A pushed-in skull and teeth of one dinosaur embedded in the other suggested a mortal confrontation between them.
Clayton Phipps, a fossil hunter who made the discovery on his neighbor's land in 2006 in the fossil-rich Hell Creek Formation, dubbed them the "Montana Dueling Dinosaurs."
Phipps and the ranch owners have put them up for sale at Bonhams auction house on Tuesday for an estimated $7 million to $9 million, a price out of the reach of most museums.
The auction record for a dinosaur fossil is slightly more than $8.36 million, which was set in October 1997 for a Tyrannosaurus rex skeleton that sold at Sotheby's. Known as Sue, the skeleton is on display at The Field Museum in Chicago.
The fossils are believed to be a Nanotyrannus lancensis, a smaller relative of the T. rex, and a newly discovered species of Chasmosaurine ceratopsian, a close relative of the Triceratops, which lived at the end of the Cretaceous age some 65 million years ago.
"I am just the lucky guy that happened to stumble out there and find this dinosaur," Phipps said. "I really appreciate the academic paleontologists that understand the importance of what us amateurs bring to the mix. I am hoping that it will be professionally and academically studied. ... I want to know more about them."
They were found fully articulated with pockets of skin tissue attached. They have been separated into four large blocks because of their total 40-ton weight and are on display in a plaza adjacent to Bonhams.
Kirk Johnson, director of the Smithsonian's National Museum of Natural History, called the dinosaurs "a significant discovery."
"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.
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