The Mother of All Extinctions

Despite the paranoia of many people today, there is really no reason to create contingency plans for velociraptor attacks. In fact, all of the velociraptors (or any other giant man-eating dinosaurs), are dead. Around 65 million years ago, there was a mass extinction you may have heard of. It was called the KT extinction because it formed the boundary between the Cretaceous and Tertiary periods. This extinction event has been studied heavily, but was only one of five major extinctions known as the "big five". The most devastating extinction actually occurred about 150 million years before that and formed the boundary between the Permian and Triassic Periods. It is known as the PT extinction and marked the beginning of the age of the dinosaurs.



The PT extinction was so massive it is commonly called the "Great Dying" or the "Mother of all Extinctions" and occurred around 250 million years ago. It's estimated that around 95% of marine species and 70% of land species went extinct and was the only known mass extinction of insects. The composition of creatures in the sea previous to the extinction was mostly shelled creatures that were attached to the sea bottom. However, events of the extinction hit these animals hard. After the extinction, the sea was more heavily composed of free-swimming creatures. Dominant land creatures in the Permian were Sauropsids ("Lizard Faces") and Therapsids ("Beast Faces"). Sauropsids were the group that gave rise to reptiles, dinosaurs, and, eventually, birds and Therapsids were "mammal-like reptiles" that were the ancestors of mammals. Other land creatures of the Permian included some of the largest insects that ever lived, including Meganeuropsis, a dragonfly that had a wingspan of 27 inches (2.25 feet)! The PT extinction prevented the Earth from being overrun by giant dragonflies and nearly all of those massive bugs went extinct. Additionally the PT extinction was what finally wiped out the last of the trilobites.

Sketch of Lystrosaurus: D. Bogdanov

After the PT extinction came the Triassic era, the first of the well-known sequence of Triassic-Jurassic-Cretaceous that enveloped the time of the dinosaurs. However, it took a long time before the Triassic gave rise to dinosaurs such as Eoraptor and Plateosaurus. The Triassic period was overrun by "disaster species", or "weedy" species that thrived in disturbed environments. For example, bivalves, clams and oysters, were rare and out-competed in Permian, but took over the sea floor after the extinction. Lystrosaurus was a therapsid (mammal-like reptile) that flourished until dinosaurs began evolving later in the Triassic period. It's believed that Lystrosaurus alone made up 90% of land vertebrates in the early Triassic. This is the only time in history where a single specie dominated the land to this extent.



 
More interesting even than the aftermath of the extinction is the debate over what caused it. It's been fairly well decided that an asteroid impact had, at the very least, begun the extinction of the dinosaurs, but the PT extinction occurred a much longer time ago. This causes some problems with finding evidence of what occurred, as the fossil layers for the PT boundary are much harder to find than the KT boundary. In fact, it wasn't until 2000 that the date of the PT extinction was confirmed when fossil layers were discovered in China. However, with what fossil evidence is available, there have been many hypotheses for the cause of this extinction. Among them is the hypothesis of an asteroid, but it is generally believed that an impact alone could not have caused anything close to the extinction levels seen in the PT extinction. Other hypotheses include increased volcanic activity, global cooling and/or warming, methane hydrate release, changes in sea levels, water anoxia (a lack of oxygen), emissions of hydrogen sulfide, and increased competition from the formation of Pangaea, which occurred in the middle of the Permian.

C.R. Scotese: Geography of the Earth in the Late Permian.

 
Each possible cause has evidence leaning for and against it. For example, a large formation of volcanoes in what is now Russia is called the Siberian Traps. This formation erupted around the same time as the PT extinction and the eruptions lasted about a million years. This eruption is the largest known volcanic eruption in Earth's history and covered over 2,000,000 sq kilometers (about 770,000 sq miles) with lava. That's equivalent to the entire area of Mexico! It is mostly agreed that this eruption had at least some part in the PT extinction, but could only have caused the world-wide extinctions seen if it were closer to the equator. The most plausible idea is that the PT extinction was caused by a domino-effect of extinction-causing phenomenon.
 
One such domino effect may have been as follows: an asteroid hits the earth near Siberia, setting off the eruption of the Siberian Traps. This eruption, lasting a million years, sends ash into the air, blocking out the sun and causing global cooling. However, the lava that spread from the Traps covers primarily shallow seas, which contain deposits of methane hydrate that now melt and are sent into the atmosphere. When the eruptions begin slowing, the ash falls out of suspension and the CO2, released in the eruption, and methane released from the seas begin to cause global warming. Eventually, the increased temperature of the earth (believed to have increased up to 6 degrees F at the equator) slows the circulation of deep water in the seas, which leads to anoxic conditions. These conditions lead to a massive increase in bacteria that don't need oxygen to survive (anaerobic bacteria), that naturally release hydrogen sulfide, a toxic gas that kills life in the ocean and on land and destroys the ozone layer.
 
But that's just one theory.