The teenage pachycephalosaur was raring to go. Like many other bird-hipped Ornithischian dinosaurs and many a modern animal too, he had reached sexual maturity before his body finished growing – we monkeys do the same.
We can’t say whether being addled by puberty hormones messed with his mind and led to his premature passing, as happens with incautious apes. But somehow he died in what is today the Gobi Desert 108 million years ago before he could grow up, Tsogtbaatar Chinzorig of the Mongolian Academy of Sciences, Lindsay Zanno of North Carolina State University and colleagues reported in Nature on Wednesday.
The youthful pachycephalosaur was a previously unknown species of dome-skulled dinosaur and is the earliest of his kind found to date, pushing back the timeline of the group by as much as 22 million years to the Early Cretaceous.
His species is now named Zavacephale rinpoche, from zava, meaning “root” or “origin” in Tibetan, and cephal, meaning “head” in Latin. Rinpoche is “precious one” in Tibetan and refers to the domed skull, which Chinzorig observed exposed on a cliff like a cabochon jewel.
This took place in the Lower Cretaceous Khuren Dukh Formation in the Gobi Desert, Mongolia, the researchers say. Today the site is stark and arid but back then it was a pleasant valley.
Young Zavacephale was wee, all of a meter long, and weighed about 5.9 kilograms, or 13 pounds, about the same as a Jack Russell Terrier. Other species of dome-headed dinosaurs in the Late Cretaceous could reach 4 to 5 meters long in adulthood and are estimated to have weighed between 370 and 450 kilograms (992 pounds).
No, from his remains, the researchers can’t extrapolate how big this one might have become, Zanno says. But at a meter tall in teen-hood, he probably wouldn’t have been a giant.
Though Zavacephale is the earliest pachycephalosaur ever found, there were earlier ones we haven’t found. Such are the vagaries of fossil hunting. His species isn’t ancestral to the group; they were already diverging, the authors say. Even so, his discovery sheds light on the evolutionary trajectory of dome-headed dinosaurs, and their personal maturation, which had been unclear due to a paucity of fossil evidence and the terrible shape of those clues.
This fossil is almost complete, so the team could deduce that his bony head-dome finished developing while the rest of him didn’t.
How teenagers do it
How do we know he was sexually mature? “The dome evolved for social interactions, most likely involving mating or territorial behaviors, thus the timing of full-dome development should correspond to sexual maturity,” Zanno explains by email from Mongolia.
How do we know he was somatically immature? “We determined that it was not fully grown by examining a thin slice of its leg bones. The fossilized tissue confirms this animal was a juvenile when it died,” she says.
“We age dinosaurs by looking at growth rings in bones, but most pachycephalosaur skeletons are just isolated, fragmentary skulls. Z. rinpoche is a spectacular find because it has limbs and a complete skull, allowing us to couple growth stage and dome development for the first time.”
So why are we calling him a teenager? Because he was sexually mature but not full-grown, and if ever there was teenage territory, that’s it.
Are the scientists sure pachycephalosaurs weren’t born, or hatched really, with domes?
“We have not yet discovered any baby pachycephalosaurs, but the smallest skulls we have generally lack domes,” Zanno says. “The dome is thought to develop as the animal matures from baby to adult.”
All about the bling
There are two base hypotheses behind the thickened skull roof of pachycephalosaurs, which have puzzled paleontologists since the discovery of the beast in 1859. Paleontology was young and the discoverer of pachycephalosaurs, Ferdinand Vandeveer Hayden, thought that the creature resembled an armadillo and named it Tylosteus, or “knob lizard.” Later, separately, other specimens would be found and named Pachycephalosaurus.
When the realization dawned that Tylosteus and Pachycephalosaurus were the same, the former name should have prevailed according to the laws of nomenclature, but the latter was better known.
The two base hypotheses are self-defense and sociosexual behavior or both. Self-defense is self-explanatory. Sociosexual behavior could have included displaying for the ladies and/or fighting with other males. It seems many of the behaviors exhibited these days on TikTok go very far back.
In short, this is exactly the same base theory for other elaborately blinged-out Ornithischian dinosaurs, including frilled and horned ceratopsians such as Triceratops and stegosaurias with their spinal plates, not to mention ankylosaurians with their body armor and mace-like tail.
Among this group too, paleontologists have identified decoupling between skeletal maturity and the bling that plausibly served to seduce – and/or fight – a characteristic also evident in extant dinosaurs, the researchers write. “Extant dinosaurs” are birds, which can reach sexual maturity before they reach full adulthood.
It’s just that this hadn’t been demonstrated in pachycephalosaurs before. Now it has been – the cranial dome stops growing before the rest of the skeleton.
“The domes wouldn’t have helped against predators or for temperature regulation, so they were most likely for showing off and competing for mates,” Zanno says. As for the early sexuality: “If you need to head-butt yourself into a relationship, it’s a good idea to start rehearsing early.”
The teenager in question was also the first pachycephalosaur in which the paleontologists also discovered gastroliths, known in chickens as gizzard stones. He was a herbivore and swallowed stones to aid in food breakdown and digestion. Gastroliths have been discovered in other herbivorous dinosaurs too.
And this teenager may have had children, who would pass down the ancient feature of domes atop their heads that they would probably use to ram into other pachycephalosaurs when feeling frisky.