The Late Cretaceous saw a spectacular proliferation of ceratopsians along the coastal floodplains of western North America, with two distinct families diversifying across the landscape, each sporting every imaginable combination of spines, horns, and frills, and using their head ornaments to signal to mates and challenge rivals.
78 million years later, ancient fossils continue to be found, leading to a modern boom in discoveries, the latest of which was published Thursday by a team in the journal PeerJ. — Lociceratops rangiformis was a five-ton herbivore with an impressive curved horn and huge blade-like spines on a metre-long frill.
The researchers say it's a new species, and that together with others like it, it suggests the region from Mexico to Alaska was a hotbed of dinosaur biodiversity, but other experts say there's insufficient evidence to draw such a conclusion based on a single fossil.
The dinosaur skull in question was discovered by a commercial paleontologist on private land in northern Montana in 2019 and was acquired by the Museum of Evolution in Maribo, Denmark.
“They preserved it by purchasing it, and now it's available for scientists to look at in perpetuity,” said Joseph Sertich, a paleontologist at the Smithsonian Tropical Research Institute and author of the study. “You can't write a paper about a fossil that's on display in someone's living room and is treated as a piece of art.”
The team initially thought they were working with the remains of a Medusaceratops, but as they began to put together the shattered skull pieces, they began to notice differences.
The animal lacked a nasal horn, had a hollow rostrum, and on the underside of its frill were curved, paddle-like horns — the largest of any horned dinosaur yet discovered — with a distinctive asymmetrical spike in the middle.
“That's when we started to get really excited,” says Mark Loewen, a paleontologist at the Natural History Museum of Utah and author of the study, “because it became clear that we had found something new.”
Because the skull was destined for a museum in Denmark, the team named the animal after the Norse god Loki: “It really does look like the helmet Loki wears,” Dr Lowen said.
The discovery sheds light on the evolution of ceratopsians in North America, Dr Sertich said. During the Late Cretaceous period, the continent was split in half by an inland sea. The western Laramidia subcontinent was home to two groups of ceratopsians. Chasmosaurinae (ancestors of Triceratops) tend to be found in the southern half of the subcontinent, while Centrosaurinae, which includes Lociceratops, are generally found further north.
Lociceratops is the fourth centrosaurine to be discovered in the Montana ecosystem.
Fossils of these species have not been found anywhere else in North America and fit into a broader pattern of ceratopsian diversity in the West, the researchers say.
“You don't find animals in Utah that lived in Canada, or animals in Utah that lived in New Mexico,” Dr. Loewen said.
The team suggests that this change may have been driven by sexual selection: different populations of female ceratopsians developed specific aesthetic preferences, which in turn drove an evolutionary explosion in local species. In modern ecosystems, this process would cause closely related birds of paradise to develop different displays despite sharing an ecological niche.
By the end of this period, centrosaurines had all but disappeared, and animals like Triceratops and T. rex lived from Mexico to Canada, suggesting the continent had become much more homogenous, Dr Sertich said.
“This has implications for the modern world: as the world warms and the climate changes, animal distributions are changing,” he added. “Studying past climates and ecosystems and how they responded will impact our understanding of what may happen in the future.”
Not everyone agrees with this explanation, or believes animals like Lociceratops represent separate species. Denver Fowler, a paleontologist at the Dickinson Museum in North Dakota who was not involved in the study, said that many of the ceratopsian species are based on limited fossils, which can lead to overinterpretation.
For example, the hollow rostrum seen in Lociceratops was also present in the earliest adult Triceratops, and the asymmetrical horn projections on the frill may have been a genetic trait, he said.
“A lot of the features we see here could be vestiges of a very mature Medusaceratops, which would be a more conservative explanation,” Dr Fowler said.
Fowler and some of his colleagues favor an alternative theory: that a few species with strong individual differences migrated gradually from Mexico to Alaska. As more fossils are discovered, it will become clearer which differences were important, he adds.
“This is a fantastic specimen and absolutely needs to be described,” Dr Fowler said. “It will really help us look into the fauna in more detail.”
Dr Sertic said that as more fossils are discovered, the team will be able to test whether Lociceratops is a unique species.
“I think we'll soon discover eight undescribed species,” Dr. Loewen says, “and I don't think we have even 1% of the true ceratopsian diversity that existed in North America.”