A new type of duckbilled dinosaur discovered by researchers at Montana State University shows how the creatures transitioned from ancestral forms that lacked a crest to a descendant which had a larger crest, shedding new light on what was a poorly understood evolutionary process.
The new species, which is described in the latest edition of the journal PLOS One, was found by adjunct professor Elizabeth Freedman Fowler and MSU paleontologist Jack Horner. Identified as Probrachylophosaurus bergei, the new dinosaur is believed to be the missing link separating the 81 million year old Acristavus from its descendants, the Brachylophosaurus.
The bones in the skull of the new dinosaur are said to be similar to those of the Acristavus and the Brachylophosaurus, the study authors explained. However, Acristavus does not have a crest and the top of its skull is flat, while Brachylophosaurus has a large flat paddle-shaped crest that covers the back of the top of its skull and Probrachylophosaurus has a crest in the middle of the skull.
“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,” explained Freedman Fowler, who received her doctorate in paleontology from MSU’s Earth Sciences Department earlier this year and is now the curator of paleontology at the Great Plains Dinosaur Museum in Malta.
Findings underscore the importance of heterochrony
The fossils were originally discovered during the summer of 2007, as Freedman Fowler led an expedition to a site in north-central Montana that was home to duckbilled dinosaur remains. The team found what wound up being the skull and postcranial bones of the new species.
Thanks to dating techniques, the researchers determined that Probrachylophosaurus lived about 79 million years ago, placing it between Acristavus and Brachylophosaurus. Based on the age of the new creature, the MSU team expected its skull and crest to be a bridge between both of those dinosaurs – which, as it turns out, it was.
Probrachylophosaurus is “a perfect example of evolution within a single lineage of dinosaurs over millions of years,” Freedman Fowler said. A fragmented juvenile Probrachylophosaurus was also found at a nearby site, she added. That discovery suggests that each generation of the Brachylophosaurus lineage grew larger crests by changing the pace of crest development.
This change in the timing or rate of development is called heterochrony, the MSU researchers explained, and it is a process that is becoming more recognized as a driving evolutionary force. Freedman Fowler called heterochrony the “key to understanding how evolution actually occurs in these dinosaurs,” but added that a large amount of specimens from several different growth stages are required to develop “a really precise time framework” of such changes.
—–
Image credit: Montana State University/ Elizabeth Freedman Fowler
Comments