A pottery shard found in a site in Texas, 1,500 miles north of Mexico, is dated to be 9,500 years old. Researchers concluded that since the desert conditions prevented human migration north from the Mexico-United States border between 12,000 and 9,000 years ago, humans must have arrived in North America more than 12,000 years ago.

Which of the following pieces of new evidence would cast doubt on the conclusion drawn above?