The Late Triassic saw the rise of the dinosaurs to fill many ecological niches in the terrestrial environment. Dinosaurs are part of a larger group called archosaurs ('ruling reptiles'). This group includes the pterosaurs and crocodiles. They are part of an even bigger group called diapsids. This group includes snakes and lizards. There were also other diapsids that were alive during the Late Triassic, but did not live to share in the good times of the Jurassic and Cretaceous.
Snakes have even more flexible jaws than lizards. This is what helps them to swallow their prey whole. Like lizards they have a poor fossil record and are first known from the Upper Cretaceous. There is still some dispute as to whether they evolved from creatures like the mosasaurs
Close relatives of the archosaurs were the rhynchosaurs. These are known from the Late Triassic of Elgin in Scotland. This deposit contains evidence of other diaspids, including some early dinosaurs and how they lived. Rhynchosaurs had beak-like mouths that they would have used to lop the tough plants of the Triassic, probably seed ferns. They were an important group of herbivores but died out during the Late Triassic mass extinction. This gave the herbiverous dinosaurs a chance to occupy the empty niches.
There were two other groups of diapsids that should be mentioned. Trilophosaurids were herbivores too. Using heavy flat teeth they sheared through their plant food in a similar manner to rhynchosaurs. Prolacertiforms are the final group to consider. They al possessed very long necks, some such as had necks twice the length of their bodies. Tanystropheus also had sharp teeth. These teeth are those of a hunter. Other features of its body indicate that it was adapted for aquatic life. It may well have been an ambush predator, using its long neck to lunge at small fish. This is just what plesiosaurs did to catch their prey. The prolacertiforms went extinct in the Late Triassic, leaving this niche open for plesiosaurs to exploit.
Once you have been through all the pages on this site you may wonder why dinosaurs became the undisputed rulers of the land. This used to be explained by the fact that the dinosaurs were more efficent than the other groups that existed alongside them during the Triassic. They were better at occupying the niches they did than the other groups and this force these other groups into extinction or into marginal niches, like being nocturnal, which is how the mammals survived through the Jurassic and Cretaceous.
This idea has been challenged in the past few years. During the Late Triassic there was a mass extinction event. Many of the groups that had been around during the rest of the Triassic became extinct. When dinosaurs first appear in the fossil record they are quite rare, making up only about 5% of the animals found. After the mass extinction the occured 225 million years ago, they had a lot of empty niches that they could occupy. Given the chance, they did. By 205 million years ago they made up between 50 and 90% of the animals in various fossil deposits. It now seems that the dinosaurs were as lucky in their chance to dominate the land as they were unlucky at the K-T boundary. Evidence like this lends itself to the idea that modern mammals did exactly the same thing after the K-T mass extinction.