No gap in the Middle Permian record of terrestrial vertebrates. 2012. M. J. Benton. Geology. posted online 2 Mar. 2012
Until recently, however, the Permian record of reptiles was said to be incomplete, with a gap of up to 5 million years. This was because paleontologists had to look at rock successions from different continents, and it seemed there was a major time hiatus between the well-known Lower Permian successions of North America and the Middle and Upper Permian successions of South Africa and Russia.
New dating evidence shows that the Russian rock record overlaps the North American record, and the gap is closed. We can study the story of change in terrestrial ecosystems through the Permian without a major lack of knowledge. link