified species, those with thinner shells and those with more ability to control their calcification tended to survive.

The world recovered very slowly after the extinction. A small number of species had large populations, rather like the handful of weed species that fill an empty lot. Fungus spores continued to be abundant. For millions of years, there were no reefs and no coal beds. Early Triassic rocks show completely undisturbed marine sediments—nothing was burrowing in the mud. Many marine species, including the dasyclad algae and calcareous sponges, disappeared from the record for millions of years, then reappeared looking just the same. Paleontologists call these Lazarus species (after the man Jesus revived from death). Presumably they lived on in sheltered places from which no rocks have been found.

Many different geologic aspects of the extinction period have been documented recently:

• Salinity in the sea fell sharply during the Permian for the first time, changing oceanic physics to make deep water circulation more difficult.
• The atmosphere went from very high oxygen content (30%) to very low (15%) during the Permian.
• The evidence shows global warming AND glaciations near the P-Tr.
• Extreme erosion of the land suggests that ground cover disappeared.
• Dead organic matter from the land flooded the seas, pulling dissolved oxygen from the water and leaving it anoxic at all levels.
• A geomagnetic reversal occurred near the P-Tr.
• A series of great volcanic eruptions was building up a gigantic body of basalt called the Siberian Traps.

Some researchers argue for a cosmic impact at P-Tr time, but the standard evidence of impacts is missing or disputed. The geologic evidence fits an impact explanation, but it does not demand one. Instead the blame seems to fall on volcanism, as it does for other mass extinctions.