The two virtually complete dinosaur skeletons, a carnivore and a herbivore, were found fossilised together in Hell Creek, Montana in 2006.
Dubbed the "Dueling Dinos," the specimens are preserved in a sensational death match between predator and prey.
Bonhams will auction the fossil on November 19, with an estimate of USD 7,000,000 to USD 9,000,000, in New York.
The dinosaurs have been identified as a Nanotyrannus lancensis, a close relative of Tyrannosaurus Rex, and a new species of Chasmosaurine Ceratopsian (similar to a Triceratops), Bonhams said in a statement.
Nanotyrannus teeth are present in the skull of the Chasmosaurine, while the Nanotyrannus' skull and chest appear to have been crushed laterally, as though kicked. The state of the skeletons indicates the pair was buried very shortly after the fatal struggle.
Their preservation in fine-grain, loosely consolidated sandstone allowed them to remain intact despite the weight of the sediment that buried them.
The specimens were removed in large, plaster-jacketed sections of earth, safeguarding the spatial relationships in which the bones were found.
Moreover, the "Dueling Dinos" may hold the key to answering one of the most puzzling questions for paleontologists today.
Currently, researchers are divided over whether Nanotyrannus' are their own genus, or whether they are simply juvenile Tyrannosaurus Rexes.
The Nanotyrannus involved in the "Dueling Dinos" is only the second example ever found, and by far the most complete, offering the best hope to date of answering this pressing scientific question.
"Offering the 'Dueling Dinos' at Bonhams in November is the highlight of my career. They are one of the most significant dinosaur discoveries ever in North America," said Thomas E Lindgren, Co-Consulting Director of the Natural History Department at Bonhams.
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