求教大全中的一题

1.       A huge flying reptile that died out with the dinosaurs some 65 million years ago, the Quetzalcoatlus had a wingspan of 36 feet, believed to be the largest flying creature the world has ever seen.

a.  and it is believed to have been

b. believed to be

用havebeen是为了说明过去是，现在不是么？

谢谢