It's in the heads: Dinosaur head size

Scientist links bony cranial ornamentation to body size


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Chris Organ, paleontologist, visiting research professor at Montana State University and MSU alumnus, is co-author of a paper published in the journal Nature Communications that connects the bony cranial ornaments of theropod dinosaurs to their body size. MSU Photo by Adrian Sanchez-Gonzalez

By Michael Becker, for the MSU News Service

Theropod dinosaurs with bony ornaments, like horns and crests, evolved to gigantic body sizes as much as 20 times faster than species without them, according to research by a Montana State University evolutionary biologist.

These ornaments perform a wide variety of functions, the most important being social communication and competition for mates.

“They are more than mere decorations; they are vital for reproductive success by determining which individuals pass along their genes to the next generation,” said Chris Organ, research professor in MSU’s Department of Earth Sciences in the College of Letters and Science. “Figuring out how ornaments evolve is important for understanding behavior in general, but will also advance the understanding of dinosaur behavior.”
Organ is one of three authors of the paper "Bony cranial ornamentation linked to rapid evolution of gigantic theropod dinosaurs," published Sept. 27, in the journal Nature Communications that applies statistical models of evolution to study how dinosaurs evolved.
Along with co-authors Terry Gates and Lindsay Zanno from North Carolina State University, Organ examined theropod dinosaurs — a group of mostly carnivorous species that walked on two legs, perhaps best known by its most famous species: Tyrannosaurus rex.
Birds are theropod dinosaurs as well, Organ said, and the only group of these Mesozoic monsters to have survived the Cretaceous-Paleogene extinction, a mass extinction of nearly three-quarters of the plant and animal species on Earth that occurred 65 million years ago.
Organ said the research project began with a simple observation: Large theropods tended to have bony head ornaments — horns, crests or knobs. Those ornaments likely served a variety of social functions, such as signaling to competitors and potential mates. Importantly, like plumage in small theropods, including modern birds, the ornaments probably affected how theropods interacted with one another and reproduced.
Organ and the other researchers wondered what effect those ornaments ultimately had on the evolution of new species, their physiology and even their extinctions.
“Essentially, we’re using statistics to quantify evolutionary patterns over many millions of years, and this helps us understand the processes that generate them,” Organ explained. "Vertebrate paleontology is getting much more sophisticated in how evolutionary processes are studied. Twenty years ago, questions like this wouldn’t have considered evolutionary relationships in a rigorous statistical framework. And in science, what is qualitative is often vague and open to many interpretations."
The researchers began examining theropod evolutionary trees of 111 species.
"If you have an evolutionary tree, you can model how traits change over time and from lineage to lineage, and this helps you understand possible drivers and constraints of evolution,” Organ said. “This type of analysis extends far beyond paleontology and is fundamental to studying anything that evolves, from languages to cancer.”
The research found that theropods weighing 80 pounds or less didn’t evolve bony ornaments. Above that weight limit, 20 out of the largest 22 theropod species -- for which skulls are known -- have some form of head ornament.

Moreover, the species that developed ornaments evolved toward gigantic body sizes — at least 2,200 pounds — as much as 20 times faster than non-ornamented species, taking big leaps in size every few million years.
Organ’s lab investigates topics ranging from genomics to human evolution. Organ earned his doctorate from MSU's Department of Cell Biology and Neuroscience in 2004. After completing a fellowship from the National Institutes of Health at Harvard University, he returned to MSU in 2013. He is also a teaching professor in MSU’s Honors College and in the WIMU Regional Program in Veterinary Medicine, a collaboration among MSU, Washington State University, Utah State University and the University of Idaho.

Organ said the findings from the paper will help paleontologists better understand how behavior evolved in dinosaurs and birds.
"You can dig up the fossils and you can describe them, but we also want to know what they were like as living animals," Organ said. "How did they behave? This research aims to better understand how they found mates and reproduced. After all, that is where life and evolution begin."

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