A massive extinction event occurred about 5 million years before the end of the Devonian. It has been estimated that 35 out of 46 families of fishes became extinct at that time. This global event took out the last of the ostracoderms, pretty much all the placoderms and 10 families of lobe-finned fishes. As well, many invertebrate groups were hit hard. It has been estimated that over 20% of all marine animal families became extinct at that time. Global sea levels rose throughout most of the Late Devonian, flooding continental margins and creating new environments for fishes and invertebrates while at the same time destroying freshwater habitats in lower lying areas. Inundation also tended to break down barriers between originally separate areas, resulting in opportunities for invasion of habitats. Major changes in global climates also occurred at that time and have been associated with a worldwide decline in the circulation of water within the ocean basins. Reduced circulation led to the build-up of bottom waters low in oxygen, which eventually accumulated so that their upper regions spilled over into te shallow seas surrounding the continents. As these shallow environments received oxygen depleted their faunas and floras suffered massive extinctions.