Before revision: (2015) point out that the real rate of extinction during previous mass extinctions is unknown, both as only some organisms leave fossil remains, and as the temporal resolution of the fossil layer is larger than the time frame of the extinction events.
After revision: (2015) point out that the real rate of extinction during previous mass extinctions is unknown, because only some organisms leave fossil remains, and also the temporal resolution of the fossil layer is larger than the time frame of the extinction events.