Zavacephale rinpoche at time of discovery. Credit: Tsogtbaatar Chinzorig (CC BY-NC-ND)
Palaeontologists have unearthed a new species of dinosaur in a “once-in-a-lifetime discovery” which pushes back the earliest record of dome-headed pachycephalosaurs by 15 million years.
The species, Zavacephale rinpoche, lived 108 million years ago (mya) during the Early Cretaceous period in what is today Mongolia’s Gobi Desert.
Pachycephalosaurs are a rare clade of small-bodied, bipedal herbivores best known for their iconic dome-shaped skulls.
“The consensus is that these dinosaurs used the dome for socio-sexual behaviours,” says Lindsay Zanno, associate research professor at North Carolina State University and corresponding author of the study published in Nature.
“The domes wouldn’t have helped against predators or for temperature regulation, so they were most likely for showing off and competing for mates.”
Artist illustration of Zavacephale rinpoche in a headbutting contest. Credit: Masaya Hattori (CC BY-NC-ND)
In other words, these dinosaurs were holding headbutting contests on the regular.
Zanno says the new specimen is a once-in-a-lifetime discovery.
“It is remarkable for being the oldest definitive pachycephalosaur, pushing back the fossil record of this group by at least 15 million years, but also because of how complete and well-preserved it is,” she says.
“It has limbs and a complete skull, allowing us to couple growth stage and dome development for the first time.
“Pachycephalosaurs are all about the bling, but we can’t use flashy signalling structures alone to figure out what species they belong to or what growth stage they’re in because some cranial ornamentation changes as animals mature.
Zavacephale rinpoche skull. Credit: Tsogtbaatar Chinzorig (CC BY-NC-ND)
“We age dinosaurs by looking at growth rings in bones, but most pachycephalosaur skeletons are just isolated, fragmentary skulls.”
The researchers used high-resolution computed tomography (CT) and examinations of thin slices of the skull and lower leg bone to determine the Z. rinpoche specimen already had a fully formed dome despite being a juvenile when it died.
“If you need to headbutt yourself into a relationship, it’s a good idea to start rehearsing early,” suggests Zanno.
The researchers estimate the specimen would have been 1m tall and weighed about 5.85kg.
Zavacephale rinpoche hand bones. Credit Alfio Alessandro Chiarenza (CC BY-NC-ND)
Lead author of the paper Tsogtbaatar Chinzorig of the Mongolian Academy of Sciences, who discovered the specimen in the Eastern Gobi Basin, adds: “Z. rinpoche is an important specimen for understanding the cranial dome development of pachycephalosaurs, which has been debated for a long time due to the absence of early diverging or pre-Late Cretaceous species and the fragmentary nature of nearly all pachycephalosaurian fossils.
“The newly recovered materials of Z. rinpoche, such as the hand elements, the stomach stones (gastroliths), and an articulated tail with covered tendons, reshape our understanding of the paleobiology, locomotion, and body plan of these ‘mysterious’ dinosaurs.”