Paleontologists have been baffled as to how horned dinosaurs such as the triceratops that once walked in ancient North America arrived in the continent. A small crow-sized dinosaur with the weight of a bunny is shedding light to this mystery.

The dinosaur called Aquilops americanus is the oldest known horned dinosaur found in North America. Scientists on Wednesday, describe the discovery of its fossils in a study published in the journal PLOS ONE on Dec, 10.

The skull of the Aquilops was first found in 1997 during an expedition to Montana led by Richard Cifelli, who is currently affiliated with the Sam Noble Oklahoma Museum of Natural History in Norman, Oklahoma. Despite its relatively small size, the 3-inch beaked skull filled the gap in the evolutionary history of a group of horned dinosaurs called ceratopsians in North America.

Paleontologists found that Aquilops, which lived in the Early Cretaceous period, is not closely related to the later ceratopsians from North America but were closely related to the dinosaurs from Asia. The similarities suggest that horned dinosaurs migrated from Asia to North America millions of years ago.

"In most features, it's virtually identical to them," said study researcher Andrew Farke, from the Raymond M. Alf Museum of Paleontology in Claremont, California. "And that's cool because it adds support for this idea that, around 110 million years ago or so, there was a big influx of animals from Asia into North America."

Despite the distance between Asia and North America, which at the time is slightly farther compared today, experts said that a land bridge may have connected the two continents and this allowed the horned dinosaurs to walk across.

"Aquilops is not closely related to later ceratopsians from North America: instead, phylogenetic and biogeographic data suggest a complex history of interchanges between North America and Asia," the researchers wrote.

Unlike the Triceratops, the Aquilops does not have horns but it has other features that are distinct to horned dinosaurs such as the presence of a toothless beak called rostral bone. The researchers likewise noted that the dinosaur's rostral bone has a little bump and could possibly have started the type of horn present in the dinosaur's later relatives.

Aquilops also have cheek spikes which the animal may have used for defending itself as this could be inflicting pain on its enemy if it gets too close.