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The Hell Creek Formation in what's now the Dakotas, Montana, and Wyoming was once home to some of the world's most beloved dinosaurs, like Triceratops and Tyrannosaurus rex (including SUE, one of the largest, most complete, and best-preserved T. rex specimens ever found).

But these giant dinosaurs weren't alone in their ecosystem, and in a paper in the journal PLOS ONE, scientists describe two new species of birds that lived alongside these dinosaurs 68 million years ago. The researchers were able to name these new species from just one bone each: the powerful foot bone that suggests these birds could have captured and carried off prey.

"Based on clues in their foot bones, we think these birds would have been able to catch and carry prey, similar to what a modern hawk or owl does," says Alex Clark, a Ph.D. student at the Field Museum and the University of Chicago and the study's lead author.

"While they might not be the first birds of prey to ever evolve, their fossils are the earliest known examples of predatory birds."

The three fossils Clark studied in this paper had been collected in the past several years by researchers at other institutions, but there hadn't been much work done on them.

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