In the Lower Carboniferous, high sea levels meant that the northern hemisphere was almost devoid of land. The island of Angaraland was the only significant landmass north of the equatorial zone. Laurentia, comprising what would become North America and northern Europe, was still separated by straits from the main landmass of Gondwanaland. North of the Antarctic ice cap, the climate was significantly warner and more humid than today, with around 40 per cent more oxygen in the atmosphere than today. This encouraged the growth of lush rainforests of swamp-loving lycophytes, and a profusion of insect life, some extremely large, like the dragonfly, Meganeura. Amphibians became the first vertebrates to colonize dry land, but the evolution of the amniote egg, which enabled terrestrial incubation and hatching to take place, would lead to the development of the higher vertebrates later in the Carboniferous.
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