From Gizmodo.com in the USA:
A new dinosaur species sheds some light on how duck-billed dinosaurs got their crests. Paleontologists say Probrachylophosaurus bergei is a missing link between two other species, and it fills in vital pieces of the story of how crests evolved.
Probrachylophosaurus bergei is a hadrosaur, one of the large crested herbivores that roamed the Earth – mostly on their hind legs – during the late Cretaceous period. Hadrosaurs are best known for their duck-like bills and their frilled, crested skulls, and now scientists know a little more about how those distinctive crests evolved.
The fossils’ age put Probrachylophosaurus right in the middle of two hadrosaur species: the older Acristavus, which had no crest on its skull, and the more recent Brachylophosaurus, which had a large, well-developed crest. “So we would predict that its crest would be intermediate between these species. And it is,” said Elizabeth Freedman Fowler, the Montana State University paleontologist who unearthed the first Probrachylophosaurus fossils in 2007 and has studied them ever since.
81 million years ago, a hadrosaur called Acristavus roamed the Late Cretaceous coastal plain that is now Montana. Unlike its descendants, Acristavus had a flat skull with no sign of a crest – but by 79 million years ago, its descendants had evolved small, triangular crests that stuck up from their skulls just slightly, right above their eyes. Otherwise, their skulls weren’t very different from their ancestor, Acristavus. This small-crested species is now called Probrachylophosaurus.
By 77.5 million years ago, those small triangular crests had evolved further, into large, flat, paddle-shaped crests covering the back portion of the top of their skulls. Paleontologists now call these hadrosaurs Brachylophosaurus, and aside from the crests, their skulls are very similar to Acristavus and Probrachylophosaurus.
It’s a classic example of a “missing link” in a field where things seldom fall into place so neatly. “It is a perfect example of evolution within a single lineage of dinosaurs over millions of years,” said Freedman Fowler. She published her findings in the journal PLOS One.