Many bird and reptile species use hissing as a threat device against potential predators. The way these species produce hissing sounds is similar enough that it is likely that this behavior developed in an early common ancestor. At the time this common ancestor would have lived, however, none of its potential predators would have yet acquired the anatomy necessary to hear hissing sounds.
The statement 'Like its potential predators, the common ancestor of bird and reptile species would have lacked the anatomy necessary to hear hissing sounds' most helps to resolve the apparent discrepancy in the information above.