The Permian-Triassic Extinction Event
How the "Great Dying" Affected Life on Earth 250 Million Years Ago
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The Cretaceous-Tertiary (K/T) Extinction--the
global cataclysm that killed the dinosaurs 65 million years ago--gets
all the press, but the fact is that the mother of all global extinctions
was the Permian-Triassic (P/T) Event that transpired about 250 million
years ago, at the end of the Permian
period.
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. (See a list of the Earth's 10 Biggest Mass Extinctions.)
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. (See a list of the Earth's 10 Biggest Mass Extinctions.)
Before getting to the causes of the Permian-Triassic Extinction, it's
worth examining its effects in closer detail. The hardest-hit organisms
were marine invertebrates possessing calcified shells, including
corals, crinoids and ammonoids, as well as various orders of
land-dwelling insects (the only time we know of that insects, usually
the hardiest of survivors, have ever succumbed to a mass extinction).
Granted, this may not seem very dramatic compared to the 10-ton and
100-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 Permian-Triassic Extinction, "only" losing two-thirds of their
numbers, by species and genera. 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 modern
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.
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 Permian-Triassic Extinction stands in stark
contrast to the leisurely pace at which it unfolded. We know that the
later K/T Extinction was precipitated 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 worldwide.
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.
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 at this time can be directly attributed to the P/T Event; the evidence either way is constrained by which animals happen to be preserved in the fossil record. Another important clue, the importance of which has yet to be fully adduced, is that it took an unusually long time for the earth to replenish its previous diversity: for the first couple of million years of the Triassic period, the earth was an arid wasteland, practically devoid of life!
The important thing to realize is that not all extinctions at this time can be directly attributed to the P/T Event; the evidence either way is constrained by which animals happen to be preserved in the fossil record. Another important clue, the importance of which has yet to be fully adduced, is that it took an unusually long time for the earth to replenish its previous diversity: for the first couple of million years of the Triassic period, the earth was an arid wasteland, practically devoid of life!
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 Permian-Triassic 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 would have 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
(created by decaying microorganisms) from the bottom of the sea floor.
The bulk of the recent evidence points to yet another possible
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 theory, 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 of the water, global warming,
and (most important of all) a drastic reduction in atmospheric and
marine oxygen levels, which resulted in the slow asphyxiation of most
marine organisms and many terrestrial ones.
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
(as witness the crises facing coral reef communities around the world).
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!
