In 1860 Bavarian quarry workers discovered the impression of a feather in a limestone slab dating to the Mesozoic era. It had previously been assumed that birds developed only after the close of the Mesozoic era and after the disappearance of pterosaurs, a species characteristic of that era. But there in limestone lay the imprint of a fully aerodynamic, three-inch-long feather. This, ttherefore, must have been the earliest bird -- certainly, the earliest found to that date. The argument assumes which one of the following? ['In the Mesozoic era, no creatures other than birds had such feathers.', 'The creature to which the feather belonged was a descendant of the pterosaurs.', 'The earliest bird flew in an awkward manner.', 'The feather belonged to a Mesozoic creature that was neither a pterosaur nor a bird, but an intermediate between them.']