No gap in the Middle Permian record of terrestrial vertebrates

During the Permian, tetrapods showed a major transition from basal synapsid-dominated faunas in the first half to therapsid-dominated faunas in the second. The transition was significant in marking the beginning of richer and more complex communities, a precursor to modern terrestrial ecosystems. This changeover may have been gradual or abrupt, but its study has been complicated by the postulated occurrence of a substantial hiatus in the fossil record, termed "Olson's Gap", which obscured the nature of the turnover. New evidence from redating of key tetrapod-bearing units of the American southwest and European Russia confirms that there is no gap in the fossil record of Permian tetrapods. Indeed, evidence for substantial sampling bias in the Permian tetrapod fossil record as a whole is queried.