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No "Olson's Gap" in Permian tetrapod fossil record

From: Ben Creisler

With the paper out yesterday announcing the discovery of tetrapods
fossils that fill in "Romer's Gap" in early Carboniferous, another new
paper addresses the so-called "Olson's Gap" in the Permian fossil
record for tetrapods.

Michael J. Benton (2012)
No gap in the Middle Permian record of terrestrial vertebrates.
Geology (advance online publication)
doi: 10.1130/G32669.1


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.

Summary for press release:

New work on fossil reptiles from Russia shows a more continuous
evolutionary record than had been assumed. A key concern about the
fossil record is that it is incomplete. Major gaps — times when no
fossils were preserved — can hide the detail of certain episodes in
the history of life. During the Permian period 300-250 million years
ago, the basis of modern terrestrial ecosystem was established.
Worldwide climates became warmer and drier, and reptiles rose in
importance. The first plant-eating reptiles appeared, and Late Permian
ecosystems were broadly comparable to modern ones. 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.