Answer: discredits the hypothesis that mounds were built by a vanished culture.
Step-by-step explanation:
According to the author, Cyrus Thomas refuted the popular idea that mounds were the result of a vanished culture. According to the author, it was believed that the mounds were raised by early Native Americans.
Therefore, the author describes the research of Cyrus Thomas most likely because it discredits the hypothesis that mounds were built by a vanished culture.