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Fossil of dog-sized dinosaur 'shows prehistoric evolutionary divide'

The fossil of a dog-sized, horned dinosaur that roamed eastern North America up to 100 million years ago, is evidence of an east-west divide in the evolution of dinosaurs on the continent, a scientist studying the fossil has said.

An artist's impression of a dog-sized horned dinosaur which roamed eastern North America up to 100 million years ago Credit: Nick Longrich/University of Bath/PA Wire

During the Late Cretaceous period, 66 to 100 million years ago, the North American continent was divided by a shallow sea into two landmasses - Laramidia in the west, and the "lost continent" of Appalachia in the east.

Dinosaurs living in Laramidia were similar to those found in Asia.

But few fossils from Appalachia have been discovered because the areas dense vegetation has made it difficult to discover and excavate them.

Dr Nick Longrich, from the Milner Centre for Evolution at the University of Bath, studied a fragment of jaw bone from a rare eastern fossil.

Although he was unable to identify the exact species accurately, he saw it had a strange twist to the jaw, causing the teeth to curve downward and outwards in a beak shape.

The jaw was also more slender than that of Ceratopsia found in western North America, suggesting the dinosaurs had different diets and evolved along distinct evolutionary paths.

Ceratopsia is a group of plant-eating horned dinosaurs that lived in the Cretaceous period.

Just as many animals and plants found in Australia today are quite different to those found in other parts of the world, it seems that animals in the eastern part of North America in the Late Cretaceous period evolved in a completely different way to those found in the western part of what is now North America due to a long period of isolation.

This adds to the theory that these two land masses were separated by a stretch of water, stopping animals from moving between them, causing the animals in Appalachia to evolve in a completely different direction, resulting in some pretty weird looking dinosaurs.

– Dr Nick Longrich, Milner Centre for Evolution

The fossil Dr Longrich studied comes from a smaller cousin of the better known Triceratops, the leptoceratopsids, which is about the size of a large dog.

The fossil is kept in the Peabody Museum at Yale University in the US.