marginocephalia – Nix Illustration

Strange Symmetries #17: Spiky Surprise

Styracosaurus albertensis was a ceratopsid dinosaur living during the late Cretaceous about 75 million years ago, in what is now Alberta, Canada. Around 5m long (~16′), it was one of the most elaborately ornamented horned dinosaurs, with a long nose horn and multiple elongated spikes on its frill.

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.

It Came From The Wastebasket #10: Struggling With Stegoceras

First described and named in the early 1900s, Stegoceras validum was a dog-sized small pachycephalosaur that lived in Alberta, Canada, during the Late Cretaceous (~77-74 million years ago).

Initially just known from its skull domes, it was one of the first pachycephalosaurs to be discovered and was very poorly understood until more complete remains were found in the 1920s. Then it spent a couple of decades being mixed up with Troodon due to similarities in tooth shape, until the discovery of Pachycephalosaurus led to pachycephalosaurs finally being recognized as a distinct group of ornithischian dinosaurs in the 1940s.

An illustration of Stegoceras, an extinct pachycephalosaur. It's a small bipedal dinosaur with tiny arms, bird-like legs, a speculative coat of fluffy protofeathers over most of its body, and a long tapering tail with speculative bristly quills. It has a large bony dome on its forehead, rimmed with short spikes, and a short snout with a stubby beak. It's mainly colored white and grey, with brighter red and yellow markings on its face and red towards the tip of its tail. — *Stegoceras validum*

For much of the 20^th century Stegoceras was treated as a wastebasket taxon for any small-to-mid-sized North American (and one Asian) pachycephalosaur, and multiple different species were named based on what were often rather dubious fragmentary fossils. But towards the start of the 21^st century this mess did start getting cleaned up, merging some dubious species into the original Stegoceras validum, and moving others to separate genera like Sphaerotholus, Colepiocephale, Hanssuesia, and Sinocephale.

By the early 2000s just the Canadian Stegoceras validum remained – but then in 2011 the new species Stegoceras novomexicanum was named based on specimens from New Mexico, USA. The validity of this second species has been debated, since the fossils are juveniles and might instead belong to Stegoceras validum or another genus like Sphaerotholus, but if it is some sort of Stegoceras then it significantly re-extends the known geographic range of this little pachycephalosaur.

Sierraceratops

In the late 1990s a partial skeleton of a ceratopsian was discovered in New Mexico, USA. These remains were initially thought to belong to Torosaurus, but after more of the specimen was recovered in the mid-2010s it became clear the bones actually represented an entirely new species of horned dinosaur – officially named in 2022 as Sierraceratops turneri.

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.

It was part of a unique lineage of ceratopsians that were endemic to southern Laramidia, with its closest known relatives being Bravoceratops from western Texas and Coahuilaceratops from northern Mexico.

Spectember 2021 – Quadrupedal Pachycephalosaurs

Today’s #Spectember concept comes from an anonymous submitter:

Prenoceratops

Although much less famous than their larger horned and frilled relatives, the leptoceratopsids were a widespread and successful group of ceratopsian dinosaurs during the Late Cretaceous, with fossils known from North America, Asia, and Europe (and, dubiously, Australia).

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.

Prenoceratops pieganensis here is known from the Two Medicine Formation bone beds in Montana, USA, dating to about 74 million years ago. Around 1.5-2m long (~5′-6’6″), it was very similar to its later relative Leptoceratops, but had a slightly lower, more sloping shape to its skull.

Weird Heads Month #18: Boneheaded Dinosaurs

Pachycephalosaurs are highly recognizable dinosaurs with their thick spiky skulls, and it’s not hugely surprising that they were the evolutionary cousins of the equally weird-headed ceratopsians.

Much like their frilled relatives they had beaks at the tips of their snouts and large gut cavities for digesting plant matter, but they also had surprisingly sharp theropod-like teeth in front of their more standard herbivore teeth further back – suggesting they may also have been opportunistic omnivores, occasionally snacking on carrion or small animals similarly to modern pigs or bears.

Their striking-looking dome heads were probably used for combat, headbutting or flank-butting each other, and many fossil skulls show evidence of injuries that would have been caused by that sort of behavior.

The eponymous Pachycephalosaurus wyomingensis lived in North America right at the end of the Cretaceous, about 70-66 million years ago. It was one of the largest of its kind, reaching lengths of around 4.5m (14’9″), and was characterized by a large bony dome-head surrounded by small blunt spikes.

But it turns out that was probably only what it looked like as a fully mature adult.

Recent discoveries of juvenile Pachycephalosaurus skulls confirmed a hypothesis proposed a few years earlier: these dinosaurs changed appearance drastically as they grew up, and younger individuals had been mistaken for separate species. They started off with domeless flat heads, bristling with long spikes (a form previously named Dracorex hogwartsia) then as they matured their domes began to grow (previously Stygimoloch spinifer) and by full maturity they had big domes with the spikes shrunk down to smaller stubbier knobs (the classic Pachycephalosaurus look).

This particular reconstruction depicts a Stygimoloch-like subadult individual, not quite fully mature and still sporting some longer spikes.

Weird Heads Month #14: Horns and Frills

We can’t go through this month without having an appearance from the most famous group of weird-headed dinosaurs: the ceratopsids!

Their distinctive-looking skulls were highly modified from those of their ancestors, with large bony frills extending from the back of their heads, various elaborate horns and spikes, enormous nasal cavities, large hooked beaks at the front of their snouts, and rows of slicing teeth further back.

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.

Medusaceratops

Medusaceratops lokii, a ceratopsid from the Late Cretaceous of Montana, USA (~77.5 mya).

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 centrosaur Albertaceratops, 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.

Unsolved Paleo Mysteries Month #16 – Strange Snoots 2: Oddball Ornithischians

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.

An illustration of the skull of an extinct horned dinosaur, showing the unusually large nasal cavity. Below is a reconstruction of the dinosaur's head in life. — *Triceratops horridus* skull and head reconstruction

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.

An illustration of the skull of an extinct armored dinosaur, showing the multiple holes inside the nasal cavity. Below is a reconstruction of the dinosaur's head in life. — *Minotaurasaurus ramachandrani* skull and head reconstruction

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.

~~Also floofy ankylosaur because I can.~~