In Mongolia’s Gobi Desert, scientists have discovered a 108-million year-old juvenile pachycephalosaur (dome-headed dinosaur) who had a fully-formed dome on its head. This discovery means two things. One, that pachycephalosaurs likely engaged in headbutting earlier in their lives, not waiting to be fully grown. And two, that pachycephalosaurs evolved approximately 15 million years before previously known.
The dome may have been a display weapon, not a weapon weapon
CT scans and bone slices show that this dinosaur had a fully formed cranial dome despite being young at death. That suggests the dome developed early, and not only in adults, challenging assumptions that domes only appear later in life.
Its dome probably wasn’t for defense against predators or heat control. The authors argue the dome’s purpose lies in socio-sexual behavior, domes were showy structures used in contests for mates, rivalries, and display. The authors suggest the dome was a display weapon. It may not have been for defense, but for signaling strength, competing for mates, and establishing dominance.
Natural position of the skeleton in field jacket. Image Credit: Nature
This positions the pachycephalosaur dome in the same class as horns or antlers in modern animals. It also strengthens the idea that display structures can evolve early in life stages, not just when animals reach full size, suggesting that sexual selection was a potent force in dinosaur evolution
What else does the discovery tell us about the dinosaur itself?
Unlike most pachycephalosaurs, which are known only from skull fragments, this dinosaur, named Zavacephale rinpoche has a near-complete skull, limb bones, hand elements, even an articulated tail with preserved tendons and stomach stones (gastroliths). Hand bones and stomach stones provide clues about locomotion and diet. The tail with tendons preserved also helps reconstruct how flexible and strong their tails were.
These data let scientists move past speculative reconstructions toward anatomically informed life reconstructions. Z. rinpoche would have stood about 1 meter tall and weighed roughly 5.85 kilograms.
Until Zavacephale, pachycephalosaurs were documented only from later Cretaceous strata. Zavacephale, however, is the oldest definitive dome-headed pachycephalosaur. Pushing the group’s record back helps fill the evolutionary gap in dome-headed dinosaurs.
“The dome-headed pachycephalosaurians are among the most enigmatic dinosaurs“, the scientists wrote in their paper published in Nature last month. “Members of the clade are considered to have evolved complex sociosexual systems. Despite their importance in understanding behavioural ecology in Dinosauria, the absence of uncontested early diverging taxa has hindered our ability to reconstruct the origin and early evolution of the clade.”