At that time, about 200 million years ago, the Earth contained one giant land mass called Pangea, which eventually broke up into the modern continents. For a long time, scientists have suspected that this breakup was somehow connected to the Triassic/Jurassic mass extinction. By studying plant and animal fossils as well as carbon isotope ratios, Whiteside and her team have documented evidence proving that this was so.
In a nutshell, as the American and African tectonic plates separated creating the basin of the Atlantic ocean, there was tremendous volcanic activity throughout this enormous fissure. This area, called the Central Atlantic Magmatic Province (CAMP) and covering a region about the size of the continental United States, experienced volcanic eruptions over a period of about 600 thousand years. Even more deadly than the lava flows and shaking were the tremendous amounts of carbon dioxide and other greenhouse gases released into the atmosphere. These gases were responsible for killing half the species alive at that time.
Among the evolutionary losers in this mass extinction were the dominant predators on the planet, the large crurotarsans (relatives of modern crocodiles and alligators). And among the winners were a group of small theropod dinosaurs that eventually gave rise to Tyrannosaurus rex and its ilk.
Pencil drawing of Gracilisuchus stipanicorum. Gracilisuchus (meaning "gracile crocodile") is the name given to a tiny (30 cm long) genus of crurotarsan (a group which includes the ancestors of crocodilians) from the Middle Triassic of Argentina.
Drawing by Nobu Tamura, Dec 25, 2006, Modified May 15, 2007
Of course, if you want to take a longer view, there still are 23 species of crocodilians alive today, and T. rex, not so much.