When the ancient fossils of a primitive land mammal were unearthed in New Zealand, they provided the first concrete evidence that the island country had once had indigenous land mammals. Until that discovery, New Zealand had no known native land mammals. The discovery thus falsifies the theory that New Zealand' s rich and varied native bird population owes its existence to the lack of competition from mammals. Which one of the following, if true, most seriously weakens the argument? ['Some other island countries that are believed to have no native land mammals in fact had indigenous land mammals at one time.', 'The unearthed land mammal is only one of several ancient land mammals that were indigenous to New Zealand.', 'Countries with rich and varied native land mammal populations do not have rich and varied native bird populations.', 'The recently discovered land mammal became extinct long before the native bird population was established.']