The Upper Devonian had significantly higher sea levels than today, with most of the Earth’s surface covered by the vast Panthalassa Ocean. Proto Europe and North America existed as islands skirting the continent of Laurentia, which had collided with Baltica in the Lower Devonian. The main landmass, Gondwanaland, lay to the south. Carbon dioxide levels in the atmosphere were much higher than today, oxygen levels appreciably lower. The climate was uniformly warm, and generally arid. Shrub-like forests of lycophytes began to appear, and the earliest known tree Wattlieza. The ‘Devonian Explosion’ in terrestrial plant biomass acted as a carbon sink, progressively reducing levels of atmospheric carbon dioxide. Lobe finned fish, the forerunners of the first amphibious land vertebrates had begun to colonize shallow coastal waters. The Devonian ended with a (primarily marine) mass extinction, resulting in the disappearance of jawless fishes, to be replaced by cartilaginous and bony species.