We've all (well, maybe most) have heard of the great dinosaur extinction that defines the Cretaceous-Tertiary periods's (called the K-T boundary) in earths geological history. But few know about an even bigger mass extinction event that occurred 250 million years ago at the end of the Permian Period. This extinction nearly wiped out all life on Earth. What survived evolved into the dinosaurs. While the mass extinction at the K-T boundary can be attributed to extensive enviornmental stress's on the dinosaurs habitat [the supercontinent "Pangea" was splitting up and drifting away from the equatorial regions], coupled with a huge meteor impact. The same cannot be said of the P-Tr extinction. Instead a multitude of events happened over the course of several million years. In effect, the worst possible geological events all happened at roughly the same time. The Permian-Triassic (P-Tr) extinction event, sometimes informally called the Great Dying, was an extinction event that occurred approximately 251 million years ago (mya), forming the boundary between the Permian and Triassic geologic periods. It was the Earth's most severe extinction event, with about 96 percent of all marine species  and 70 percent of terrestrial vertebrate species becoming extinct. Timing of the extinction It used to be thought that rock sequences spanning the Permian-Triassic boundary were too few and contained too many gaps for scientists to estimate reliably when the extinction occurred, how long it took or whether it happened at the same time all over the world. But newly discovered rock sequences in China and improvements in radiometric dating have made scientists confident that the end of the extinction can be dated to somewhere between 251.2 and 250.8MYA (millions of years ago). There is evidence worldwide of an abrupt and massive change in the ratio of carbon-13 to carbon-12. Scientists are confident that rocks which show this change were formed at the same time. It has also recently been discovered that many rocks of about the right age, both from continental shelf and from terrestrial environments (at the time), contain evidence of a "fungal spike", an enormous increase in the abundance of fungal spores. Since fungi feed on the remains of dead organisms, especially plants, the fungal spike is interpreted as marking the time of the end-Permian extinction and the boundary between the Permian and the Triassic. This helps in dating rocks which are not suitable for radiometric dating. There is evidence that the extinction took a few million years but with a very sharp peak in the last 1 million years of the Permian (possibly in a period of under 60,000 years). This applies both to marine organisms (see the diagram "Marine Genus Biodiversity") and terrestrial organisms. In fact many scientists believe: that there were two major extinction pulses 5M years apart, separated by a period of extinctions well above the background level; and that the final extinction killed off "only" about 80% of marine species alive at that time while the other losses occurred during the first pulse or the interval between pulses. According to this theory the first of these extinction pulses occurred at the end of the Guadalupian epoch of the Permian. Terrestrial losses It is harder to produce such detailed statistics for land, river, swamp and lake environments because good Permian-Triassic rock sequences from terrestrial environments are extremely rare (the Karoo is by far the best). Even so, there is enough evidence to indicate that: * Over two-thirds of terrestrial amphibian, sauropsid ("reptile") and therapsid ("mammal-like reptile") families became extinct. Large herbivores suffered the heaviest losses. All Permian anapsid reptiles died out except the procolophonids (testudines have anapsid skulls but are most often thought to have evolved later, from diapsid ancestors). * The end-Permian is the only known mass extinction of insects. * Many land plants became extinct, including groups which had been very abundant such as Cordaites (gymnosperms) and Glossopteris (seed ferns). Notes: * Pelycosaurs died out before the end of the Permian. * Too few Permian diapsid fossils have been found to support any conclusion about the effect of the Permian extinction on diapsids (the "reptile" group from which lizards, snakes, crocodilians, dinosaurs and birds evolved). Survivors The groups that survived suffered very heavy losses, and some very nearly became extinct at the end-Permian. Some of the survivors did not last for long, but some of those which barely survived produced diverse and long-lasting lineages. "Dead clades walking" which became extinct in the Triassic include: many bryozoa; Orthocerida (a group of nautiloids); the