New research published today in Nature described a teenaged pachycephalosaur from Mongolia’s Gobi Desert named Zavacephale rinpoche, the oldest and most complete skeleton of this dinosaur group found to date. The specimen has been dated to about 108 million years ago in the Early Cretaceous, pushing back the pachycephalosaur fossil record by at least 15 million years. Researchers defined the pronunciation as zah-vah-SEF-al-ay rin-POH-shay.
The fossil was unearthed in 2019 at the Khuren Dukh locality in the Eastern Gobi Basin, where the top of the skull was visible on a cliff like a cabochon jewel. Zavacephale rinpoche means ‘precious one’ in Tibetan and refers to the exposed dome; the authors combined Tibetan and Latin to mean precious root head, according to Scientific American. At the time, the area where the animal lived was a valley dotted with lakes and surrounded by cliffs.
About 90 percent of the body’s bony elements were preserved in the skeleton, including complete limbs, an articulated tail with covered tendons, gastroliths, and the first documented pachycephalosaur hand. The animal was small and lightly built, with long legs, short arms, and small hands. It measured a little over 1 meter and weighed about 6 kilograms. “It was a small animal - about three feet or less than one meter long - and the most skeletally complete specimen yet found,” said Tsogtbaatar Chinzorig, lead author at the Mongolian Academy of Sciences.
Almost all pachycephalosaur skull caps dated to the Late Cretaceous, leaving early evolution poorly sampled. By filling that earlier interval, the fossil offered new clues about the first steps toward the characteristic cranial dome and suggested the lineage may have originated in Asia before dispersing.
The team used a thin slice of a lower leg bone to count growth rings. The tissue preserved two rings and indicated the animal was still growing at death, according to New Scientist. Although not yet adult, the skull already showed a well-formed dome with less of the additional ornamentation found in later species. In Zavacephale rinpoche, the dome was formed mostly by a single skull bone, unlike later pachycephalosaurs in which two bones dominated. The cranium largely lacked knobby horns, though small spikes and nodes encircled the dome and the back of the skull, and dimples covered the dome surface. CT scans of dome sutures, combined with the growth record in the limb bones, allowed the team to link, for the first time in a pachycephalosaur, the age of an animal with the developmental stage of its cranial dome. The study also used growth rings to identify when the dome fully developed.
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“The consensus is that these dinosaurs used the dome for socio-sexual behaviors,” said Lindsay Zanno, associate research professor at North Carolina State University. “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 Zanno. “If you need to headbutt yourself into a relationship, it’s a good idea to start rehearsing early,” explained Zanno.
“Pachycephalosaurs are iconic dinosaurs, but they were also rare and mysterious,” said Zanno. “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,” said Zanno. “We’ve been missing transitional forms that can shed light on how their bizarre cranial domes evolved and resolve their relationships to other dinosaurs,” she said. “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,” she said.
“It tells us that the dome head evolved early in pachycephalosaur evolution and arose long before Zavacephale’s bigger cousins started littering the latest Cretaceous fossil record with their skull caps,” said Stephen Brusatte, a paleontologist at the University of Edinburgh who was not involved in the study.
The fossil was found alongside remains of fish, turtles, crocodile relatives, and other plant-eating dinosaurs, underscoring a lake-dotted river valley ecosystem in the Early Cretaceous Gobi. The discovery reinforced the role of the Gobi Desert as a major region for paleontology; since the 1920s, the area yielded key fossils for understanding dinosaur evolution, including some of the first known nests with eggs and embryos.