Two prehistoric predators are going up for auction at Sotheby's in New York. The fossil skeletons of a Pteranodon and a Plesiosaur are both going up for sale. The Pteranodon being offered is expecting to fetch an estimate of $4-6 million. With a wingspan of nearly 20 feet, the Pteranodon is one of the more complete large skeletons from the age of dinosaurs to ever come to market. Cassandra Hatton, a Senior Vice President at Sotheby's, said the market for the Pteranodon is for both private collectors and institutions. The other highlight of the Geek Week auction is one of the best preserved Plesiosaur skeleton's ever discovered. Known as "Nessie," the Jurassic-era Plesiosaur is expected to fetch between $600,000- 800,000. Both skeletons will Headline Sotheby's Live Natural History Auction on July 26th.