Well-preserved remains of the oldest known pachycephalosaur have been discovered in Mongolia. The find, approximately 108 million years old, will help understand how this mysterious group of dinosaurs evolved, UNN reports, citing Reuters.
Details
Researchers stated that the fossil represents a "juvenile of a previously unknown species, which they named Zavacephale rinpoche."
"It dates back approximately 108 million years ago, to the Cretaceous period. This individual was small and lightly built, with long legs, short 'arms,' and small hands – overall comparable to a German Shepherd," the publication states.
Zavacephale is also the oldest of all known pachycephalosaurs, approximately 15 million years older than previously known specimens. Very little is known about this group, which inhabited Asia and North America during the last period of the dinosaur era, and the fossils found so far are usually incomplete – sometimes only with a skull. However, the new Zavacephale specimen provides the first comprehensive insight into the anatomy of these dinosaurs, the publication writes.
"This Zavacephale individual was at least two years old at the time of death. Although it was not yet fully adult – the equivalent of a human teenager – it already had a well-developed cranial dome, consisting of thickened bone on top of the skull," the publication notes.
Peculiarity of Pachycephalosaurs
In Zavacephale, the dome was formed primarily by a single skull bone. In later pachycephalosaurs, it was formed primarily by two bones. Zavacephale had decorative elements on the dome and the back of the skull – small spikes and nodules forming a kind of ornament.
Researchers determined the age of the individual by examining growth rings in the limb bones, similar to the annual growth rings formed in trees. This study was the first to use growth rings to determine when the skull dome fully developed in a pachycephalosaur's life.
The purpose of the dome remains a subject of debate. Scientists, among other things, have hypothesized that these dinosaurs butted heads – dome to dome – like bighorn sheep, possibly to establish dominance and secure mating rights.
"The general consensus is that these dinosaurs used the dome for socio-sexual behavior. Domes would not help against predators or for temperature regulation, so most likely they were used for display and competition for mates," said paleontologist Lindsay Zanno of North Carolina State University and the North Carolina Museum of Natural Sciences.
A fully formed dome in a juvenile individual suggests that pachycephalosaurs engaged in social and likely mating behaviors before reaching full adulthood, another researcher says.
Addition
Adults of the largest pachycephalosaurs, such as late North American representatives, reached approximately 4.3 meters in length, while adult Zavacephale were smaller – no more than half that size. Zavacephale lived in a valley dotted with lakes and surrounded by cliffs. Remains of numerous other herbivorous dinosaurs, as well as fish, turtles, and crocodile relatives, were found nearby.
The name Zavacephale combines the Tibetan word for "origin" and the Latin for "head," emphasizing that it is the oldest known representative of dome-headed dinosaurs.
"Before the advent of Zavacephale, our data on pachycephalosaurs were almost exclusively limited to their indestructible domes. With such meager skeletons, we were left with gaps in our knowledge of their anatomy, including such basic things as what their 'arms' looked like, how their digestive system functioned, and how the cranial dome evolved over time," explained paleontologist Lindsay Zanno.
The fossils also preserved gastroliths – stomach stones that helped the dinosaur grind plant food. Scientists are trying to determine the evolutionary origin of pachycephalosaurs.
"We lacked transitional forms that could shed light on the evolution of their bizarre cranial domes and clarify their relationships with other dinosaurs," Zanno said.