Best fossil of a dome-headed dinosaur is unearthed in Mongolia

By Will Dunham

(Reuters) -Pachycephalosaurs, two-legged plant eaters known for their bony dome-shaped skulls, are among the quirkiest dinosaurs known, and also among the least understood. A fossil found peeking out of a cliffside in the Gobi Desert of Mongolia - the oldest and most complete of any dinosaur from this group - is now changing that.

Researchers said the fossil represents a juvenile of a previously unknown species they have named Zavacephale rinpoche (pronounced zah-vah-SEF-al-ay rin-POH-shay), dating to about 108 million years ago during the Cretaceous Period. This individual was small and lightly built, with long legs, short arms and small hands - comparable in size overall to a German shepherd.

Very little is known about pachycephalosaurs (pak-ee-SEF-al-oh-sors), a group that inhabited Asia and North America during the final period of the age of dinosaurs, with the fossils known until now generally being quite incomplete - sometimes with just the skull.

The Zavacephale specimen, however, is largely complete, providing the first comprehensive look at the anatomy of these dinosaurs. Zavacephale also is the oldest of any known pachycephalosaur by approximately 15 million years.

This Zavacephalee individual was at least two years old at the time of death. Even though it was not fully grown - the equivalent of a human teenager - it already had a well-developed cranial dome, made up of thickened bone atop the skull. This feature in Zavacephale differed a bit from those of pachycephalosaurs from later in the Cretaceous. In Zavacephale, the dome was formed mostly by a single skull bone. In later pachycephalosaurs, it was formed mostly by two bones.

Zavacephale exhibited ornamentation accompanying the dome in the form of small spikes and nodes that encircled the dome and the back of its skull.

The researchers determined the individual's age by examining growth rings in the limb bones, akin to the annual growth rings formed in trees. The study was the first to use growth rings to identify when in the life of a pachycephalosaur the cranial dome fully develops.

The dome's purpose remains a matter of debate. Scientists among other things have hypothesized that these dinosaurs butted heads - with dome-on-dome collisions - like bighorn sheep, perhaps to establish dominance and secure mating rights.

"The consensus is that these dinosaurs used the dome for socio-sexual behaviors. The domes wouldn't have helped against predators or for temperature regulation, so they were most likely for showing off and competing for mates," said paleontologist Lindsay Zanno of North Carolina State University and the North Carolina Museum of Natural Sciences, senior author of the study published on Wednesday in the journal Nature.

"Were they for head-butting, grappling or just showing off?" Zanno asked. "At the end of the day, you have to hand it to pachycephalosaurs. When it comes to flashy headgear, they didn't phone it in."

A completely formed dome on a juvenile indicates that pachycephalosaurs engaged in social and likely mating behaviors before becoming fully grown, Zanno said.

Adults of the largest pachycephalosaurs, like the later Pachycephalosaurus from North America, could grow to around 14 feet long (4.3 meters), though adults of Zavacephale may have been less than half of that length.

The environment in which Zavacephale lived was a valley dotted with lakes and surrounded by cliffs. The remains of multiple other plant-eating dinosaurs have been found nearby as well as various fish, turtles and croc relatives.

The name Zavacephale combines the Tibetan word for "origin" with the Latin word for "head," owing to the fact that this species is the earliest-known example of dome-headed dinosaurs.

"Before Zavacephale, our record of pachycephalosaurs was almost exclusively limited to their indestructible domes. With such scanty skeletons, we were left with holes in our knowledge of their anatomy, including basic things like what their arms would have looked like, how their digestive system functioned and how the cranial dome evolved over time," Zanno said.

The fossil preserved gastroliths, the stomach stones that helped the dinosaur grind the plants it ate.

Scientists are trying to figure out the evolutionary ancestry of pachycephalosaurs.

"We've been missing transitional forms that can shed light on how their bizarre cranial domes evolved and resolve their relationships to other dinosaurs," Zanno said.

(Reporting by Will Dunham in Washington, Editing by Rosalba O'Brien)