"They are a superb pair of specimens and are certainly of great scientific and display value," he said. "This pair is certainly a unique find" for the Hell Creek Formation.
Thomas Lindgren, Bonhams co-consulting director of natural history, said scientists will have to determine whether the ceratopsian was indeed a new species, but either way, it would "still be one of the rarest ceratopsians of all time."
"It is either the most complete and oldest triceratops that had lived at the end of the Cretaceous or it's a brand-new species," he said.
But Jack Horner, a paleontologist at Montana State University, called the promoters' claims a means "to enhance the price of the specimen."
"These fossils are not worth anything because they were collected to sell and not specifically for their science," he said.
Johnson, who plans to see the fossils before the auction, said the skeletons would need to be extracted from their enclosing sandstone and compared to other skeletons in various museums to determine their "actual completeness."
Finding a carnivore and herbivore together is "very unusual," said Johnson, whose museum is scheduled to open a new dinosaur hall in 2019 but has no plans to bid on the skeletons.
Phipps said he hopes a wealthy buyer donates the skeletons to a public institution, similar to how The Field Museum came to own Sue, the T. rex discovered in South Dakota in 1990.
"Any time you have a complete skeleton you can do things that can help you understand the biology of the species," said Pete Mokozicky, associate curator of dinosaurs at The Field Museum.