The fossil record does not perfectly and completely record what organisms were alive at a given time-- there is a bias in favor of more common organisms and ones with more durable bodies. The last appearance in the fossil record might not be at the time of the mass extinction even if the organism were present at that time, especially if it was less common overall.
Work Step by Step
Review the chapter, paying special attention to any mention of bias in the fossil record and the discussion of how complete it is. Rarer species have fewer chances to leave fossils behind and are less likely to leave a complete record.