Despite becoming extinct millions of years ago, new dinosaur species are still being discovered. Photo Credit: Lucas George Wendt via Unsplash
In the spring of 2019, commercial fossil hunter Mark Eatman unearthed a dinosaur’s skull and part of its skeleton in the Judith River Formation in Montana, near the USA-Canada border. 78 million years ago, the now-arid landscape was a swampy, marsh-like floodplain adjacent to the Western Interior Seaway, which divided North America into two island continents, Laramidia and Appalachia. At the time, during the Late Cretaceous, a distinctive family of dinosaurs roamed Laramidia. Ceratopsidae, which includes the well-known Triceratops spp., sported elaborate frills and horns and travelled in herds. The newly discovered specimen, named Lokiceratops rangiformis, has been added to the Centrosaurine subfamily.
The newly discovered specimen, named Lokiceratops rangiformis, has been added to the Centrosaurine subfamily.
Following cleaning and restoration of Eatman’s specimen, a team of paleontologists performed structural and phylogenetic analysis, the results of which have been published in a new article in PeerJ. The team estimates that L. rangiformis was a 6.7-metre-long herbivore that weighed five tonnes. But the new species’ most striking feature was a pair of blade-like, curved horns protruding from the back of its frill.
But the new species’ most striking feature was a pair of blade-like, curved horns protruding from the back of its frill.
L. rangiformis’ physical features informed its name, chosen by co-lead authors Joseph Sertich (Smithsonian Tropical Research Institute) and Mark Loewen (Natural History Museum of Utah). The generic name alludes to Loki, the trickster god of Norse mythology, who in some depictions is horned. The choice of a Norse god is especially fitting given the dinosaur now resides permanently in Denmark’s Museum of Evolution. Meanwhile, the specific name refers to the asymmetry of the horns at the peak of the frill, which resembles the asymmetry of antlers seen in the reindeer genus Rangifer.
The narrow coastal portion of Laramidia where L. rangiformis roamed was populated by three other Centrosaurine species, including one whose name is similarly mythology-inspired—Medusaceratops lokii. The limited geographical range of these dinosaurs contrasts the pattern we see nowadays, where large animals typically occupy correspondingly larger ranges.
A number of marked morphological differences distinguish L. rangiformis, M. lokii, and the other Centrosaurine species: Lokiceratops was 20% larger, lacked a nose horn, and had a ‘distinct kink’ in the ischium, a pelvic bone. This unprecedentedly high level of speciation (with four distinct horned dinosaur species living in the same place at the same time) could have been driven by small regional variations in climate. Alternatively, latitudinal variation in floral gradients might have led to diet specialisation, with different groups of herbivorous dinosaurs preferring to eat distinct foods. But in a PeerJ video interview, Loewen suggests it is more likely that speciation occurred by sexual selection, with L. rangiformis and other ceratopsids using their horns as ornaments to attract mates. Other paleontologists who were not directly involved in the study have supported this view: David Norman, a University of Cambridge professor, likened the phenomenon to the case of the Paradisaeidae family of birds (“birds of paradise”), where sexual selection has resulted in the development of ‘extremely flamboyant’ features.
This unprecedentedly high level of speciation (with four distinct horned dinosaur species living in the same place at the same time) could have been driven by small regional variations in climate.
Other researchers are not so convinced about L. rangiformis. Jordan Mallon, a paleontologist at the Canadian Museum of Nature, told Science it is ‘contentious as to whether [L. rangiformis] represents a new species or not’. He notes that if northern Laramidia did support the hypothesised level of diversity, we would expect different species to adopt distinct feeding patterns to avoid competition. The jaws and teeth of ceratopsids are too similar for this to have been the case. It is possible that differences in horns between L. rangiformis and the three other Centrosaurine species simply reflect morphological variation between individuals of a single species at one point in time, just as dogs can appear vastly different whilst all belonging to the Canis familiaris species. Denver Fowler (Dickinson Museum Center) suggests this could arise from age-related changes in morphology—the L. rangiformis specimen may have been a very mature individual.
It is possible that differences in horns between L. rangiformis and the three other Centrosaurine species simply reflect morphological variation between individuals of a single species
This debate is not unprecedented. In the late 1800s, Yale paleontologist Othniel Charles Marsh first named Triceratops and then, two years later, Torosaurus. The two genera could be distinguished by the absence or presence, respectively, of large holes (fenestrae) in their frills. But in 2010, professor Jack Horner and his doctoral student (Montana State University) asserted that Triceratops and Torosaurus simply represented growth stages of a single genus. Through analysis of skull anatomy and bone microstructure, they found regions of reduced bone thickness in Triceratops skulls that they argued would open up into actual fenestrae. One fossil (named Nedoceratops), seemingly showing an intermediate anatomy, supported their argument. Their idea, however, was refuted by Nicholas Longrich and Daniel Field (Yale University), who performed structural analysis of 35 skulls and identified Torosaurus specimens with unfused skull bones, indicating immaturity. The finding that the position of Triceratops skull depressions did not align with Torosaurus skull openings further poked a hole in Horner’s Nedoceratops intermediate theory.
Whilst today Triceratops and Torosaurus are regarded as distinct species, it is still critical that paleontologists comparing the morphology of specimens consider other sources of variation. Ontogeny (the course of development of an organism) is one such source, but differences in specimens can also reflect evolutionary changes within a single species over time.
Given that fossils no longer contain sufficiently preserved DNA molecules for genetic analysis, the paleontologist can define species only by morphology (shape and structure of the specimen). We cannot conclude with certainty that L. rangiformis was genetically separate from its fellow northern Laramidia-dwelling ceratopsids but, for now, the scientific community remains optimistic—a replica of the new dinosaur can already be found at the Natural History Museum of Utah. As Joseph Sertich says, the discovery of Lokiceratops ‘changes the way we think about evolution—at least of horned dinosaurs’.