Goniatitida and Prolecanitida orders of ammonites; procolophonids (the last of the Permian anapsid reptiles). Articulate brachiopods (those with a hinge) have declined slowly ever since the P-Tr extinction. Groups which very nearly became extinct but later became abundant and diverse include: the Cerititida order of ammonites; crinoids ("sea lilies"). Paleontologists have found very few fossils from the Permian of archosaurs (or archosauriformes as some prefer to describe the Permian specimens), but in the Triassic the archosaurs took over all the medium to large terrestrial vertebrate niches, and were the ancestors of crocodilians, dinosaurs and birds. After the extinction event Very slow recovery "Normal" levels of biodiversity do not appear until about 6 million years after the end of the Permian, and in fact recovery was extremely slow for the first 5 million years. This pattern is seen in land plants, marine invertebrates and land vertebrates. . The early Triassic shows well-known signs of how long the recovery took: * The coal gap - throughout the early Triassic (8M years) there were insufficient large plants to form coal deposits, and hence little food for large animals. * Each major segment of the ecosystem - plant and animal, marine and terrestrial - was dominated by a small number of genera, which appeared virtually world-wide, for example: the herbivorous therapsid Lystrosaurus (which accounted for about 90% of early Triassic land verterbrates) and the bivalves Claraia, Eumorphotis, Unionites and Promylina. A healthy ecosystem has a much larger number of genera, each living in a few preferred types of habitat. * "Disaster taxa" (opportunist organisms) took advantage of the devastated ecosystem and enjoyed a temporary population boom and increase in their territory, for example: Lingula (a brachiopod); stromatolites, which had been confined to marginal environments since the Ordovician; Pleuromeia (a small, weedy plant); Dicrodium (a seed fern). * River patterns in the Karoo changed from meandering to braided, indicating that vegetation there was very sparse for a long time. Changes in marine ecosystems Before the extinction about 67% of marine animals were sessile, but during the Mesozoic only about 50% were sessile. Analysis of a survey of marine fossils from the period showed a decrease in the abundance of sessile epifaunal suspension feeders (animals anchored to the ocean floor such as brachiopods and sea lilies), and an increase in more complex mobile species such as snails, urchins and crabs. Before the Permian mass extinction event some 251 million years ago, both complex and simple marine ecosystems were equally common, but after the recovery from the mass extinction the complex communities outnumbered the simple communities by nearly three to one. Bivalves were fairly rare before the P-Tr extinction but became numerous and diverse in the Triassic and one group, the rudist clams, became the Mesozoic's main reef-builders. Some researchers think much of this change happened in the 5 million years between the two major extinction pulses. Fungal spike For some time after the P-Tr extinction, fungal species were the dominant form of terrestrial life. Though they only made up approximately 10% of remains found before and just after the extinction horizon, fungal species subsequently grew rapidly to make up nearly 100% of the available fossil record. Fungi flourish where there are large amounts of dead organic matter. However, some researchers argue that fungal species did not dominate terrestrial life, even though their remains have only been found in shallow marine deposits. Alternatively, others argue that fungal hyphae are simply better suited for preservation and survival in the environment, creating an inaccurate representation of certain species in the fossil record. Land vertebrates Before the extinction, therapsids ("mammal-like reptiles") were the dominant terrestrial vertebrates. Lystrosaurus (a herbivorous therapsid) was the only large land animal to survive the event, becoming the most populous land animal on the planet for a time. Early in the Triassic, archosaurs became the dominant terrestrial vertebrates, until they were overtaken by their descendants the dinosaurs. Archosaurs quickly took over all the ecological niches previously occupied by therapsids (including eventually the lystrosaurs' vegetarian niche), and therapsids and their mammaliform descendants could only survive as small insectivores. Some temnospondyl amphibians also made a relatively quick recovery after being nearly exterminated - capitosauria and trematosauria were the main aquatic and semi-aquatic predators for most of the Triassic, some specializing to prey on tetrapods and other on fish.