There was actually quite a lot of variation in the frills of Styracosaurus, with varying numbers of long spikes and extra hook-like projections present on some individuals. But one recently-discovered specimen nicknamed “Hannah” is especially surprising – it had a noticeable amount of asymmetry in its skull. The left and right sides show different numbers and arrangements of spikes, so much so that if the two halves had been discovered separately they might have been identified as belonging to two completely different species.
Frill arrangements are often used to define different ceratopsids, so if this level of individual variation and asymmetry existed in other species, too, then we may need to reevaluate some of them.
At first much of this dinosaur’s anatomy was poorly understood, and at first it was misidentified as a hadrosaur. The skull remains were fragmentary and ceratopsians hadn’t yet been identified as a group, so Monoclonius‘ horns weren’t even recognized as being horns and a piece of the frill was initially misinterpreted as part of a breastbone.
Once the much better-preserved Triceratops was discovered in 1889, and the existence of ceratopsians was recognized, Monoclonius was re-examined and identified as a similar dinosaur – and three more species were quickly described within the genus, also based on very fragmentary fossils.
Then for a while afterwards every ceratopsid fossil that wasn’t clearly a Triceratops was then just dumped into Monoclonius, quickly turning the genus into a wastebasket full of dubious indistinct remains.
Centrosaurus apertus was named in 1904, from the similarly-aged Dinosaur Park Formation in southern Alberta, Canada. It had originally been one of the various species of Monoclonius, but was now claimed to be different enough to deserve its own separate genus name – and this started a decades-long controversy between several paleontologists.
Over the new few decades arguments went back and forth over whether Centrosaurus was actually valid or if it was just a junior synonym of Monoclonius. As more and better ceratopsid fossil material was discovered several other Monoclonius species were eventually split off into their own separate genera, too, creating Styracosaurus, Chasmosaurus, and the somewhat dubious Brachyceratops. But other new species also continued to be lumped into Monocloniusup until 1990, meaning that over its century of existence this wastebasket taxon had at one point or another contained at least 16 different species.
During the 1990s opinion began to turn against Monoclonius, increasingly regarding it as a dubious name. Its original type specimen was a chimera of multiple different individuals (and possibly multiple different species), and it just didn’t have any distinct enough anatomical features to distinguish it from other ceratopsids.
Centrosaurus, meanwhile, was further validated by the discovery of huge bonebeds containing thousands of individuals, making it into one of the best-known of all ceratopsians.
Today Monoclonius‘ name remains attached to a few fossil specimens, but only the ones that are too indistinct to classify as anything else. Some “Monoclonius” have also turned out to actually be juveniles and subadults of other ceratopsians – it seems many young centrosaurines had a Monoclonius-like stage in their growth, before they went on to develop their own species’ distinctive horn and frill shapes.
So Monoclonius may never have been a distinct genus at all – it was just a bunch of different ceratopsian teenagers!
Sierraceratops lived during the Late Cretaceous, around 72 million years ago, in what at the time was the southern region of the island continent of Laramidia. About 4.6m long (~15′), it had fairly short chunky brow horns, long pointed cheek horns, and a relatively large frill.
They were fairly small stocky quadrupedal dinosaurs, sort of pig-like, with short deep jaws and powerful beaks adapted for eating fibrous low-level plants like ferns and cycads – and to process such tough food they even evolved a chewing style similar to mammals like rodents.
And while typically depicted as purely herbivorous, ceratopsids’ powerful parrot-like beaks and lack of grinding teeth suggest they may actually have been somewhat more omnivorous – the Cretaceous equivalent of pigs – still feeding mainly on plant matter but also munching on carrion and opportunistically eating smaller animals when they got the chance.
Machairoceratops cronusi here lived during the late Cretaceous of Utah, USA, about 77 million years ago. Only one partial skull has ever been found belonging to an individual about 4.5m long (14’9″), but it wasn’t fully grown and so probably reached slightly larger sizes.
It had two long spikes at the top of its frill, similar to its close relative Diabloceratops but curving dramatically forward and downwards above its face. Whether they were purely for display or used in horn-locking shoving matches is unknown, but either way it was a unique arrangement compared to all other known ceratopsids.
About 6m long (19′8″), it had long brow horns and large curved spikes on its frill – an arrangement very similar in appearance to the centrosaurAlbertaceratops, and initially its fossils were misidentified as belonging to that particular ceratopsid. But in 2010 it was recognized as a different genus, and based on some partial frill remains it was classified as a very early chasmosaur (a different branch of the ceratopsids which includes Triceratops), related to other early forms like Mercuriceratops.
Its genus name was based on the snake-haired Medusa from Greek mythology, while its species name comes from the Norse trickster god Loki – both in reference to the years of confusion about the identity of Medusaceratops’ fossils, and the distinctive curved horns on the helmet of Marvel’s Loki.
And, true to its name, the confusion wasn’t over yet.
Recently more fossil material and a new study have shown it was still being misclassified. Now it seems like Medusaceratops was actually part of the centrosaur lineage all along, and was indeed a very close relative of Albertaceratops.
It also turns out that what were thought to be numerous Albertaceratops fossils found in the same location were all just even more Medusaceratops. Instead of a mixture of two different ceratopsids there’s a single big bonebed representing some sort of mass-mortality event of only this one animal.
Similar mass bonebeds have been found for other centrosaurs in the same area and around the same age. Perhaps there were frequent flash floods at the time, or they were attempting to migrate across fast-flowing rivers like some modern animals, but we still don’t actually know for certain why they died en masse so frequently.
Those extinct horses weren’t the only ancient creatures with unexplained noses. Some dinosaurs had equally weird things going on with their snouts – and while hadrosaurs’ big honkin’ snoots are fairly well-known, there were other ornithischians with their own bizarre nasal anatomy.
Many ceratopsids had an enormous nasal opening forming a giant bony “window” through their snout, with the chasmosaurines like the famous Triceratops having additional bony projections and hollowed regions within these holes. They probably supported some huge elaborate cartilage structures in life, but what they were for is still a mystery. They may have helped with heat dissipation or moisture conservation, aided sound production, provided a highly sensitive sense of smell, housed a vomeronasal organ, held part of an air-filled pneumatic system… or, getting more speculative, possibly even some sort of inflatable nasal display structure.
Some ankylosaurids, meanwhile, went with multiple holes instead. Minotaurasaurus here had two additional openings around its nostrils, and Pinacosaurus could have up to five – the purpose of which is unknown. Many ankylosaurs also had forward-facing nostrils (a rare trait in archosaurs) and incredibly complex looping airways through their skulls. These may have allowed for mammal-like “air conditioning”, regulating the heat and moisture content of each breath, or perhaps enhanced their sense of smell or served some sort of resonance chamber function. Or, again, maybe even nose balloons.