The first dinosaurs appear during the Triassic, along with some of the first mammals, but the dominant animals at the beginning of this epoch are reptiles like turtles and "fish lizards" called ichthyosaurs.

In the late Triassic, there is evidence that Pterosaurs had developed; these were the first vertebrates to fly, long before birds and bats had appeared. With their ability to fly, these hollow-boned reptiles colonized many of the continents throughout the Mezozoic Era and developed a huge number of shapes and sizes. The smallest were the size of a sparrow; the largest had a wingspan of about 40 feet. They ate insects and fish and, like the migratory birds of today, it may be that they fed on the eggs of the horseshoe crab.

At the end of the Triassic period, about 35% of the animal families become extinct. Most of the marine reptiles vanish during this extinction (though not the horseshoe crab). This allowed the dinosaurs to expand into many niches and become the dominant species throughout the next two epochs in the Mezozoic Era.