Dinosaurs are land reptiles that include some of the biggest terrestrial life forms ever on Earth. We know dinosaurs from fossilized bones, teeth, footprints, eggs, and occasionally even soft tissues, although complete skeletons are rare. The earliest known dinosaurs are from the late Triassic (about 230 million years ago), when they were still overshadowed by other animals, such as synapsids (related to the ancestors of mammals). By the end of the Triassic, dinosaurs had begun their remarkable diversification, with several major lineages present. The Jurassic and Cretaceous periods saw dinosaurs dominate many land environments, reach enormous sizes, and diversify into thousands of species (including the first birds). Paleontologists have named more than 1200 species of dinosaurs, and that’s probably just a fraction of all the dinosaur species that existed. Most dinosaurs went extinct abruptly 66 million years ago (end of the Cretaceous), along with many other species. A period of severe environmental stress, punctuated by an asteroid impact, was likely responsible for this mass extinction.

Reptiles first appeared on Earth about 300 million years ago (during the Pennsylvanian). Before then, all back-boned animals (vertebrates) were tied to the water, living in or near swamps. Their eggs were laid underwater. A set of gradual adaptations, such as thicker, scaly skin, allowed life on land to emerge. The biggest change was the development of an egg containing layers of membranes (amniotic membranes) to retain water, supply food, and store waste for an embryo. Two further innovations – the eggshell and internal fertilization – marked the first reptiles. Fertilization inside the female and shelled, amniotic eggs freed reptiles from aquatic life. They diversified into dinosaurs, crocodiles, lizards, snakes, turtles, tuataras, and birds as they colonized a variety of land habitats. For more than 100 million years, reptiles dominated the Earth. The largest reptiles were wiped out in the extinction event at the end of the Mesozoic, but their smaller descendants live on today.

Tetrapods (from tetra = “four”, and pous = “foot”) include amphibians, reptiles (including dinosaurs), birds, and mammals. The first well-documented tetrapod remains are known from the latest Devonian (about 360 million years ago) although tracks belonging to this group are known from rocks that are 395 million years old. Scientists agree that tetrapods evolved from a group called the lobe-finned fishes. Fins turned into legs even before the first tetrapods moved onto land. Lobe-finned fishes developed the first lungs, allowing them to breathe air. The first tetrapods still had both gills and lungs, suggesting a gradual transition from breathing under water to air-breathing. The move to land also required a stronger backbone to support the body while walking. Among present-day animals, lungfishes are the closest living relatives of tetrapods.