The extinction that occurred 245 million years ago was in the late Permian period. The causes are thought to be fluctuations in sea-level, a change in the salinity of the ocean, and volcanic activity.

Interesting site on mass extinction:

mass extinction in history

As you've probably noticed while doing your research, there are many hypotheses about why mass extinctions occur.

Just a side note: All the divisions of the geologic time scale are based on extinction events determined by the disappearance of certain fossils from rocks of a given age. At the boundary between the Permian and Triassic (245 million years ago), something happened that caused the extinction of about 95% of all marine invertebrates and 70% of terrestrial vertebrates. Here are two of the most widely accepted hypotheses for what happened:

1. Formation of the supercontinent Pangea:
The joining of multiple continents into one continent would decrease the amount of shallow marine shelf environment available as habitat. Most marine organisms live in shallow environments. In addition, the center of the supercontinent was probably dry and hot, providing miserable habitat for terrestrial animals.

2. Climate change and carbon dioxide buildup:
This hypothesis is also related to Pangaea. The idea is that the formation of Pangea drastically changed ocean circulation patterns so that circulation became slow and the oceans were poorly mixed (i.e., deep water stays in the depths instead of upwelling). Photosynthetic marine organisms like phytoplankton sink to the bottom of the ocean when they die, and aerobic bacteria at the bottom of the ocean consume them, producing carbon dioxide. In the late Permian, this carbon dioxide was stuck on the bottom of the ocean instead of being driven to the surface and released to the atmosphere. The atmosphere, therefore, became depleted in carbon dioxide.

Carbon dioxide is a greenhouse gas that helps keep the atmosphere warm, and when it was depleted, the climate cooled. Climatic cooling led to glaciation, which caused the ocean surface temperatures to decrease. This led to sinking of ocean surface water because the colder surface water was denser than the underlying, warmer water. As the oceans began to mix, all that carbon dioxide trapped at the bottom was released into the atmosphere (think of a big carbon dioxide belch). Oxygen-breathing creatures on land and in the sea were suffocated by the huge amount of carbon dioxide released.