From the sands of Mongolia’s Gobi Desert emerges a new, exquisitely preserved fossil that could revolutionize our understanding of pachycephalosaurs – those iconic dinosaurs sporting a dome of thick bone at the top of their heads.
The newly discovered Zavacephale rinpoche is the oldest known pachycephalosaur ever found, with an age dating back at least 108 million years ago, pushing back the fossil record by 14 million years. The specimen is also the most complete skeleton of its kind ever found.
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Its extraordinary features include a spectacularly preserved skull, the first-ever example of a pachycephalosaur hand, the stones in its stomach that aided digestion, and a full tail complete with tendons.
“The first time I saw Zavacephale, it literally took my breath away,” paleontologist Lindsay Zanno of North Carolina State University told ScienceAlert. “Everyone who has seen it is in awe.”
The pachycephalosaurs are a group of dinosaurs that mostly lived in the Late Cretaceous, from about 86 to 66 million years ago. Their elaborate head adornments included large, prominent domes of thickened, fused bone, and bony horns and knobs. Scientists think that they used these elements for social and mating purposes.
However, there are a lot of open questions about this group of animals. Pachycephalosaur fossils are rare, and most of the examples found to date are dramatically incomplete, usually just consisting of part of the skull.
Zavacephale was discovered in the Khuren Dukh formation by a team led by paleontologist Tsogtbaatar Chinzorig of the Mongolian Academy of Sciences. The researchers immediately knew they were looking at something extraordinary.
The skeleton is not fully complete – it’s missing some parts, such as the neck, most of its backbone, and parts of its limbs – but it’s far and away the most complete pachycephalosaur skeleton ever discovered.
“The quality of fossilization of the preserved skeletal elements of Zavacephale is extraordinarily well-preserved compared to other recovered specimens from the same locality, such as holotypes of the iguanodontian dinosaur Choyrodon and the primitive ornithomimosaur Harpymimus,” Chinzorig told ScienceAlert.
“In addition, the skull of Zavacephale is exquisitely and three-dimensionally preserved (no signs of pre- or post-mortem distortion).”
That remarkable preservation, Zanno says, is a product of the sandstone fossilization process, which would have buried the remains quickly in relatively soft material that cushioned the bones against crushing pressures.
In life, the animal was small, measuring just a meter (3 feet) or so in length, and weighing around 5.85 kilograms (12.9 pounds). And here’s where it gets exciting… or rather, even more exciting.
Paleontologists can determine the age of a dinosaur when it died by examining bone growth rings in its limbs, a technique that the researchers employed for Zavacephale.
This specimen was a juvenile, an adolescent of its species. But its head dome was fully mature.
“The domed and ornamented heads of some pachycephalosaurs are so baroque, and we still do not completely understand their purpose. Were they for head-to-head combat, showing off, a little of both?” Zanno said.
“To answer these questions, we need to determine when, in the life of these animals, the dome completely forms. Because Zavacephale is so complete, we were able to test the relative developmental stage of the dome and the rest of the skeleton in the same animal for the first time.
“We examined the sutures inside the dome with CT scanning and coupled this with the growth record preserved in the limb bones. What we found is that Zavacephale was fully armed for showing off and competing with its kin during its teenage years.”
Because it’s the earliest pachycephalosaur on record, Zavacephale is also something of a Rosetta Stone for understanding how these animals evolved. The researchers found that the dome in its skull mainly consisted of the bones in the forehead, an interesting contrast to later species whose domes incorporate more of the skull.
“What’s really interesting to me is that in some of those later species, the dome transitions through the Zavacephale condition as the animal grows from a baby to an adult. Meaning that Zavacephale is a link between evolutionary change and developmental change,” Zanno said.
Future work on this once-in-a-lifetime discovery is already underway. Paleontologists are already examining the dome in greater detail to try to understand its function. The tiny hands and stomach stones of Zavacephale are also of intense interest, as is the tendon that runs along its tail.
“Zavacephale is the most complete pachycephalosaur yet discovered, and we expect our colleagues to be clambering to get a peek at it,” Zanno said. “There are literally decades of research to come.”
The discovery has been published in Nature.