The duck-billed dinosaurs that roamed North America some 77 million years ago were giant herbivores with a flat, paddle-shaped bony crest that completely covered the back of the top of the skull. But the ancestral form that lived just a few million years earlier has no crest. Now, researchers have unearthed the fossils of a new, transitional duck-billed species that bridges the older, crestless dinosaurs with the later, large-crested ones. Probrachylophosaurus bergei has a small crest that extends over the skull just above the eyes. It’s described in PLOS ONE this week.
The crestless Acristavus gagslarsoni from the Two Medicine Formation of Montana lived around 81 million years ago, and it’s thought to be the ancestral member of this lineage of hadrosaurs (or duck-billed dinos). The younger, large-crested Brachylophosaurus canadensis is known from the Oldman Formation of Alberta.
In the summer of 2007, teams led by Montana State University’s Elizabeth Freedman Fowler and John Horner were excavating in the Judith River Formation in north-central Montana. At 79 million years old, this area is intermediate in age. "The first bones we uncovered were the pelvis and parts of the legs; which were so large it led to the site being given the nickname ‘Superduck,’" Freedman Fowler says in a statement. Then they found the jaws, parts of a braincase, and after cleaning the fossils back in the lab, they realized they had most of the skeleton of a new species.
Because of its age, the team predicted that the skull and crest would be intermediate between Acristavus and Brachylophosaurus. "And it is. It is a perfect example of evolution within a single lineage of dinosaurs over millions of years,” Freedman Fowler adds. "The crest of Probrachylophosaurus is small and triangular, and would have only poked up a little bit on the top of the head, above the eyes."
Visual abstract depicting evolution of Probrachylophosaurus into Brachylophosaurus. Elizabeth Freedman Fowler
The new genus name combines "pro," Latin for before, and "brachylophosaurus," Greek for short-crested lizard. It refers to its discovery in a lower (older) layer than Brachylophosaurus. The new species name honors Sam Berge, co-owner of the land where the fossils were found.
While Probrachylophosaurus is a "missing link" between a crestless ancestor and a large-crested descendent, the new species isn’t the ancestor of every crested duck-billed dinosaur, Freedman Fowler tells IFLScience over email. "It seems that several different subgroups of the hadrosaur family independently evolved different crests, all from different crestless ancestors," she adds. "So the elaborate crests of duck-billed dinosaurs probably evolved at least three times from scratch, maybe more."
Furthermore, a fragmentary Probrachylophosaurus juvenile suggests that successive generations of the Brachylophosaurus lineage were able to grow larger crests by changing the timing or the pace of crest development as they matured – a process called heterochrony. Juveniles of many animals retain ancestral characteristics that aren’t expressed by the time they reach adulthood. (That’s why fossils of juveniles are often mistaken for more primitive species.) The crest of Brachylophosaurus, for example, enlarges late in its development. "Heterochrony is key to understanding how evolution actually occurs in these dinosaurs," Freedman Fowler says.
Museum of the Rockies field crew digging in the Superduck quarry. Elizabeth Freedman Fowler