The Cretaceous-Tertiary (K/T) Extinction--the one that killed the dinosaurs 65 million years ago--gets all the press, but the mother of all global extinctions was the Permian-Triassic (P/T) Event that transpired about 250 million years ago. Within the space of a million years or so, over 90 percent of the earth's marine organisms were rendered extinct, along with more than 70 percent of their terrestrial counterparts. In fact, as far as we know, the P/T Extinction was as close as life has ever come to being completely wiped off the planet, and it had a profound effect on the plants and animals that survived into the ensuing Triassic period.
Before getting to the causes of the P/T Extinction, it's worth examining its effects in closer detail. The hardest-hit organisms were marine invertebrates with calcified shells, including corals, crinoids and ammonoids, as well as various orders of land-dwelling insects (the only time we know of that insects have ever succumbed to a mass extinction). Granted, this may not seem very dramatic compared to the 10-ton dinosaurs that went defunct after the K/T Extinction, but these invertebrates dwelt close to the bottom of the food chain, with disastrous effects for vertebrates higher up the evolutionary ladder.
Terrestrial organisms (other than insects) were spared the full brunt of the P/T Extinction, "only" losing two-thirds of their numbers. The end of the Permian period witnessed the extinction of most plus-sized amphibians and sauropsid reptiles (i.e., lizards), as well as the majority of the therapsids, or mammal-like reptiles (the scattered survivors of this group evolved into the first mammals during the ensuing Triassic period). Most anapsid reptiles also disappeared, with the exception of the ancient ancestors of turtles and tortoises, like Procolophon. It's uncertain how much of an effect the P/T Extinction had on diapsid reptiles, the family from which crocodiles, pterosaurs and dinosaurs evolved, but clearly a sufficient number of diapsids survived to spawn these three major reptile families millions of years later.
The Permian-Triassic Extinction Was a Long, Drawn-Out Event
The severity of the P/T Extinction stands in stark contrast to the leisurely pace at which it unfolded. We know that the later K/T Extinction was caused by the impact of an asteroid on Mexico's Yucatan Peninsula, which spewed millions of tons of dust and ash into the air and led, within a couple of hundred (or couple of thousand) years, to the extinction of dinosaurs, pterosaurs and marine reptiles. By contrast, the P/T Extinction was much less dramatic; by some estimates, this "event" actually spanned as much as five million years during the late Permian period.
Further complicating our assessment of the P/T Extinction, many types of animals were already on the decline before this cataclysm started in earnest. For example, pelycosaurs--the family of prehistoric reptiles best represented by Dimetrodon--had mostly disappeared off the face of the earth by the early Permian period, with a few straggling survivors succumbing millions of years later. The important thing to realize is that not all extinctions can be traced directly to the P/T Event; the evidence either way is constrained by which fossil specimens have persisted down to the present day.
What Caused the Permian-Triassic Extinction?
Now we come to the million-dollar question: what was the proximate cause of the "Great Dying," as the P/T Extinction is called by some paleontologists? The slow pace with which the process unfolded points to a variety of interrelated factors, rather than a single, global catastrophe. Scientists have proposed everything from a series of major asteroid strikes (the evidence for which has been erased by over 200 million years of erosion) to a calamitous change in ocean chemistry, perhaps caused by the sudden release of huge methane deposits through the sea floor.
The bulk of the recent evidence has pointed to a different culprit--a series of gigantic volcanic eruptions in the region of Pangea that today corresponds to modern-day eastern Russia (i.e., Siberia) and northern China. According to this model, these eruptions released a huge amount of carbon dioxide into the earth's atmosphere, which gradually leached down into the oceans. The disastrous effects were threefold: acidification, warming, and (most important of all) a drastic reduction in oxygen levels, which resulted in the asphyxiation of most marine organisms. (By the same token, the sudden surge of carbon dioxide didn't do any favors for the earth's terrestrial organisms, which also had to cope with the disappearance of their accustomed food sources.)
Could a disaster on the scale of the Permian-Triassic Extinction ever happen again? It may well be happening right now, but in super-slow-motion: the levels of carbon dioxide in the earth's atmosphere are indisputably increasing, thanks partly to our burning of fossil fuels, and life in the oceans is beginning to be affected as well. It's unlikely that global warming will cause human beings to go extinct anytime soon, but the prospects are less sanguine for the rest of the plants and animals with which we share the planet!