Additionally, the location of the tooth and its ancestry suggest that the two halves of North America, formerly separated by a giant seaway, were connected before the Dinosaur Age came to a dramatic close. Phillips explained that the tooth has a distinctive double root, which is unique to western North American horned dinosaurs known as ceratopsids.
Although paleontologists have suspected for a while that horned dinosaurs once lived in what are now Maryland and North Carolina, based on fossil fragments, those possible remains belonged to more primitive species that likely lived in the area before the Western Interior Seaway separated it from western North America. Farke therefore believes that it is "highly unlikely" that the Mississippi animal belonged to a unique eastern evolutionary group.
The researchers instead propose that the ancestors of the Triceratops-like dinosaur came from western North America.
The Western Interior Seaway that once split North America was estimated at its largest to have been 2,500 feet deep, 600 miles wide, and over 2,000 miles long. Such a massive barrier probably would have prevented a terrestrial animal — and particularly one with little or no water navigation skills — to have swum across it.
The Western Interior Seaway is long gone, but exactly when it receded has been a mystery. The dinosaur tooth shows that the seaway must have done so before dinosaurs went extinct 66 million years ago.
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“The Western Interior Seaway receded from the continent in either a southerly and/or northerly direction as a result of an actively growing Rocky Mountains, which were the size of mere foothills when the last of the dinosaurs were roaming the earth,” Phillips said. “Evidence for this retreat of the seaway lies in the types of sediments deposited and stacked in the ancient basin once occupied by the seaway and are now exposed at higher elevations with the growth of the Rockies.”
Since the Triceratops-like animal wound up in bay water — not the seaway — after its death, the dinosaur’s tooth fossil was found alongside the remains of water dwellers, such as ancient clams, crabs, fishes, and giant marine lizards. During the dinosaur’s lifetime, these animals were all in the large bay, which was about the size of today’s Mobile Bay in Alabama.