Birds evolved from theropod dinosaurs during the Late Jurassic period, however, our knowledge of the initial stages of Avialae's evolution is limited due to a scarcity of Jurassic fossils.
Insects and terrestrial arthropods have inhabited the Earth since before the time of the dinosaurs, growing much larger to their contemporary equivalents during the Carboniferous period, due in part to a surplus of oxygen in the Earth’s atmosphere.